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<h3>
Richard Eisenberg pushed to branch wip/cfuneqcan-refactor
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<h4>
Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/cbdfe1b9957e3c1f83feaf10e344e21864c202f2">cbdfe1b9</a></strong>
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<span>by Richard Eisenberg</span>
<i>at 2020-10-11T16:42:36-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Super skolems are really super.
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<h4>2 changed files:</h4>
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<a href="#975dc08a8e7942b32d621f617d5a9c1b668601dd">
compiler/GHC/Core/Coercion/Opt.hs
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<a href="#89a2e0e1a3095c7a42b10f2819049908908080e8">
compiler/GHC/Core/Unify.hs
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<h4>Changes:</h4>
<li id="975dc08a8e7942b32d621f617d5a9c1b668601dd">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/cbdfe1b9957e3c1f83feaf10e344e21864c202f2#975dc08a8e7942b32d621f617d5a9c1b668601dd"><strong>compiler/GHC/Core/Coercion/Opt.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">@@ -27,7 +27,6 @@ import GHC.Types.Var.Env</td>
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<pre style="margin: 0;"> <span id="LC27" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Data.Pair</span></span>
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<pre style="margin: 0;"> <span id="LC28" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Data.List.SetOps</span> <span class="p">(</span> <span class="nf" style="color: #900; font-weight: 600;">getNth</span> <span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC29" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Core.Unify</span></span>
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<pre style="margin: 0;">-<span id="LC30" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">GHC.Core.InstEnv</span></span>
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<pre style="margin: 0;"> <span id="LC30" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">Control.Monad</span> <span class="p">(</span> <span class="nf" style="color: #900; font-weight: 600;">zipWithM</span> <span class="p">)</span></span>
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31
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<pre style="margin: 0;"> <span id="LC31" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC32" 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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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1002" 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="1001" 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">@@ -1002,7 +1001,7 @@ checkAxInstCo (AxiomInstCo ax ind cos)</td>
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1002
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<td class="diff-line-num new_line" data-linenumber="1001" 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">
1001
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<pre style="margin: 0;"> <span id="LC1001" class="line" lang="haskell"> <span class="n" style="color: #333;">check_no_conflict</span> <span class="kr" style="font-weight: 600;">_</span> <span class="kt" style="color: #458; font-weight: 600;">[]</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">Nothing</span></span>
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1003
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<td class="diff-line-num new_line" data-linenumber="1002" 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">
1002
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<pre style="margin: 0;"> <span id="LC1002" class="line" lang="haskell"> <span class="n" style="color: #333;">check_no_conflict</span> <span class="n" style="color: #333;">flat</span> <span class="p">(</span><span class="n" style="color: #333;">b</span><span class="o" style="font-weight: 600;">@</span><span class="kt" style="color: #458; font-weight: 600;">CoAxBranch</span> <span class="p">{</span> <span class="n" style="color: #333;">cab_lhs</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">lhs_incomp</span> <span class="p">}</span> <span class="o" style="font-weight: 600;">:</span> <span class="n" style="color: #333;">rest</span><span class="p">)</span></span>
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<td class="diff-line-num old_line" data-linenumber="1004" 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">
1004
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1003
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<pre style="margin: 0;"> <span id="LC1003" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Apartness] in GHC.Core.FamInstEnv</span></span>
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<td class="diff-line-num old old_line" data-linenumber="1005" 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">
1005
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<td class="diff-line-num new_line old" data-linenumber="1004" 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="LC1005" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">SurelyApart</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcUnifyTysFG</span> <span class="n" style="color: #333;"><span class="idiff left right">instanceBindFun</span></span> <span class="n" style="color: #333;">flat</span> <span class="n" style="color: #333;">lhs_incomp</span></span>
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<td class="diff-line-num new old_line" data-linenumber="1006" 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="1004" 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">
1004
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<pre style="margin: 0;">+<span id="LC1004" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">|</span> <span class="kt" style="color: #458; font-weight: 600;">SurelyApart</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcUnifyTysFG</span> <span class="p"><span class="idiff left">(</span></span><span class="n" style="color: #333;"><span class="idiff">const</span></span><span class="idiff"> </span><span class="kt" style="color: #458; font-weight: 600;"><span class="idiff">BindMe</span></span><span class="p"><span class="idiff right">)</span></span> <span class="n" style="color: #333;">flat</span> <span class="n" style="color: #333;">lhs_incomp</span></span>
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1006
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1005
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<pre style="margin: 0;"> <span id="LC1005" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">check_no_conflict</span> <span class="n" style="color: #333;">flat</span> <span class="n" style="color: #333;">rest</span></span>
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1007
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1006
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<pre style="margin: 0;"> <span id="LC1006" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">otherwise</span></span>
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1008
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1007
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<pre style="margin: 0;"> <span id="LC1007" 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;">b</span></span>
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<li id="89a2e0e1a3095c7a42b10f2819049908908080e8">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/cbdfe1b9957e3c1f83feaf10e344e21864c202f2#89a2e0e1a3095c7a42b10f2819049908908080e8"><strong>compiler/GHC/Core/Unify.hs</strong></a>
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<td class="diff-line-num unfold js-unfold new_line" data-linenumber="366" 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">@@ -366,9 +366,29 @@ indexed-types/should_compile/Overlap14.</td>
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366
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<pre style="margin: 0;"> <span id="LC366" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [Unification with skolems]</span></span>
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367
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<pre style="margin: 0;"> <span id="LC367" 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="LC368" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">If we discover that two types unify if and only if a skolem variable is</span></span>
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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;">substituted, we can't properly unify the types. But, that skolem variable</span></span>
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370
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<pre style="margin: 0;">-<span id="LC370" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">may later be instantiated with a unifyable type. So, we return maybeApart</span></span>
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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;">in these cases.</span></span>
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<pre style="margin: 0;">+<span id="LC369" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">substituted, we can't properly unify the types. Perhaps surprisingly,</span></span>
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370
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<pre style="margin: 0;">+<span id="LC370" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">we say that these types are SurelyApart. This is to allow an example</span></span>
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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;">like the following:</span></span>
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372
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<pre style="margin: 0;">+<span id="LC372" class="line" lang="haskell"></span>
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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;"> class C a b where</span></span>
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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;"> meth :: a -> b -> ()</span></span>
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375
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<pre style="margin: 0;">+<span id="LC375" class="line" lang="haskell"></span>
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376
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<pre style="margin: 0;">+<span id="LC376" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> instance C a a where ...</span></span>
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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;"> instance C a Int where</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;"> meth = meth -- a bit silly, but this is meant to be a small example</span></span>
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379
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<pre style="margin: 0;">+<span id="LC379" class="line" lang="haskell"></span>
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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;">NB: No -XOverlappingInstances or overlapp(ing|able|ed) pragmas.</span></span>
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381
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<pre style="margin: 0;">+<span id="LC381" class="line" lang="haskell"></span>
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382
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<pre style="margin: 0;">+<span id="LC382" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">The recursive call within the definition of `meth` is actually ambiguous:</span></span>
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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;">if the instance variable `a` becomes Int, then the first instance would</span></span>
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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;">match. But this is silly, because if `a` were Int, then we would never</span></span>
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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;">have ended up in the second instance in the first place.</span></span>
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386
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<pre style="margin: 0;">+<span id="LC386" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC387" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">We thus say that an unbindable variable is SurelyApart from other</span></span>
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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;">types. This allows us to accept the program above.</span></span>
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389
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<pre style="margin: 0;">+<span id="LC389" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC390" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See also Note [Binding when looking up instances] in GHC.Core.InstEnv.</span></span>
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391
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<pre style="margin: 0;">+<span id="LC391" class="line" lang="haskell"></span>
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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;">-}</span></span>
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373
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393
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<pre style="margin: 0;"> <span id="LC393" class="line" lang="haskell"></span>
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374
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394
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<pre style="margin: 0;"> <span id="LC394" class="line" lang="haskell"><span class="cd">-- | Simple unification of two types; all type variables are bindable</span></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1163" 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="1183" 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">@@ -1163,12 +1183,12 @@ uUnrefined env tv1' ty2 ty2' kco</td>
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1163
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1183
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<pre style="margin: 0;"> <span id="LC1183" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- How could this happen? If we're only matching and if</span></span>
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1164
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1184
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<pre style="margin: 0;"> <span id="LC1184" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- we're comparing forall-bound variables.</span></span>
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1165
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1185
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<pre style="margin: 0;"> <span id="LC1185" class="line" lang="haskell"></span>
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1166
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<td class="diff-line-num new_line old" data-linenumber="1186" 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="LC1166" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">_</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;"><span class="idiff left right">maybe</span>Apart</span> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Unification with skolems]</span></span>
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1186
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<pre style="margin: 0;">+<span id="LC1186" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">_</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;"><span class="idiff left right">surely</span>Apart</span> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Unification with skolems]</span></span>
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1167
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1187
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<pre style="margin: 0;"> <span id="LC1187" class="line" lang="haskell"> <span class="p">}}}}</span></span>
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1168
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1188
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<pre style="margin: 0;"> <span id="LC1188" class="line" lang="haskell"></span>
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1169
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1189
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<pre style="margin: 0;"> <span id="LC1189" 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="kr" style="font-weight: 600;">_</span> <span class="n" style="color: #333;">kco</span> <span class="c1" style="color: #998; font-style: italic;">-- ty2 is not a type variable</span></span>
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1170
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1190
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<pre style="margin: 0;"> <span id="LC1190" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="kr" style="font-weight: 600;">case</span> <span class="n" style="color: #333;">tvBindFlag</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">tv1'</span> <span class="kr" style="font-weight: 600;">of</span></span>
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1171
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<pre style="margin: 0;">-<span id="LC1171" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Skolem</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;"><span class="idiff left right">maybe</span>Apart</span> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Unification with skolems]</span></span>
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1191
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<pre style="margin: 0;">+<span id="LC1191" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Skolem</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;"><span class="idiff left right">surely</span>Apart</span> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Unification with skolems]</span></span>
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1172
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1192
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<pre style="margin: 0;"> <span id="LC1192" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">BindMe</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">bindTv</span> <span class="n" style="color: #333;">env</span> <span class="n" style="color: #333;">tv1'</span> <span class="p">(</span><span class="n" style="color: #333;">ty2</span> <span class="p">`</span><span class="n" style="color: #333;">mkCastTy</span><span class="p">`</span> <span class="n" style="color: #333;">mkSymCo</span> <span class="n" style="color: #333;">kco</span><span class="p">)</span></span>
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1173
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1193
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<pre style="margin: 0;"> <span id="LC1193" class="line" lang="haskell"></span>
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1174
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1194
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<pre style="margin: 0;"> <span id="LC1194" class="line" lang="haskell"><span class="n" style="color: #333;">bindTv</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">UMEnv</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">OutTyVar</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Type</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">UM</span> <span class="nb" style="color: #0086b3;">()</span></span>
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