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GitLab

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<h3>

 Marge Bot pushed to branch wip/marge_bot_batch_merge_job

at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>

</h3>

<h4>

Commits:

</h4>

<ul>

<li>

<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a182ec32bf5edfc5ca100772ebd15eaa7ca7475e">a182ec32</a></strong>

<div>

<span>by Sylvain Henry</span>

<i>at 2020-08-12T14:22:40-04:00</i>

</div>

<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Rewrite and move the monad-state hack note

The note has been rewritten by @simonpj in !3851

[skip ci]

</pre>

</li>

<li>

<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/855f71a9c639a72833b63cd03061cc92785c2228">855f71a9</a></strong>

<div>

<span>by Alan Zimmerman</span>

<i>at 2020-08-12T14:22:41-04:00</i>

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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">ApiAnnotations: Fix parser for new GHC 9.0 features

</pre>

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</ul>

<h4>5 changed files:</h4>

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<li class="file-stats">

<a href="#c3967bb9d3e8f5aae2dd111b5a335b48c21c1999">

compiler/GHC/Core/Opt/Arity.hs

</a>

</li>

<li class="file-stats">

<a href="#37879a759bc2ca5cc8f1f35235fe96e3e55b57c9">

compiler/GHC/Core/Opt/Simplify/Monad.hs

</a>

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<li class="file-stats">

<a href="#89a2e0e1a3095c7a42b10f2819049908908080e8">

compiler/GHC/Core/Unify.hs

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<li class="file-stats">

<a href="#bf951467d4a9aa443cb109cb4c84a2891945649b">

compiler/GHC/Parser.y

</a>

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<li class="file-stats">

<a href="#cb4ad425e8ed846dfcc5d3793b08b95f6864e545">

compiler/GHC/Utils/Monad.hs

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<h4>Changes:</h4>

<li id="c3967bb9d3e8f5aae2dd111b5a335b48c21c1999">

<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/a4a1660cfea13d5b918c8d9240c76026c411b04e...855f71a9c639a72833b63cd03061cc92785c2228#c3967bb9d3e8f5aae2dd111b5a335b48c21c1999"><strong>compiler/GHC/Core/Opt/Arity.hs</strong></a>

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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1031" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="diff-line-num unfold js-unfold new_line" data-linenumber="1031" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -1031,7 +1031,7 @@ one-shot flag from the inner \s{osf}.  By expanding with the</td>

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1031

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<pre style="margin: 0;"> <span id="LC1031" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">ArityType gotten from analysing the RHS, we achieve this neatly.</span></span>

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<td class="diff-line-num old_line" data-linenumber="1032" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1032

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<td class="diff-line-num new_line" data-linenumber="1032" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1032

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<pre style="margin: 0;"> <span id="LC1032" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="1033" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1033

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<td class="diff-line-num new_line" data-linenumber="1033" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1033

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<pre style="margin: 0;"> <span id="LC1033" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">This makes a big difference to the one-shot monad trick;</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1034" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1034

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<td class="diff-line-num new_line old" data-linenumber="1034" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1034" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">see Note [The one-shot state monad trick] in GHC.<span class="idiff left right">Core.Unify</span>.</span></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="1035" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="1034" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

1034

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<pre style="margin: 0;">+<span id="LC1034" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">see Note [The one-shot state monad trick] in GHC.<span class="idiff left right">Utils.Monad</span>.</span></span>

</pre>

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1035

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<td class="diff-line-num new_line" data-linenumber="1035" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1035

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<pre style="margin: 0;"> <span id="LC1035" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span>

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<td class="diff-line-num old_line" data-linenumber="1036" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1036

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<pre style="margin: 0;"> <span id="LC1036" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="1037" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1037

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<td class="diff-line-num new_line" data-linenumber="1037" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1037

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<pre style="margin: 0;"> <span id="LC1037" class="line" lang="haskell"><span class="cd">-- | @etaExpand n e@ returns an expression with</span></span>

</pre>

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<br>

</li>

<li id="37879a759bc2ca5cc8f1f35235fe96e3e55b57c9">

<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/a4a1660cfea13d5b918c8d9240c76026c411b04e...855f71a9c639a72833b63cd03061cc92785c2228#37879a759bc2ca5cc8f1f35235fe96e3e55b57c9"><strong>compiler/GHC/Core/Opt/Simplify/Monad.hs</strong></a>

<hr>

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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="71" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="diff-line-num unfold js-unfold new_line" data-linenumber="71" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -71,7 +71,7 @@ pattern SM :: (SimplTopEnv -> UniqSupply -> SimplCount</td>

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<td class="diff-line-num old_line" data-linenumber="71" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC71" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- This pattern synonym makes the simplifier monad eta-expand,</span></span>

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<td class="diff-line-num old_line" data-linenumber="72" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<td class="diff-line-num new_line" data-linenumber="72" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC72" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- which as a very beneficial effect on compiler performance</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="73" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

73

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<td class="diff-line-num new_line" data-linenumber="73" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC73" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- (worth a 1-2% reduction in bytes-allocated).  See #18202.</span></span>

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<td class="diff-line-num old old_line" data-linenumber="74" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

74

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<td class="diff-line-num new_line old" data-linenumber="74" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC74" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick] in GHC.<span class="idiff left right">Core.Unify</span></span></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="75" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="74" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<pre style="margin: 0;">+<span id="LC74" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick] in GHC.<span class="idiff left right">Utils.Monad</span></span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="75" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

75

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<td class="diff-line-num new_line" data-linenumber="75" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC75" class="line" lang="haskell"><span class="n" style="color: #333;">pattern</span> <span class="kt" style="color: #458; font-weight: 600;">SM</span> <span class="n" style="color: #333;">m</span> <span class="o" style="font-weight: 600;"><-</span> <span class="kt" style="color: #458; font-weight: 600;">SM'</span> <span class="n" style="color: #333;">m</span></span>

</pre>

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<pre style="margin: 0;"> <span id="LC76" class="line" lang="haskell">  <span class="kr" style="font-weight: 600;">where</span></span>

</pre>

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<pre style="margin: 0;"> <span id="LC77" class="line" lang="haskell">    <span class="kt" style="color: #458; font-weight: 600;">SM</span> <span class="n" style="color: #333;">m</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">SM'</span> <span class="p">(</span><span class="n" style="color: #333;">oneShot</span> <span class="n" style="color: #333;">m</span><span class="p">)</span></span>

</pre>

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<br>

</li>

<li id="89a2e0e1a3095c7a42b10f2819049908908080e8">

<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/a4a1660cfea13d5b918c8d9240c76026c411b04e...855f71a9c639a72833b63cd03061cc92785c2228#89a2e0e1a3095c7a42b10f2819049908908080e8"><strong>compiler/GHC/Core/Unify.hs</strong></a>

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<pre style="margin: 0;"> <span id="LC1212" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">************************************************************************</span></span>

</pre>

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<pre style="margin: 0;"> <span id="LC1213" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span>

</pre>

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<pre style="margin: 0;"> <span id="LC1214" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1215" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [The one-shot state monad trick]</span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1216" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1217" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Many places in GHC use a state monad, and we really want those</span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1218" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">functions to be eta-expanded (#18202).  Consider</span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1219" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1220" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    newtype M a = MkM (State -> (State, a))</span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1221" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1222" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    instance Monad M where</span></span>

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<pre style="margin: 0;">-<span id="LC1223" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       mf >>= k = MkM (\s -> case mf  of MkM f  -></span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1224" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                             case f s of (s',r) -></span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1225" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                             case k r of MkM g  -></span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1226" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                             g s')</span></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1227" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">-<span id="LC1228" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    foo :: Int -> M Int</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1229" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1229

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1229" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    foo x = g y >>= \r -> h r</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1230" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1230

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1230" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      where</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1231" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1231

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1231" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">        y = expensive x</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1232" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1232

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1232" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1233" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1233

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1233" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">In general, you might say (map (foo 4) xs), and expect (expensive 4)</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1234" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1234

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1234" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">to be evaluated only once.  So foo should have arity 1 (not 2).</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1235" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1235

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1235" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">But that's rare, and if you /aren't/ re-using (M a) values it's much</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1236" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1236

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1236" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">more efficient to make foo have arity 2.</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1237" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1237

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1237" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1238" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1238

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1238" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See https://www.joachim-breitner.de/blog/763-Faster_Winter_5__Eta-Expanding_ReaderT</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1239" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1239

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1239" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1240" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1240

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1240" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">So here is the trick.  Define</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1241" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1241

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1241" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1242" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1242

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1242" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    data M a = MkM' (State -> (State, a))</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1243" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1243

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1243" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    pattern MkM f <- MkM' f</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1244" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1244

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1244" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      where</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1245" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1245

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1245" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">        MkM f = MkM' (oneShot f)</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1246" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1246

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1246" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1247" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1247

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1247" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The patten synonm means that whenever we write (MkM f), we'll</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1248" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1248

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1248" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">actually get (MkM' (oneShot f)), so we'll pin a one-shot flag</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1249" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1249

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<pre style="margin: 0;">-<span id="LC1249" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">on f's lambda-binder. Now look at foo:</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1250" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1250

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1250" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1251" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1251

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1251" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  foo = \x. g (expensive x) >>= \r -> h r</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1252" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1252

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1252" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      = \x. let mf = g (expensive x)</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1253" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1253

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1253" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                k  = \r -> h r</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1254" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1254

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1254" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            in MkM' (oneShot (\s -> case mf  of MkM' f  -></span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1255" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1255

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1255" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                                    case f s of (s',r) -></span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1256" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1256

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1256" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                                    case k r of MkM' g  -></span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1257" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1257

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1257" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                                    g s'))</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1258" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1258

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1258" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      -- The MkM' are just newtype casts nt_co</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1259" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1259

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1259" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      = \x. let mf = g (expensive x)</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1260" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1260

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1260" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                k  = \r -> h r</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1261" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1261

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1261" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            in (\s{os}. case (mf |> nt_co) s of (s',r) -></span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1262" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1262

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1262" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                        (k r) |> nt_co s')</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1263" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1263

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1263" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">               |> sym nt_co</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1264" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1264

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1264" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1265" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1265

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1265" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      -- Float into that \s{os}</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1266" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1266

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1266" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      = \x. (\s{os}. case (g (expensive x) |> nt_co) s of (s',r) -></span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1267" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1267

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<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC1267" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                     h r |> nt_co s')</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1268" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1268

</td>

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<pre style="margin: 0;">-<span id="LC1268" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            |> sym nt_co</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1269" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1269

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1269" class="line" lang="haskell"></span>

</pre>

</td>

</tr>

<tr class="line_holder old" id="" style="line-height: 1.6;">

<td class="diff-line-num old old_line" data-linenumber="1270" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1270

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1270" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">and voila!  In summary:</span></span>

</pre>

</td>

</tr>

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<td class="diff-line-num old old_line" data-linenumber="1271" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1271

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1271" class="line" lang="haskell"></span>

</pre>

</td>

</tr>

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<td class="diff-line-num old old_line" data-linenumber="1272" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1272

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1272" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It's a very simple, two-line change</span></span>

</pre>

</td>

</tr>

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<td class="diff-line-num old old_line" data-linenumber="1273" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1273

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1273" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1274" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1274

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1274" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It eta-expands all uses of the monad, automatically</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1275" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1275

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1275" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1276" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1276

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1276" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It is very similar to the built-in "state hack" (see</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1277" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1277

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1277" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  GHC.Core.Opt.Arity Note [The state-transformer hack]) but the trick</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1278" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1278

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1278" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  described here is applicable on a monad-by-monad basis under</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1279" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1279

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1279" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  programmer control.</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1280" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1280

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1280" class="line" lang="haskell"></span>

</pre>

</td>

</tr>

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<td class="diff-line-num old old_line" data-linenumber="1281" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1281

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1281" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Beware: itt changes the behaviour of</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1282" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1282

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1282" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">     map (foo 3) xs</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1283" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1283

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

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<pre style="margin: 0;">-<span id="LC1283" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  ToDo: explain what to do if you want to do this</span></span>

</pre>

</td>

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<tr class="line_holder old" id="" style="line-height: 1.6;">

<td class="diff-line-num old old_line" data-linenumber="1284" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1284

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1284" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1285" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1285

</td>

<td class="diff-line-num new_line old" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1285" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1286" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1286

</td>

<td class="diff-line-num new_line" data-linenumber="1215" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1215

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<pre style="margin: 0;"> <span id="LC1215" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">data</span> <span class="kt" style="color: #458; font-weight: 600;">UMEnv</span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1287" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1287

</td>

<td class="diff-line-num new_line" data-linenumber="1216" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1216

</td>

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<pre style="margin: 0;"> <span id="LC1216" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">UMEnv</span> <span class="p">{</span> <span class="n" style="color: #333;">um_unif</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">AmIUnifying</span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1288" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1288

</td>

<td class="diff-line-num new_line" data-linenumber="1217" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1217

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1217" class="line" lang="haskell"></span>

</pre>

</td>

</tr>

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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1311" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="diff-line-num unfold js-unfold new_line" data-linenumber="1240" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -1311,11 +1240,11 @@ data UMState = UMState</td>

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<td class="diff-line-num old_line" data-linenumber="1311" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1311

</td>

<td class="diff-line-num new_line" data-linenumber="1240" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1240

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1240" class="line" lang="haskell"></span>

</pre>

</td>

</tr>

<tr class="line_holder" id="" style="line-height: 1.6;">

<td class="diff-line-num old_line" data-linenumber="1312" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1312

</td>

<td class="diff-line-num new_line" data-linenumber="1241" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1241

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1241" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">newtype</span> <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="n" style="color: #333;">a</span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1313" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1313

</td>

<td class="diff-line-num new_line" data-linenumber="1242" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1242

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1242" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">UM'</span> <span class="p">{</span> <span class="n" style="color: #333;">unUM</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">UMState</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">UnifyResultM</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">UMState</span><span class="p">,</span> <span class="n" style="color: #333;">a</span><span class="p">)</span> <span class="p">}</span></span>

</pre>

</td>

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<tr class="line_holder old" id="" style="line-height: 1.6;">

<td class="diff-line-num old old_line" data-linenumber="1314" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1314

</td>

<td class="diff-line-num new_line old" data-linenumber="1243" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1314" class="line" lang="haskell">    <span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick]</span></span>

</pre>

</td>

</tr>

<tr class="line_holder new" id="" style="line-height: 1.6;">

<td class="diff-line-num new old_line" data-linenumber="1315" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

</td>

<td class="diff-line-num new new_line" data-linenumber="1243" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

1243

</td>

<td class="line_content new" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#ecfdf0">

<pre style="margin: 0;">+<span id="LC1243" class="line" lang="haskell">    <span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick]<span class="idiff left right"> in GHC.Utils.Monad</span></span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1315" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1315

</td>

<td class="diff-line-num new_line" data-linenumber="1244" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1244

</td>

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<pre style="margin: 0;"> <span id="LC1244" class="line" lang="haskell">  <span class="kr" style="font-weight: 600;">deriving</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">Functor</span><span class="p">)</span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1316" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1316

</td>

<td class="diff-line-num new_line" data-linenumber="1245" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1245

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1245" class="line" lang="haskell"></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1317" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1317

</td>

<td class="diff-line-num new_line" data-linenumber="1246" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1246

</td>

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<pre style="margin: 0;"> <span id="LC1246" class="line" lang="haskell"><span class="n" style="color: #333;">pattern</span> <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">UMState</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">UnifyResultM</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">UMState</span><span class="p">,</span> <span class="n" style="color: #333;">a</span><span class="p">))</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="n" style="color: #333;">a</span></span>

</pre>

</td>

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<td class="diff-line-num old old_line" data-linenumber="1318" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1318

</td>

<td class="diff-line-num new_line old" data-linenumber="1247" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

</td>

<td class="line_content old" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#fbe9eb">

<pre style="margin: 0;">-<span id="LC1318" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick]</span></span>

</pre>

</td>

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<td class="diff-line-num new old_line" data-linenumber="1319" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

</td>

<td class="diff-line-num new new_line" data-linenumber="1247" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

1247

</td>

<td class="line_content new" style="padding-left: 0.5em; padding-right: 0.5em;" bgcolor="#ecfdf0">

<pre style="margin: 0;">+<span id="LC1247" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick]<span class="idiff left right"> in GHC.Utils.Monad</span></span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1319" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1319

</td>

<td class="diff-line-num new_line" data-linenumber="1248" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1248

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1248" class="line" lang="haskell"><span class="n" style="color: #333;">pattern</span> <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="n" style="color: #333;">m</span> <span class="o" style="font-weight: 600;"><-</span> <span class="kt" style="color: #458; font-weight: 600;">UM'</span> <span class="n" style="color: #333;">m</span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1320" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1320

</td>

<td class="diff-line-num new_line" data-linenumber="1249" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1249

</td>

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<pre style="margin: 0;"> <span id="LC1249" class="line" lang="haskell">  <span class="kr" style="font-weight: 600;">where</span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1321" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1321

</td>

<td class="diff-line-num new_line" data-linenumber="1250" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1250

</td>

<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1250" class="line" lang="haskell">    <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="n" style="color: #333;">m</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">UM'</span> <span class="p">(</span><span class="n" style="color: #333;">oneShot</span> <span class="n" style="color: #333;">m</span><span class="p">)</span></span>

</pre>

</td>

</tr>

</table>

<br>

</li>

<li id="bf951467d4a9aa443cb109cb4c84a2891945649b">

<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/a4a1660cfea13d5b918c8d9240c76026c411b04e...855f71a9c639a72833b63cd03061cc92785c2228#bf951467d4a9aa443cb109cb4c84a2891945649b"><strong>compiler/GHC/Parser.y</strong></a>

<hr>

<table class="code white" style="font-family: monospace; font-size: 90%;" bgcolor="#fff" width="100%" cellpadding="0" cellspacing="0">

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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1961" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="diff-line-num unfold js-unfold new_line" data-linenumber="1961" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -1961,7 +1961,7 @@ type :: { LHsType GhcPs }</td>

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<td class="diff-line-num old_line" data-linenumber="1961" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1961

</td>

<td class="diff-line-num new_line" data-linenumber="1961" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1961

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<td class="line_content" style="padding-left: 0.5em; padding-right: 0.5em;">

<pre style="margin: 0;"> <span id="LC1961" class="line" lang="haskell"></span>

</pre>

</td>

</tr>

<tr class="line_holder" id="" style="line-height: 1.6;">

<td class="diff-line-num old_line" data-linenumber="1962" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1962

</td>

<td class="diff-line-num new_line" data-linenumber="1962" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1962

</td>

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<pre style="margin: 0;"> <span id="LC1962" class="line" lang="haskell">        <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">btype</span> <span class="sc" style="color: #d14;">'#</span><span class="err" style="color: #a61717; background-color: #e3d2d2;">-</span><span class="o" style="font-weight: 600;">></span><span class="sc" style="color: #d14;">' </span><span class="err" style="color: #a61717; background-color: #e3d2d2;">c</span><span class="kr" style="font-weight: 600;">type</span>             <span class="p">{</span><span class="o" style="font-weight: 600;">%</span> <span class="n" style="color: #333;">hintLinear</span> <span class="p">(</span><span class="n" style="color: #333;">getLoc</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">2</span><span class="p">)</span> <span class="o" style="font-weight: 600;">>></span></span>

</pre>

</td>

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<td class="diff-line-num old_line" data-linenumber="1963" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1963

</td>

<td class="diff-line-num new_line" data-linenumber="1963" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1963

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<pre style="margin: 0;"> <span id="LC1963" class="line" lang="haskell">                                         <span class="n" style="color: #333;">ams</span> <span class="p">(</span><span class="n" style="color: #333;">sLL</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="o" style="font-weight: 600;">$></span> <span class="o" style="font-weight: 600;">$</span> <span class="kt" style="color: #458; font-weight: 600;">HsFunTy</span> <span class="n" style="color: #333;">noExtField</span> <span class="kt" style="color: #458; font-weight: 600;">HsLinearArrow</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">3</span><span class="p">)</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="1964" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

1964

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<pre style="margin: 0;">-<span id="LC1964" class="line" lang="haskell">                                             <span class="p">[</span><span class="n" style="color: #333;">mu</span> <span class="kt" style="color: #458; font-weight: 600;">Ann<span class="idiff left right">Rarrow</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">2</span><span class="p">]</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="1965" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="1964" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

1964

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<pre style="margin: 0;">+<span id="LC1964" class="line" lang="haskell">                                             <span class="p">[</span><span class="n" style="color: #333;">mu</span> <span class="kt" style="color: #458; font-weight: 600;">Ann<span class="idiff left right">Lolly</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">2</span><span class="p">]</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="1965" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1965

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<td class="diff-line-num new_line" data-linenumber="1965" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1965

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<pre style="margin: 0;"> <span id="LC1965" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="1966" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1966

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<td class="diff-line-num new_line" data-linenumber="1966" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1966

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<pre style="margin: 0;"> <span id="LC1966" class="line" lang="haskell"><span class="n" style="color: #333;">mult</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">{</span> <span class="kt" style="color: #458; font-weight: 600;">LHsType</span> <span class="kt" style="color: #458; font-weight: 600;">GhcPs</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="1967" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1967

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<td class="diff-line-num new_line" data-linenumber="1967" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

1967

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<pre style="margin: 0;"> <span id="LC1967" class="line" lang="haskell">        <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">btype</span>                  <span class="p">{</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="2089" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="diff-line-num unfold js-unfold new_line" data-linenumber="2089" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -2089,10 +2089,10 @@ tv_bndrs :: { [LHsTyVarBndr Specificity GhcPs] }</td>

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2089

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2089

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<pre style="margin: 0;"> <span id="LC2089" class="line" lang="haskell"><span class="n" style="color: #333;">tv_bndr</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">{</span> <span class="kt" style="color: #458; font-weight: 600;">LHsTyVarBndr</span> <span class="kt" style="color: #458; font-weight: 600;">Specificity</span> <span class="kt" style="color: #458; font-weight: 600;">GhcPs</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="2090" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

2090

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2090

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<pre style="margin: 0;"> <span id="LC2090" class="line" lang="haskell">        <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">tv_bndr_no_braces</span>             <span class="p">{</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="2091" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

2091

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<td class="diff-line-num new_line" data-linenumber="2091" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

2091

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<pre style="margin: 0;"> <span id="LC2091" class="line" lang="haskell">        <span class="o" style="font-weight: 600;">|</span> <span class="sc" style="color: #d14;">'{'</span> <span class="n" style="color: #333;">tyvar</span> <span class="sc" style="color: #d14;">'}'</span>                 <span class="p">{</span><span class="o" style="font-weight: 600;">%</span> <span class="n" style="color: #333;">ams</span> <span class="p">(</span><span class="n" style="color: #333;">sLL</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="o" style="font-weight: 600;">$></span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">UserTyVar</span> <span class="n" style="color: #333;">noExtField</span> <span class="kt" style="color: #458; font-weight: 600;">InferredSpec</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">2</span><span class="p">))</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="2092" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

2092

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<td class="diff-line-num new_line old" data-linenumber="2092" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC2092" class="line" lang="haskell">                                               <span class="p">[</span><span class="n" style="color: #333;">mo<span class="idiff left">p</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff">$</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff">,</span></span><span class="idiff"> </span><span class="n" style="color: #333;"><span class="idiff right">mcp</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">3</span><span class="p">]</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="2093" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="2092" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

2092

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<pre style="margin: 0;">+<span id="LC2092" class="line" lang="haskell">                                               <span class="p">[</span><span class="n" style="color: #333;">mo<span class="idiff left">c</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff">$</span></span><span class="mi" style="color: #099;"><span class="idiff">1</span></span><span class="p"><span class="idiff">,</span></span><span class="idiff"> </span><span class="n" style="color: #333;"><span class="idiff right">mcc</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">3</span><span class="p">]</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="2093" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

2093

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<td class="diff-line-num new_line" data-linenumber="2093" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

2093

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<pre style="margin: 0;"> <span id="LC2093" class="line" lang="haskell">        <span class="o" style="font-weight: 600;">|</span> <span class="sc" style="color: #d14;">'{'</span> <span class="n" style="color: #333;">tyvar</span> <span class="sc" style="color: #d14;">':</span><span class="err" style="color: #a61717; background-color: #e3d2d2;">:</span><span class="sc" style="color: #d14;">' </span><span class="err" style="color: #a61717; background-color: #e3d2d2;">k</span><span class="n" style="color: #333;">ind</span> <span class="sc" style="color: #d14;">'}'</span>       <span class="p">{</span><span class="o" style="font-weight: 600;">%</span> <span class="n" style="color: #333;">ams</span> <span class="p">(</span><span class="n" style="color: #333;">sLL</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="o" style="font-weight: 600;">$></span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">KindedTyVar</span> <span class="n" style="color: #333;">noExtField</span> <span class="kt" style="color: #458; font-weight: 600;">InferredSpec</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">2</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">4</span><span class="p">))</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="2094" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

2094

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<td class="diff-line-num new_line old" data-linenumber="2094" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC2094" class="line" lang="haskell">                                               <span class="p">[</span><span class="n" style="color: #333;">mo<span class="idiff left right">p</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span><span class="p">,</span><span class="n" style="color: #333;">mu</span> <span class="kt" style="color: #458; font-weight: 600;">AnnDcolon</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">3</span></span>

</pre>

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<td class="diff-line-num old old_line" data-linenumber="2095" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

2095

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<td class="diff-line-num new_line old" data-linenumber="2094" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #fac5cd; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#f9d7dc">

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<pre style="margin: 0;">-<span id="LC2095" class="line" lang="haskell">                                               <span class="p">,</span><span class="n" style="color: #333;">mc<span class="idiff left right">p</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">5</span><span class="p">]</span> <span class="p">}</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="2094" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<pre style="margin: 0;">+<span id="LC2094" class="line" lang="haskell">                                               <span class="p">[</span><span class="n" style="color: #333;">mo<span class="idiff left right">c</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span><span class="p">,</span><span class="n" style="color: #333;">mu</span> <span class="kt" style="color: #458; font-weight: 600;">AnnDcolon</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">3</span></span>

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<td class="diff-line-num new old_line" data-linenumber="2096" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="2095" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<pre style="margin: 0;">+<span id="LC2095" class="line" lang="haskell">                                               <span class="p">,</span><span class="n" style="color: #333;">mc<span class="idiff left right">c</span></span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">5</span><span class="p">]</span> <span class="p">}</span></span>

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2096

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<pre style="margin: 0;"> <span id="LC2096" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="2097" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

2097

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2097

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<pre style="margin: 0;"> <span id="LC2097" class="line" lang="haskell"><span class="n" style="color: #333;">tv_bndr_no_braces</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">{</span> <span class="kt" style="color: #458; font-weight: 600;">LHsTyVarBndr</span> <span class="kt" style="color: #458; font-weight: 600;">Specificity</span> <span class="kt" style="color: #458; font-weight: 600;">GhcPs</span> <span class="p">}</span></span>

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2098

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2098

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<pre style="margin: 0;"> <span id="LC2098" class="line" lang="haskell">        <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">tyvar</span>                         <span class="p">{</span> <span class="n" style="color: #333;">sL1</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">UserTyVar</span> <span class="n" style="color: #333;">noExtField</span> <span class="kt" style="color: #458; font-weight: 600;">SpecifiedSpec</span> <span class="o" style="font-weight: 600;">$</span><span class="mi" style="color: #099;">1</span><span class="p">)</span> <span class="p">}</span></span>

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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="3728" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="diff-line-num unfold js-unfold new_line" data-linenumber="3728" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -3728,6 +3728,7 @@ isUnicode (L _ (ITcparenbar      iu)) = iu == UnicodeSyntax</td>

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<pre style="margin: 0;"> <span id="LC3728" class="line" lang="haskell"><span class="n" style="color: #333;">isUnicode</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">L</span> <span class="kr" style="font-weight: 600;">_</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">ITopenExpQuote</span> <span class="kr" style="font-weight: 600;">_</span> <span class="n" style="color: #333;">iu</span><span class="p">))</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">iu</span> <span class="o" style="font-weight: 600;">==</span> <span class="kt" style="color: #458; font-weight: 600;">UnicodeSyntax</span></span>

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<td class="diff-line-num old_line" data-linenumber="3729" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC3729" class="line" lang="haskell"><span class="n" style="color: #333;">isUnicode</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">L</span> <span class="kr" style="font-weight: 600;">_</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">ITcloseQuote</span>     <span class="n" style="color: #333;">iu</span><span class="p">))</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">iu</span> <span class="o" style="font-weight: 600;">==</span> <span class="kt" style="color: #458; font-weight: 600;">UnicodeSyntax</span></span>

</pre>

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<td class="diff-line-num old_line" data-linenumber="3730" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC3730" class="line" lang="haskell"><span class="n" style="color: #333;">isUnicode</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">L</span> <span class="kr" style="font-weight: 600;">_</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">ITstar</span>           <span class="n" style="color: #333;">iu</span><span class="p">))</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">iu</span> <span class="o" style="font-weight: 600;">==</span> <span class="kt" style="color: #458; font-weight: 600;">UnicodeSyntax</span></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="3731" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="3731" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<pre style="margin: 0;">+<span id="LC3731" class="line" lang="haskell"><span class="n" style="color: #333;">isUnicode</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">L</span> <span class="kr" style="font-weight: 600;">_</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">ITlolly</span>          <span class="n" style="color: #333;">iu</span><span class="p">))</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">iu</span> <span class="o" style="font-weight: 600;">==</span> <span class="kt" style="color: #458; font-weight: 600;">UnicodeSyntax</span></span>

</pre>

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<td class="diff-line-num new_line" data-linenumber="3732" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC3732" class="line" lang="haskell"><span class="n" style="color: #333;">isUnicode</span> <span class="kr" style="font-weight: 600;">_</span>                           <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">False</span></span>

</pre>

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<td class="diff-line-num new_line" data-linenumber="3733" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC3733" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num new_line" data-linenumber="3734" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">

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<pre style="margin: 0;"> <span id="LC3734" class="line" lang="haskell"><span class="n" style="color: #333;">hasE</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">Located</span> <span class="kt" style="color: #458; font-weight: 600;">Token</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Bool</span></span>

</pre>

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<br>

</li>

<li id="cb4ad425e8ed846dfcc5d3793b08b95f6864e545">

<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/a4a1660cfea13d5b918c8d9240c76026c411b04e...855f71a9c639a72833b63cd03061cc92785c2228#cb4ad425e8ed846dfcc5d3793b08b95f6864e545"><strong>compiler/GHC/Utils/Monad.hs</strong></a>

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<td class="diff-line-num unfold js-unfold new_line" data-linenumber="226" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>

<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -226,3 +226,175 @@ unlessM condM acc = do { cond <- condM</td>

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<pre style="margin: 0;"> <span id="LC226" class="line" lang="haskell"><span class="n" style="color: #333;">filterOutM</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">Applicative</span> <span class="n" style="color: #333;">m</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=></span> <span class="p">(</span><span class="n" style="color: #333;">a</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">m</span> <span class="kt" style="color: #458; font-weight: 600;">Bool</span><span class="p">)</span> <span class="o" style="font-weight: 600;">-></span> <span class="p">[</span><span class="n" style="color: #333;">a</span><span class="p">]</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">m</span> <span class="p">[</span><span class="n" style="color: #333;">a</span><span class="p">]</span></span>

</pre>

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<pre style="margin: 0;"> <span id="LC227" class="line" lang="haskell"><span class="n" style="color: #333;">filterOutM</span> <span class="n" style="color: #333;">p</span> <span class="o" style="font-weight: 600;">=</span></span>

</pre>

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<pre style="margin: 0;"> <span id="LC228" class="line" lang="haskell">  <span class="n" style="color: #333;">foldr</span> <span class="p">(</span><span class="nf" style="color: #900; font-weight: 600;">\</span> <span class="n" style="color: #333;">x</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">liftA2</span> <span class="p">(</span><span class="nf" style="color: #900; font-weight: 600;">\</span> <span class="n" style="color: #333;">flg</span> <span class="o" style="font-weight: 600;">-></span> <span class="kr" style="font-weight: 600;">if</span> <span class="n" style="color: #333;">flg</span> <span class="kr" style="font-weight: 600;">then</span> <span class="n" style="color: #333;">id</span> <span class="kr" style="font-weight: 600;">else</span> <span class="p">(</span><span class="n" style="color: #333;">x</span><span class="o" style="font-weight: 600;">:</span><span class="p">))</span> <span class="p">(</span><span class="n" style="color: #333;">p</span> <span class="n" style="color: #333;">x</span><span class="p">))</span> <span class="p">(</span><span class="n" style="color: #333;">pure</span> <span class="kt" style="color: #458; font-weight: 600;">[]</span><span class="p">)</span></span>

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<pre style="margin: 0;">+<span id="LC229" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC230" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [The one-shot state monad trick]</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC231" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC232" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Summary: many places in GHC use a state monad, and we really want those</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC233" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">functions to be eta-expanded (#18202).</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC234" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC235" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The problem</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC236" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC237" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Consider</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC238" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    newtype M a = MkM (State -> (State, a))</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC239" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC240" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    instance Monad M where</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC241" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       mf >>= k = MkM (\s -> case mf  of MkM f  -></span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC242" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                             case f s of (s',r) -></span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC243" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                             case k r of MkM g  -></span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC244" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                             g s')</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC245" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC246" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    fooM :: Int -> M Int</span></span>

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<pre style="margin: 0;">+<span id="LC247" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    fooM x = g y >>= \r -> h r</span></span>

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<pre style="margin: 0;">+<span id="LC248" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      where</span></span>

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<pre style="margin: 0;">+<span id="LC249" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">        y = expensive x</span></span>

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<pre style="margin: 0;">+<span id="LC250" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC251" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Now suppose you say (repeat 20 (fooM 4)), where</span></span>

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<pre style="margin: 0;">+<span id="LC252" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  repeat :: Int -> M Int -> M Int</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC253" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">performs its argument n times.  You would expect (expensive 4) to be</span></span>

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<pre style="margin: 0;">+<span id="LC254" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">evaluated only once, not 20 times.  So foo should have arity 1 (not 2);</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC255" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">it should look like this (modulo casts)</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC256" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC257" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  fooM x = let y = expensive x in</span></span>

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<pre style="margin: 0;">+<span id="LC258" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">           \s -> case g y of ...</span></span>

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<pre style="margin: 0;">+<span id="LC259" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC260" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">But creating and then repeating, a monadic computation is rare.  If you</span></span>

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<pre style="margin: 0;">+<span id="LC261" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">/aren't/ re-using (M a) value, it's /much/ more efficient to make</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC262" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">foo have arity 2, thus:</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC263" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC264" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  fooM x s = case g (expensive x) of ...</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC265" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC266" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Why more efficient?  Because now foo takes its argument both at once,</span></span>

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<pre style="margin: 0;">+<span id="LC267" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">rather than one at a time, creating a heap-allocated function closure. See</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC268" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">https://www.joachim-breitner.de/blog/763-Faster_Winter_5__Eta-Expanding_ReaderT</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC269" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">for a very good explanation of the issue which led to these optimisations</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC270" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">into GHC.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC271" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC272" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The trick</span></span>

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<pre style="margin: 0;">+<span id="LC273" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC274" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">With state monads like M the general case is that we *aren't* reusing</span></span>

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<pre style="margin: 0;">+<span id="LC275" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(M a) values so it is much more efficient to avoid allocating a</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC276" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">function closure for them. So the state monad trick is a way to keep</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC277" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">the monadic syntax but to make GHC eta-expand functions like `fooM`.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC278" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">To do that we use the "oneShot" magic function.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC279" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC280" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Here is the trick:</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC281" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  * Define a "smart constructor"</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC282" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       mkM :: (State -> (State,a)) -> M a</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC283" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       mkM f = MkM (oneShot m)</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC284" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC285" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  * Never call MkM directly, as a constructor.  Instead, always call mkM.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC286" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC287" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">And that's it!  The magic 'oneShot' function does this transformation:</span></span>

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<pre style="margin: 0;">+<span id="LC288" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   oneShot (\s. e)  ==>   \s{os}. e</span></span>

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<pre style="margin: 0;">+<span id="LC289" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">which pins a one-shot flag {os} onto the binder 's'.  That tells GHC</span></span>

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<pre style="margin: 0;">+<span id="LC290" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">that it can assume the lambda is called only once, and thus can freely</span></span>

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<pre style="margin: 0;">+<span id="LC291" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">float computations in and out of the lambda.</span></span>

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<pre style="margin: 0;">+<span id="LC292" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC293" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">To be concrete, let's see what happens to fooM:</span></span>

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<pre style="margin: 0;">+<span id="LC294" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC295" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> fooM = \x. g (expensive x) >>= \r -> h r</span></span>

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<pre style="margin: 0;">+<span id="LC296" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      = \x. let mf = g (expensive x)</span></span>

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<pre style="margin: 0;">+<span id="LC297" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                k  = \r -> h r</span></span>

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<pre style="margin: 0;">+<span id="LC298" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            in MkM (oneShot (\s -> case mf  of MkM' f  -></span></span>

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<pre style="margin: 0;">+<span id="LC299" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                                   case f s of (s',r) -></span></span>

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<pre style="margin: 0;">+<span id="LC300" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                                   case k r of MkM' g  -></span></span>

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<pre style="margin: 0;">+<span id="LC301" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                                   g s'))</span></span>

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<pre style="margin: 0;">+<span id="LC302" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      -- The MkM' are just newtype casts nt_co</span></span>

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<pre style="margin: 0;">+<span id="LC303" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      = \x. let mf = g (expensive x)</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC304" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                k  = \r -> h r</span></span>

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<pre style="margin: 0;">+<span id="LC305" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            in (\s{os}. case (mf |> nt_co) s of (s',r) -></span></span>

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<pre style="margin: 0;">+<span id="LC306" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                        (k r) |> nt_co s')</span></span>

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<pre style="margin: 0;">+<span id="LC307" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">               |> sym nt_co</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC308" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC309" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      -- Crucial step: float let-bindings into that \s{os}</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC310" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      = \x. (\s{os}. case (g (expensive x) |> nt_co) s of (s',r) -></span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC311" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">                     h r |> nt_co s')</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC312" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            |> sym nt_co</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC313" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC314" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">and voila! fooM has arity 2.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC315" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC316" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The trick is very similar to the built-in "state hack"</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC317" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(see Note [The state-transformer hack] in "GHC.Core.Opt.Arity") but is</span></span>

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<pre style="margin: 0;">+<span id="LC318" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">applicable on a monad-by-monad basis under programmer control.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC319" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC320" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Using pattern synonyms</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC321" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC322" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Using a smart constructor is fine, but there is no way to check that we</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC323" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">have found *all* uses, especially if the uses escape a single module.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC324" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">A neat (but more sophisticated) alternative is to use pattern synonyms:</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC325" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC326" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   -- We rename the existing constructor.</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC327" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   newtype M a = MkM' (State -> (State, a))</span></span>

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<pre style="margin: 0;">+<span id="LC328" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC329" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   -- The pattern has the old constructor name.</span></span>

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<pre style="margin: 0;">+<span id="LC330" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   pattern MkM f <- MkM' f</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC331" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      where</span></span>

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<pre style="margin: 0;">+<span id="LC332" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">        MkM f = MkM' (oneShot f)</span></span>

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<pre style="margin: 0;">+<span id="LC333" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC334" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Now we can simply grep to check that there are no uses of MkM'</span></span>

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<pre style="margin: 0;">+<span id="LC335" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">/anywhere/, to guarantee that we have not missed any.  (Using the</span></span>

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<pre style="margin: 0;">+<span id="LC336" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">smart constructor alone we still need the data constructor in</span></span>

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<pre style="margin: 0;">+<span id="LC337" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">patterns.)  That's the advantage of the pattern-synonym approach, but</span></span>

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<pre style="margin: 0;">+<span id="LC338" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">it is more elaborate.</span></span>

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<pre style="margin: 0;">+<span id="LC339" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC340" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The pattern synonym approach is due to Sebastian Graaf (#18238)</span></span>

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<pre style="margin: 0;">+<span id="LC341" class="line" lang="haskell"></span>

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<pre style="margin: 0;">+<span id="LC342" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Derived instances</span></span>

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<pre style="margin: 0;">+<span id="LC343" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~</span></span>

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<pre style="margin: 0;">+<span id="LC344" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">One caveat of both approaches is that derived instances don't use the smart</span></span>

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<pre style="margin: 0;">+<span id="LC345" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">constructor /or/ the pattern synonym. So they won't benefit from the automatic</span></span>

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<pre style="margin: 0;">+<span id="LC346" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">insertion of "oneShot".</span></span>

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<pre style="margin: 0;">+<span id="LC347" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC348" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   data M a = MkM' (State -> (State,a))</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC349" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">            deriving (Functor) <-- Functor implementation will use MkM'!</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC350" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC351" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Conclusion: don't use 'derviving' in these cases.</span></span>

</pre>

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352

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<pre style="margin: 0;">+<span id="LC352" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC353" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Multi-shot actions (cf #18238)</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC354" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="355" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<pre style="margin: 0;">+<span id="LC355" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Sometimes we really *do* want computations to be shared! Remember our</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC356" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">example (repeat 20 (fooM 4)). See Note [multiShotIO] in GHC.Types.Unique.Supply</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC357" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC358" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">We can force fooM to have arity 1 using multiShot:</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC359" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC360" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    fooM :: Int -> M Int</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC361" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    fooM x = multiShotM (g y >>= \r -> h r)</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC362" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      where</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC363" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">        y = expensive x</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC364" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC365" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    multiShotM :: M a -> M a</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC366" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    {-# INLINE multiShotM #-}</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC367" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">    multiShotM (MkM m) = MkM (\s -> inline m s)</span></span>

</pre>

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368

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<pre style="margin: 0;">+<span id="LC368" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">         -- Really uses the data constructor,</span></span>

</pre>

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369

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<pre style="margin: 0;">+<span id="LC369" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">         -- not the smart constructor!</span></span>

</pre>

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370

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<pre style="margin: 0;">+<span id="LC370" class="line" lang="haskell"></span>

</pre>

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371

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<pre style="margin: 0;">+<span id="LC371" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Now we can see how fooM optimises (ignoring casts)</span></span>

</pre>

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372

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<pre style="margin: 0;">+<span id="LC372" class="line" lang="haskell"></span>

</pre>

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373

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<pre style="margin: 0;">+<span id="LC373" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   multiShotM (g y >>= \r -> h r)</span></span>

</pre>

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374

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<pre style="margin: 0;">+<span id="LC374" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   ==> {inline (>>=)}</span></span>

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375

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<pre style="margin: 0;">+<span id="LC375" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       multiShotM (\s{os}. case g y s of ...)</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC376" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   ==> {inline multiShotM}</span></span>

</pre>

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377

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<pre style="margin: 0;">+<span id="LC377" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       let m = \s{os}. case g y s of ...</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC378" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       in \s. inline m s</span></span>

</pre>

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<pre style="margin: 0;">+<span id="LC379" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   ==> {inline m}</span></span>

</pre>

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380

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<pre style="margin: 0;">+<span id="LC380" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">       \s. (\s{os}. case g y s of ...) s</span></span>

</pre>

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381

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<pre style="margin: 0;">+<span id="LC381" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">   ==> \s. case g y s of ...</span></span>

</pre>

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382

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<pre style="margin: 0;">+<span id="LC382" class="line" lang="haskell"></span>

</pre>

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383

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<pre style="margin: 0;">+<span id="LC383" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">and voila! the one-shot flag has gone.  It's possible that y has been</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="384" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

384

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<pre style="margin: 0;">+<span id="LC384" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">replaced by (expensive x), but full laziness should pull it back out.</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="385" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

385

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<pre style="margin: 0;">+<span id="LC385" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(This part seems less robust.)</span></span>

</pre>

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386

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<pre style="margin: 0;">+<span id="LC386" class="line" lang="haskell"></span>

</pre>

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<pre style="margin: 0;">+<span id="LC387" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The magic `inline` function does two things</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="388" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

388

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<pre style="margin: 0;">+<span id="LC388" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It prevents eta reduction.  If we wrote just</span></span>

</pre>

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</td>

<td class="diff-line-num new new_line" data-linenumber="389" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

389

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<pre style="margin: 0;">+<span id="LC389" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">      multiShotIO (IO m) = IO (\s -> m s)</span></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="229" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

</td>

<td class="diff-line-num new new_line" data-linenumber="390" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

390

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<pre style="margin: 0;">+<span id="LC390" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  the lamda would eta-reduce to 'm' and all would be lost.</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="391" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

391

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<pre style="margin: 0;">+<span id="LC391" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="392" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

392

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<pre style="margin: 0;">+<span id="LC392" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It helps ensure that 'm' really does inline.</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="393" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

393

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<pre style="margin: 0;">+<span id="LC393" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num new old_line" data-linenumber="229" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

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<td class="diff-line-num new new_line" data-linenumber="394" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

394

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<pre style="margin: 0;">+<span id="LC394" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note that 'inline' evaporates in phase 0.  See Note [inlineIdMagic]</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="395" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

395

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<pre style="margin: 0;">+<span id="LC395" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">in GHC.Core.Opt.ConstantFold.match_inline.</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="396" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

396

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<pre style="margin: 0;">+<span id="LC396" class="line" lang="haskell"></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="397" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

397

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<pre style="margin: 0;">+<span id="LC397" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The INLINE pragma on multiShotM is very important, else the</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="398" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

398

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<pre style="margin: 0;">+<span id="LC398" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">'inline' call will evaporate when compiling the module that</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="399" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

399

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<pre style="margin: 0;">+<span id="LC399" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">defines 'multiShotM', before it is ever exported.</span></span>

</pre>

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<td class="diff-line-num new new_line" data-linenumber="400" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #c7f0d2; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#ddfbe6">

400

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<pre style="margin: 0;">+<span id="LC400" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span></pre>

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