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<h3>
Simon Peyton Jones pushed to branch wip/T17775
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/b4d28d52e62db37a11f58a4c035d5414c4ed9209">b4d28d52</a></strong>
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<span>by Simon Peyton Jones</span>
<i>at 2020-05-01T17:38:45+01:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Wibbles
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<h4>1 changed file:</h4>
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<a href="#54436e21ea530a66a8a71bcc01c836f4740b8f02">
compiler/GHC/Tc/Solver.hs
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<h4>Changes:</h4>
<li id="54436e21ea530a66a8a71bcc01c836f4740b8f02">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/b4d28d52e62db37a11f58a4c035d5414c4ed9209#54436e21ea530a66a8a71bcc01c836f4740b8f02"><strong>compiler/GHC/Tc/Solver.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">@@ -2531,13 +2531,15 @@ floatEqualities skols given_ids ev_binds_var no_given_eqs</td>
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2531
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<pre style="margin: 0;"> <span id="LC2531" class="line" lang="haskell"> <span class="n" style="color: #333;">is_float_eq_candidate</span> <span class="n" style="color: #333;">ct</span></span>
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2532
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2532
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<pre style="margin: 0;"> <span id="LC2532" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">|</span> <span class="n" style="color: #333;">pred</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">ctPred</span> <span class="n" style="color: #333;">ct</span></span>
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2533
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<pre style="margin: 0;"> <span id="LC2533" class="line" lang="haskell"> <span class="p">,</span> <span class="kt" style="color: #458; font-weight: 600;">EqPred</span> <span class="kt" style="color: #458; font-weight: 600;">NomEq</span> <span class="n" style="color: #333;">ty1</span> <span class="n" style="color: #333;">ty2</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">classifyPredType</span> <span class="n" style="color: #333;">pred</span></span>
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2534
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<pre style="margin: 0;">-<span id="LC2534" class="line" lang="haskell"> <span class="p">,</span> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">tv1</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">tcGetTyVar_maybe</span> <span class="n" style="color: #333;">ty1</span></span>
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2535
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<pre style="margin: 0;">-<span id="LC2535" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">isMetaTyVar</span> <span class="n" style="color: #333;">tv1</span></span>
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2536
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<pre style="margin: 0;">-<span id="LC2536" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">not</span> <span class="p">(</span><span class="n" style="color: #333;">isTyVarTyVar</span> <span class="n" style="color: #333;">tv1</span><span class="p">)</span> <span class="o" style="font-weight: 600;">||</span> <span class="n" style="color: #333;">isTyVarTy</span> <span class="n" style="color: #333;">ty2</span></span>
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2537
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<pre style="margin: 0;">-<span id="LC2537" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">True</span></span>
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<pre style="margin: 0;">+<span id="LC2534" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">float_eq</span> <span class="n" style="color: #333;">ty1</span> <span class="n" style="color: #333;">ty2</span> <span class="o" style="font-weight: 600;">||</span> <span class="n" style="color: #333;">float_eq</span> <span class="n" style="color: #333;">ty2</span> <span class="n" style="color: #333;">ty1</span></span>
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<pre style="margin: 0;"> <span id="LC2535" 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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<pre style="margin: 0;"> <span id="LC2536" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">False</span></span>
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<pre style="margin: 0;"> <span id="LC2537" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC2538" class="line" lang="haskell"> <span class="n" style="color: #333;">float_eq</span> <span class="n" style="color: #333;">ty1</span> <span class="n" style="color: #333;">ty2</span></span>
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<pre style="margin: 0;">+<span id="LC2539" 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;">getTyVar_maybe</span> <span class="n" style="color: #333;">ty1</span> <span class="kr" style="font-weight: 600;">of</span></span>
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<pre style="margin: 0;">+<span id="LC2540" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Just</span> <span class="n" style="color: #333;">tv1</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">isMetaTyVar</span> <span class="n" style="color: #333;">tv1</span></span>
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<pre style="margin: 0;">+<span id="LC2541" class="line" lang="haskell"> <span class="o" style="font-weight: 600;">&&</span> <span class="p">(</span><span class="n" style="color: #333;">not</span> <span class="p">(</span><span class="n" style="color: #333;">isTyVarTyVar</span> <span class="n" style="color: #333;">tv1</span><span class="p">)</span> <span class="o" style="font-weight: 600;">||</span> <span class="n" style="color: #333;">isTyVarTy</span> <span class="n" style="color: #333;">ty2</span><span class="p">)</span></span>
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<pre style="margin: 0;">+<span id="LC2542" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Nothing</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">False</span></span>
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<pre style="margin: 0;"> <span id="LC2543" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC2544" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [Float equalities from under a skolem binding]</span></span>
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<pre style="margin: 0;"> <span id="LC2545" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</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">@@ -2568,15 +2570,17 @@ Which equalities should we float? We want to float ones where there</td>
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<pre style="margin: 0;"> <span id="LC2570" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">is a decent chance that floating outwards will allow unification to</span></span>
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<pre style="margin: 0;"> <span id="LC2571" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">happen. In particular, float out equalities that are:</span></span>
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2572
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<pre style="margin: 0;"> <span id="LC2572" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC2571" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Of form (alpha ~# ty), where</span></span>
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<pre style="margin: 0;">+<span id="LC2573" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Of form (alpha ~# ty)<span class="idiff left right"> or (ty ~# alpha)</span>, where</span></span>
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2574
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<pre style="margin: 0;"> <span id="LC2574" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> * alpha is a meta-tyvar.</span></span>
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<pre style="margin: 0;"> <span id="LC2575" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> * And 'alpha' is not a TyVarTv with 'ty' being a non-tyvar. In that</span></span>
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2576
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<pre style="margin: 0;"> <span id="LC2576" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> case, floating out won't help either, and it may affect grouping</span></span>
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2577
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<pre style="margin: 0;"> <span id="LC2577" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> of error messages.</span></span>
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2578
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<pre style="margin: 0;"> <span id="LC2578" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC2577" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> NB: we will never have (skol ~ alpha), with the meta-tyvar on</span></span>
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<pre style="margin: 0;">-<span id="LC2578" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> the right. See Note [Unification variables on the left]</span></span>
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<pre style="margin: 0;">-<span id="LC2579" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> in GHC.Tc.Utils.Unify</span></span>
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<pre style="margin: 0;">+<span id="LC2579" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> NB: generally we won't see (ty ~ alpha), with alpha on the right because</span></span>
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<pre style="margin: 0;">+<span id="LC2580" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> of Note [Unification variables on the left] in GHC.Tc.Utils.Unify.</span></span>
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<pre style="margin: 0;">+<span id="LC2581" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> But if we start with (F tys ~ alpha), it will orient as (fmv ~ alpha),</span></span>
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<pre style="margin: 0;">+<span id="LC2582" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> and unflatten back to (F tys ~ alpha). So we must look for alpha on</span></span>
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<pre style="margin: 0;">+<span id="LC2583" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> the right too. Example T4494.</span></span>
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2584
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<pre style="margin: 0;"> <span id="LC2584" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC2585" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Nominal. No point in floating (alpha ~R# ty), because we do not</span></span>
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2582
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2586
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<pre style="margin: 0;"> <span id="LC2586" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> unify representational equalities even if alpha is touchable.</span></span>
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