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<h3>
Richard Eisenberg pushed to branch wip/T11715-2020
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<h4>
Commits:
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<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/171356f1d63464ee7b55fe69c8884e843a5348d1">171356f1</a></strong>
<div>
<span>by Richard Eisenberg</span>
<i>at 2020-08-05T13:09:58-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Use tcView, not coreView, in the pure unifier.

Addresses a lingering point within #11715.
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<h4>2 changed files:</h4>
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<a href="#84c9d3ba34173297e7d7747e02caa2e004b164fc">
compiler/GHC/Core/Type.hs
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<a href="#89a2e0e1a3095c7a42b10f2819049908908080e8">
compiler/GHC/Core/Unify.hs
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<h4>Changes:</h4>
<li id="84c9d3ba34173297e7d7747e02caa2e004b164fc">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/171356f1d63464ee7b55fe69c8884e843a5348d1#84c9d3ba34173297e7d7747e02caa2e004b164fc"><strong>compiler/GHC/Core/Type.hs</strong></a>
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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">@@ -357,10 +357,28 @@ But in Core these two are treated as identical.</td>
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<pre style="margin: 0;"> <span id="LC357" class="line" lang="haskell"></span>
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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 implement this by making 'coreView' convert 'Constraint' to 'TYPE</span></span>
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<pre style="margin: 0;"> <span id="LC359" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">LiftedRep' on the fly.  The function tcView (used in the type checker)</span></span>
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<pre style="margin: 0;">-<span id="LC360" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">does not do this.</span></span>
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<pre style="margin: 0;">+<span id="LC360" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">does not do this. Accordingly, tcView is used in type-checker-oriented</span></span>
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<pre style="margin: 0;">+<span id="LC361" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">functions (including the pure unifier, used in instance resolution),</span></span>
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<pre style="margin: 0;">+<span id="LC362" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">while coreView is used during e.g. optimisation passes.</span></span>
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<pre style="margin: 0;"> <span id="LC363" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC364" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See also #11715, which tracks removing this inconsistency.</span></span>
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<pre style="margin: 0;"> <span id="LC365" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC366" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The inconsistency actually leads to a potential soundness bug, in that</span></span>
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<pre style="margin: 0;">+<span id="LC367" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Constraint and Type are considered *apart* by the type family engine.</span></span>
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<pre style="margin: 0;">+<span id="LC368" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">To wit, we can write</span></span>
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<pre style="margin: 0;">+<span id="LC369" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC370" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  type family F a</span></span>
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<pre style="margin: 0;">+<span id="LC371" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  type instance F Type = Bool</span></span>
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<pre style="margin: 0;">+<span id="LC372" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">  type instance F Constraint = Int</span></span>
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<pre style="margin: 0;">+<span id="LC373" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC374" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">and (because Type ~# Constraint in Core), thus build a coercion between</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;">Int and Bool. I (Richard E) conjecture that this never happens in practice,</span></span>
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<pre style="margin: 0;">+<span id="LC376" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">but it's very uncomfortable. This, essentially, is the root problem</span></span>
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<pre style="margin: 0;">+<span id="LC377" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">underneath #11715, which is quite resistant to an easy fix. The best</span></span>
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<pre style="margin: 0;">+<span id="LC378" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">idea is to have roles in kind coercions, but that has yet to be implemented.</span></span>
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<pre style="margin: 0;">+<span id="LC379" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See also "A Role for Dependent Types in Haskell", ICFP 2019, which describes</span></span>
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<pre style="margin: 0;">+<span id="LC380" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">how roles in kinds might work out.</span></span>
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<pre style="margin: 0;">+<span id="LC381" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC382" 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="LC383" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC384" class="line" lang="haskell"><span class="cd">-- | Gives the typechecker view of a type. This unwraps synonyms but</span></span>
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<br>
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<li id="89a2e0e1a3095c7a42b10f2819049908908080e8">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/171356f1d63464ee7b55fe69c8884e843a5348d1#89a2e0e1a3095c7a42b10f2819049908908080e8"><strong>compiler/GHC/Core/Unify.hs</strong></a>
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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">@@ -957,7 +957,7 @@ unify_ty :: UMEnv</td>
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<pre style="margin: 0;"> <span id="LC957" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Respects newtypes, PredTypes</span></span>
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<pre style="margin: 0;"> <span id="LC958" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC959" class="line" lang="haskell"><span class="n" style="color: #333;">unify_ty</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">ty1</span> <span class="n" style="color: #333;">ty2</span> <span class="n" style="color: #333;">kco</span></span>
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<td class="diff-line-num new_line old" data-linenumber="960" 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="LC960" class="line" lang="haskell">    <span class="c1" style="color: #998; font-style: italic;">-- <span class="idiff left right">TODO: More commentary needed here</span></span></span>
</pre>
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<pre style="margin: 0;">+<span id="LC960" class="line" lang="haskell">    <span class="c1" style="color: #998; font-style: italic;">-- <span class="idiff left right">Use tcView, not coreView. See Note [coreView vs tcView] in GHC.Core.Type.</span></span></span>
</pre>
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<pre style="margin: 0;"> <span id="LC961" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">ty1'</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcView</span> <span class="n" style="color: #333;">ty1</span>   <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">unify_ty</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">ty1'</span> <span class="n" style="color: #333;">ty2</span> <span class="n" style="color: #333;">kco</span></span>
</pre>
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<pre style="margin: 0;"> <span id="LC962" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">ty2'</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcView</span> <span class="n" style="color: #333;">ty2</span>   <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">unify_ty</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">ty1</span> <span class="n" style="color: #333;">ty2'</span> <span class="n" style="color: #333;">kco</span></span>
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963
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963
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<pre style="margin: 0;"> <span id="LC963" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">CastTy</span> <span class="n" style="color: #333;">ty1'</span> <span class="n" style="color: #333;">co</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">ty1</span>     <span class="o" style="font-weight: 600;">=</span> <span class="kr" style="font-weight: 600;">if</span> <span class="n" style="color: #333;">um_unif</span> <span class="n" style="color: #333;">env</span></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1121" 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="1121" 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">@@ -1121,7 +1121,8 @@ uUnrefined :: UMEnv</td>
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1121
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<pre style="margin: 0;"> <span id="LC1121" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- We know that tv1 isn't refined</span></span>
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1122
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1122
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<pre style="margin: 0;"> <span id="LC1122" class="line" lang="haskell"></span>
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<td class="diff-line-num old_line" data-linenumber="1123" 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">
1123
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1123
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<pre style="margin: 0;"> <span id="LC1123" class="line" lang="haskell"><span class="n" style="color: #333;">uUnrefined</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">tv1'</span> <span class="n" style="color: #333;">ty2</span> <span class="n" style="color: #333;">ty2'</span> <span class="n" style="color: #333;">kco</span></span>
</pre>
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<td class="diff-line-num old old_line" data-linenumber="1124" 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">
1124
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<td class="diff-line-num new_line old" data-linenumber="1124" 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="LC1124" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">ty2''</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">coreView</span> <span class="n" style="color: #333;">ty2'</span></span>
</pre>
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<td class="diff-line-num new old_line" data-linenumber="1125" 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="LC1124" class="line" lang="haskell">    <span class="c1" style="color: #998; font-style: italic;">-- Use tcView, not coreView. See Note [coreView vs tcView] in GHC.Core.Type.</span></span>
</pre>
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<td class="diff-line-num new old_line" data-linenumber="1125" 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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1125
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<pre style="margin: 0;">+<span id="LC1125" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">ty2''</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcView</span> <span class="n" style="color: #333;">ty2'</span></span>
</pre>
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1125
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<td class="diff-line-num new_line" data-linenumber="1126" 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">
1126
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<pre style="margin: 0;"> <span id="LC1126" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">uUnrefined</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">tv1'</span> <span class="n" style="color: #333;">ty2</span> <span class="n" style="color: #333;">ty2''</span> <span class="n" style="color: #333;">kco</span>    <span class="c1" style="color: #998; font-style: italic;">-- Unwrap synonyms</span></span>
</pre>
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1126
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<td class="diff-line-num new_line" data-linenumber="1127" 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">
1127
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<pre style="margin: 0;"> <span id="LC1127" class="line" lang="haskell">                <span class="c1" style="color: #998; font-style: italic;">-- This is essential, in case we have</span></span>
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1127
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<td class="diff-line-num new_line" data-linenumber="1128" 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">
1128
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<pre style="margin: 0;"> <span id="LC1128" class="line" lang="haskell">                <span class="c1" style="color: #998; font-style: italic;">--      type Foo a = a</span></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1483" 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="1484" 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">@@ -1483,6 +1484,8 @@ ty_co_match :: MatchEnv   -- ^ ambient helpful info</td>
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1483
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1484
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<pre style="margin: 0;"> <span id="LC1484" class="line" lang="haskell">            <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Maybe</span> <span class="kt" style="color: #458; font-weight: 600;">LiftCoEnv</span></span>
</pre>
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1485
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<pre style="margin: 0;"> <span id="LC1485" class="line" lang="haskell"><span class="n" style="color: #333;">ty_co_match</span> <span class="n" style="color: #333;">menv</span> <span class="n" style="color: #333;">subst</span> <span class="n" style="color: #333;">ty</span> <span class="n" style="color: #333;">co</span> <span class="n" style="color: #333;">lkco</span> <span class="n" style="color: #333;">rkco</span></span>
</pre>
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1486
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<pre style="margin: 0;"> <span id="LC1486" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">ty'</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">coreView</span> <span class="n" style="color: #333;">ty</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">ty_co_match</span> <span class="n" style="color: #333;">menv</span> <span class="n" style="color: #333;">subst</span> <span class="n" style="color: #333;">ty'</span> <span class="n" style="color: #333;">co</span> <span class="n" style="color: #333;">lkco</span> <span class="n" style="color: #333;">rkco</span></span>
</pre>
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<td class="diff-line-num new new_line" data-linenumber="1487" 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">
1487
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<pre style="margin: 0;">+<span id="LC1487" class="line" lang="haskell">     <span class="c1" style="color: #998; font-style: italic;">-- why coreView here, not tcView? Because we're firmly after type-checking.</span></span>
</pre>
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<pre style="margin: 0;">+<span id="LC1488" class="line" lang="haskell">     <span class="c1" style="color: #998; font-style: italic;">-- This function is used only during coercion optimisation.</span></span>
</pre>
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<td class="diff-line-num old_line" data-linenumber="1486" 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">
1486
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<td class="diff-line-num new_line" data-linenumber="1489" 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">
1489
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<pre style="margin: 0;"> <span id="LC1489" class="line" lang="haskell"></span>
</pre>
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<td class="diff-line-num old_line" data-linenumber="1487" 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">
1487
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<td class="diff-line-num new_line" data-linenumber="1490" 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">
1490
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<pre style="margin: 0;"> <span id="LC1490" class="line" lang="haskell">  <span class="c1" style="color: #998; font-style: italic;">-- handle Refl case:</span></span>
</pre>
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<td class="diff-line-num old_line" data-linenumber="1488" 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">
1488
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<td class="diff-line-num new_line" data-linenumber="1491" 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">
1491
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<pre style="margin: 0;"> <span id="LC1491" class="line" lang="haskell">  <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">tyCoVarsOfType</span> <span class="n" style="color: #333;">ty</span> <span class="p">`</span><span class="n" style="color: #333;">isNotInDomainOf</span><span class="p">`</span> <span class="n" style="color: #333;">subst</span></span>
</pre>
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