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<h3>
Simon Peyton Jones pushed to branch wip/oneshot-unify
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/1c4726012b4b3057465c54f5c7d65fc25a22c56d">1c472601</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-06-18T13:14:20+01:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">GHC.Core.Unify: Make UM actions one-shot by default
This MR makes the UM monad in GHC.Core.Unify into a one-shot
monad. See the long Note [The one-shot state monad trick].
See also #18202 and !3309, which applies this to all Reader/State-like
monads in GHC for compile-time perf improvements. The pattern used
here enables something similar to the state-hack, but is applicable to
user-defined monads, not just `IO`.
Metric Decrease 'runtime/bytes allocated' (test_env='i386-linux-deb9'):
haddock.Cabal
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<h4>1 changed file:</h4>
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<a href="#89a2e0e1a3095c7a42b10f2819049908908080e8">
compiler/GHC/Core/Unify.hs
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<h4>Changes:</h4>
<li id="89a2e0e1a3095c7a42b10f2819049908908080e8">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/1c4726012b4b3057465c54f5c7d65fc25a22c56d#89a2e0e1a3095c7a42b10f2819049908908080e8"><strong>compiler/GHC/Core/Unify.hs</strong></a>
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<pre style="margin: 0;"> <span id="LC1" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- (c) The University of Glasgow 2006</span></span>
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<pre style="margin: 0;"> <span id="LC2" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC3" class="line" lang="haskell"><span class="cp" style="color: #999; font-weight: 600;">{-# LANGUAGE ScopedTypeVariables #-}</span></span>
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<pre style="margin: 0;">+<span id="LC3" class="line" lang="haskell"><span class="cp" style="color: #999; font-weight: 600;">{-# LANGUAGE ScopedTypeVariables<span class="idiff left right">, PatternSynonyms</span> #-}</span></span>
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<pre style="margin: 0;"> <span id="LC4" class="line" lang="haskell"><span class="cp" style="color: #999; font-weight: 600;">{-# LANGUAGE CPP #-}</span></span>
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<pre style="margin: 0;"> <span id="LC5" class="line" lang="haskell"><span class="cp" style="color: #999; font-weight: 600;">{-# LANGUAGE DeriveFunctor #-}</span></span>
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<pre style="margin: 0;"> <span id="LC6" class="line" lang="haskell"></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="44" 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="44" 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">@@ -44,6 +44,7 @@ import GHC.Data.Pair</td>
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<pre style="margin: 0;"> <span id="LC44" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Utils.Outputable</span></span>
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<pre style="margin: 0;"> <span id="LC45" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Types.Unique.FM</span></span>
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<pre style="margin: 0;"> <span id="LC46" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Types.Unique.Set</span></span>
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<pre style="margin: 0;">+<span id="LC47" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Exts</span><span class="p">(</span> <span class="n" style="color: #333;">oneShot</span> <span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC48" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC49" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">Control.Monad</span></span>
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<pre style="margin: 0;"> <span id="LC50" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">Control.Applicative</span> <span class="k" style="font-weight: 600;">hiding</span> <span class="p">(</span> <span class="nf" style="color: #900; font-weight: 600;">empty</span> <span class="p">)</span></span>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -1209,6 +1210,77 @@ data BindFlag</td>
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<pre style="margin: 0;"> <span id="LC1210" 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="LC1211" 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="LC1212" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC1213" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [The one-shot state monad trick]</span></span>
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<pre style="margin: 0;">+<span id="LC1214" 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="LC1215" 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>
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1216
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<pre style="margin: 0;">+<span id="LC1216" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">functions to be eta-expanded (#18202). Consider</span></span>
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1217
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<pre style="margin: 0;">+<span id="LC1217" class="line" lang="haskell"></span>
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1218
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<pre style="margin: 0;">+<span id="LC1218" 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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1219
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<pre style="margin: 0;">+<span id="LC1219" class="line" lang="haskell"></span>
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1220
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<pre style="margin: 0;">+<span id="LC1220" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> instance Monad M where</span></span>
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1221
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<pre style="margin: 0;">+<span id="LC1221" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> mf >>= k = MkM (\s -> case mf of MkM f -></span></span>
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1222
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<pre style="margin: 0;">+<span id="LC1222" 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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1223
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<pre style="margin: 0;">+<span id="LC1223" 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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1224
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<pre style="margin: 0;">+<span id="LC1224" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> g s')</span></span>
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1225
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<pre style="margin: 0;">+<span id="LC1225" class="line" lang="haskell"></span>
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1226
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<pre style="margin: 0;">+<span id="LC1226" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> foo :: Int -> M Int</span></span>
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1227
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<pre style="margin: 0;">+<span id="LC1227" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> foo x = g y >>= \r -> h r</span></span>
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1228
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<pre style="margin: 0;">+<span id="LC1228" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> where</span></span>
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1229
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<pre style="margin: 0;">+<span id="LC1229" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> y = expensive x</span></span>
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1230
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<pre style="margin: 0;">+<span id="LC1230" class="line" lang="haskell"></span>
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<td class="diff-line-num new new_line" data-linenumber="1231" 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">
1231
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<pre style="margin: 0;">+<span id="LC1231" 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>
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<td class="diff-line-num new new_line" data-linenumber="1232" 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">
1232
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<pre style="margin: 0;">+<span id="LC1232" 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>
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<td class="diff-line-num new new_line" data-linenumber="1233" 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">
1233
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<pre style="margin: 0;">+<span id="LC1233" 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>
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<td class="diff-line-num new new_line" data-linenumber="1234" 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">
1234
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<pre style="margin: 0;">+<span id="LC1234" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">more efficient to make foo have arity 2.</span></span>
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1235
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<pre style="margin: 0;">+<span id="LC1235" class="line" lang="haskell"></span>
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<td class="diff-line-num new new_line" data-linenumber="1236" 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">
1236
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<pre style="margin: 0;">+<span id="LC1236" 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>
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1237
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<pre style="margin: 0;">+<span id="LC1237" class="line" lang="haskell"></span>
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<td class="diff-line-num new new_line" data-linenumber="1238" 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">
1238
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<pre style="margin: 0;">+<span id="LC1238" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">So here is the trick. Define</span></span>
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1239
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<pre style="margin: 0;">+<span id="LC1239" class="line" lang="haskell"></span>
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1240
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<pre style="margin: 0;">+<span id="LC1240" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> data M a = MkM' (State -> (State, a))</span></span>
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1241
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<pre style="margin: 0;">+<span id="LC1241" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> pattern MkM f <- MkM' f</span></span>
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1242
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<pre style="margin: 0;">+<span id="LC1242" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> where</span></span>
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1243
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<pre style="margin: 0;">+<span id="LC1243" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> MkM f = MkM' (oneShot f)</span></span>
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1244
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<pre style="margin: 0;">+<span id="LC1244" class="line" lang="haskell"></span>
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1245
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<pre style="margin: 0;">+<span id="LC1245" 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>
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1246
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<pre style="margin: 0;">+<span id="LC1246" 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>
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1247
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<pre style="margin: 0;">+<span id="LC1247" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">on f's lambda-binder. Now look at foo:</span></span>
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1248
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<pre style="margin: 0;">+<span id="LC1248" class="line" lang="haskell"></span>
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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;"> foo = \x. g (expensive x) >>= \r -> h r</span></span>
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1250
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<pre style="margin: 0;">+<span id="LC1250" 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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1251
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<pre style="margin: 0;">+<span id="LC1251" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> k = \r -> h r</span></span>
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1252
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<pre style="margin: 0;">+<span id="LC1252" 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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1253
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<pre style="margin: 0;">+<span id="LC1253" 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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1254
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<pre style="margin: 0;">+<span id="LC1254" 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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1255
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<pre style="margin: 0;">+<span id="LC1255" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> g s'))</span></span>
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1256
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<pre style="margin: 0;">+<span id="LC1256" 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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1257
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<pre style="margin: 0;">+<span id="LC1257" 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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1258
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<pre style="margin: 0;">+<span id="LC1258" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> k = \r -> h r</span></span>
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1259
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<pre style="margin: 0;">+<span id="LC1259" 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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1260
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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) |> nt_co s')</span></span>
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1261
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<pre style="margin: 0;">+<span id="LC1261" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> |> sym nt_co</span></span>
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1262
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<pre style="margin: 0;">+<span id="LC1262" class="line" lang="haskell"></span>
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1263
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<pre style="margin: 0;">+<span id="LC1263" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> -- Float into that \s{os}</span></span>
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1264
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<pre style="margin: 0;">+<span id="LC1264" 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>
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1265
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<pre style="margin: 0;">+<span id="LC1265" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> h r |> nt_co s')</span></span>
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1266
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<pre style="margin: 0;">+<span id="LC1266" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> |> sym nt_co</span></span>
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1267
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<pre style="margin: 0;">+<span id="LC1267" class="line" lang="haskell"></span>
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1268
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<pre style="margin: 0;">+<span id="LC1268" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">and voila! In summary:</span></span>
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1269
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<pre style="margin: 0;">+<span id="LC1269" class="line" lang="haskell"></span>
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1270
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<pre style="margin: 0;">+<span id="LC1270" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It's a very simple, two-line change</span></span>
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1271
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<pre style="margin: 0;">+<span id="LC1271" class="line" lang="haskell"></span>
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1272
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<pre style="margin: 0;">+<span id="LC1272" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* It eta-expands all uses of the monad, automatically</span></span>
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1273
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<pre style="margin: 0;">+<span id="LC1273" class="line" lang="haskell"></span>
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1274
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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 is very similar to the built-in "state hack" (see</span></span>
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1275
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<pre style="margin: 0;">+<span id="LC1275" 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>
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1276
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<pre style="margin: 0;">+<span id="LC1276" 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>
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1277
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<pre style="margin: 0;">+<span id="LC1277" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> programmer control.</span></span>
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1278
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<pre style="margin: 0;">+<span id="LC1278" class="line" lang="haskell"></span>
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1279
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<pre style="margin: 0;">+<span id="LC1279" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Beware: itt changes the behaviour of</span></span>
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1280
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<pre style="margin: 0;">+<span id="LC1280" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> map (foo 3) xs</span></span>
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1281
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<pre style="margin: 0;">+<span id="LC1281" 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>
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1282
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<pre style="margin: 0;">+<span id="LC1282" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span>
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1283
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<pre style="margin: 0;">+<span id="LC1283" class="line" lang="haskell"></span>
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1212
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1284
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<pre style="margin: 0;"> <span id="LC1284" 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>
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1213
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1285
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<pre style="margin: 0;"> <span id="LC1285" 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>
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1214
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1286
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<pre style="margin: 0;"> <span id="LC1286" class="line" lang="haskell"></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1235" 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="1307" 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">@@ -1235,8 +1307,16 @@ data UMState = UMState</td>
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1235
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1307
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<pre style="margin: 0;"> <span id="LC1307" class="line" lang="haskell"> <span class="p">{</span> <span class="n" style="color: #333;">um_tv_env</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">TvSubstEnv</span></span>
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1308
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<pre style="margin: 0;"> <span id="LC1308" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">um_cv_env</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">CvSubstEnv</span> <span class="p">}</span></span>
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1237
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1309
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<pre style="margin: 0;"> <span id="LC1309" class="line" lang="haskell"></span>
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1238
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<pre style="margin: 0;">-<span id="LC1238" 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 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>
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1239
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<pre style="margin: 0;">-<span id="LC1239" 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>
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1310
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<pre style="margin: 0;">+<span id="LC1310" 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>
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1311
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<pre style="margin: 0;">+<span id="LC1311" 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>
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1312
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<pre style="margin: 0;">+<span id="LC1312" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick]</span></span>
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1313
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<pre style="margin: 0;">+<span id="LC1313" 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>
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1314
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<pre style="margin: 0;">+<span id="LC1314" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC1315" 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>
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1316
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<pre style="margin: 0;">+<span id="LC1316" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [The one-shot state monad trick]</span></span>
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1317
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<pre style="margin: 0;">+<span id="LC1317" 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>
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1318
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<pre style="margin: 0;">+<span id="LC1318" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">where</span></span>
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1319
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<pre style="margin: 0;">+<span id="LC1319" 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>
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1240
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1320
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<pre style="margin: 0;"> <span id="LC1320" class="line" lang="haskell"></span>
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1241
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1321
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<pre style="margin: 0;"> <span id="LC1321" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">instance</span> <span class="kt" style="color: #458; font-weight: 600;">Applicative</span> <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="kr" style="font-weight: 600;">where</span></span>
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1242
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1322
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<pre style="margin: 0;"> <span id="LC1322" class="line" lang="haskell"> <span class="n" style="color: #333;">pure</span> <span class="n" style="color: #333;">a</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="nf" style="color: #900; font-weight: 600;">\</span><span class="n" style="color: #333;">s</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">pure</span> <span class="p">(</span><span class="n" style="color: #333;">s</span><span class="p">,</span> <span class="n" style="color: #333;">a</span><span class="p">))</span></span>
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