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<h3>
Ryan Scott pushed to branch wip/T17313
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/0bae7ba256acaf48aa7c1d586fdad8e9fc43b4cd">0bae7ba2</a></strong>
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<span>by Ryan Scott</span>
<i>at 2020-03-31T10:36:54-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Clean up "Eta reduction for data families" Notes
Before, there were two distinct Notes named
"Eta reduction for data families". This renames one of them to
"Implementing eta reduction for data families" to disambiguate the
two and fixes references in other parts of the codebase to ensure
that they are pointing to the right place.
Fixes #17313.
[ci skip]
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<h4>7 changed files:</h4>
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<li class="file-stats">
<a href="#783e5dae6e86931f06700fc088fb7d48c8a07386">
compiler/GHC/Core/Coercion.hs
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<li class="file-stats">
<a href="#f10ed7a2470454dfdd8691a08beba67d8b78ee70">
compiler/GHC/Core/Coercion/Axiom.hs
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<li class="file-stats">
<a href="#91648438362e5a35363d2bb7abb04016dedd7d7e">
compiler/GHC/Core/FamInstEnv.hs
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<li class="file-stats">
<a href="#4aad0050db1a8a20db8bbca149111de99cb299c9">
compiler/GHC/Core/TyCon.hs
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<li class="file-stats">
<a href="#e7f3c171c8b724084e7dad0d8b951d66cc9c0751">
compiler/typecheck/TcDeriv.hs
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<li class="file-stats">
<a href="#4c1af4850cb90ab2963edb06b69b57e87ccdf9c1">
compiler/typecheck/TcInstDcls.hs
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<li class="file-stats">
<a href="#4f8d5e20ad75b577322cf46f2e146466d905fe43">
compiler/typecheck/TcSplice.hs
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<h4>Changes:</h4>
<li id="783e5dae6e86931f06700fc088fb7d48c8a07386">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0bae7ba256acaf48aa7c1d586fdad8e9fc43b4cd#783e5dae6e86931f06700fc088fb7d48c8a07386"><strong>compiler/GHC/Core/Coercion.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">@@ -245,7 +245,7 @@ ppr_co_ax_branch ppr_rhs fam_tc branch</td>
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<pre style="margin: 0;"> <span id="LC245" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC246" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Eta-expand LHS and RHS types, because sometimes data family</span></span>
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<pre style="margin: 0;"> <span id="LC247" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- instances are eta-reduced.</span></span>
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<pre style="margin: 0;">-<span id="LC248" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">FamInstEnv</span>.</span></span>
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<pre style="margin: 0;">+<span id="LC248" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">Coercion.Axiom</span>.</span></span>
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<pre style="margin: 0;"> <span id="LC249" class="line" lang="haskell"> <span class="p">(</span><span class="n" style="color: #333;">ee_tvs</span><span class="p">,</span> <span class="n" style="color: #333;">ee_lhs</span><span class="p">,</span> <span class="n" style="color: #333;">ee_rhs</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">etaExpandCoAxBranch</span> <span class="n" style="color: #333;">branch</span></span>
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<pre style="margin: 0;"> <span id="LC250" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC251" class="line" lang="haskell"> <span class="n" style="color: #333;">pp_lhs</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">pprIfaceTypeApp</span> <span class="n" style="color: #333;">topPrec</span> <span class="p">(</span><span class="n" style="color: #333;">toIfaceTyCon</span> <span class="n" style="color: #333;">fam_tc</span><span class="p">)</span></span>
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<li id="f10ed7a2470454dfdd8691a08beba67d8b78ee70">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0bae7ba256acaf48aa7c1d586fdad8e9fc43b4cd#f10ed7a2470454dfdd8691a08beba67d8b78ee70"><strong>compiler/GHC/Core/Coercion/Axiom.hs</strong></a>
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<td class="diff-line-num unfold js-unfold new_line" data-linenumber="235" 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">@@ -235,7 +235,7 @@ data CoAxBranch</td>
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<pre style="margin: 0;"> <span id="LC235" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">cab_tvs</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">[</span><span class="kt" style="color: #458; font-weight: 600;">TyVar</span><span class="p">]</span> <span class="c1" style="color: #998; font-style: italic;">-- Bound type variables; not necessarily fresh</span></span>
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<pre style="margin: 0;"> <span id="LC236" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">cab_eta_tvs</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">[</span><span class="kt" style="color: #458; font-weight: 600;">TyVar</span><span class="p">]</span> <span class="c1" style="color: #998; font-style: italic;">-- Eta-reduced tyvars</span></span>
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<pre style="margin: 0;"> <span id="LC237" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [CoAxBranch type variables]</span></span>
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<pre style="margin: 0;">-<span id="LC238" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- cab_tvs and cab_lhs may be eta-redu<span class="idiff left right">d</span>ed; see</span></span>
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<pre style="margin: 0;">+<span id="LC238" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- cab_tvs and cab_lhs may be eta-redu<span class="idiff left right">c</span>ed; see</span></span>
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<pre style="margin: 0;"> <span id="LC239" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Note [Eta reduction for data families]</span></span>
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<pre style="margin: 0;"> <span id="LC240" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">cab_cvs</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">[</span><span class="kt" style="color: #458; font-weight: 600;">CoVar</span><span class="p">]</span> <span class="c1" style="color: #998; font-style: italic;">-- Bound coercion variables</span></span>
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<pre style="margin: 0;"> <span id="LC241" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Always empty, for now.</span></span>
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<pre style="margin: 0;"> <span id="LC443" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(See #9692, #14179, and #15845 for examples of what can go wrong if</span></span>
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<pre style="margin: 0;"> <span id="LC444" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">we don't eta-expand when showing things to the user.)</span></span>
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<pre style="margin: 0;"> <span id="LC445" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC446" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(See also Note [Newtype eta] in GHC.Core.TyCon. This is notionally separate</span></span>
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<pre style="margin: 0;">-<span id="LC447" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">and deals with the axiom connecting a newtype with its representation</span></span>
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<pre style="margin: 0;">-<span id="LC448" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">type; but it too is eta-reduced.)</span></span>
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<pre style="margin: 0;">+<span id="LC446" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See also:</span></span>
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<pre style="margin: 0;">+<span id="LC447" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC448" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Note [Newtype eta] in GHC.Core.TyCon. This is notionally separate</span></span>
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<pre style="margin: 0;">+<span id="LC449" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> and deals with the axiom connecting a newtype with its representation</span></span>
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<pre style="margin: 0;">+<span id="LC450" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> type; but it too is eta-reduced.</span></span>
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<pre style="margin: 0;">+<span id="LC451" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Note [Implementing eta reduction for data families] in TcInstDcls. This</span></span>
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<pre style="margin: 0;">+<span id="LC452" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> describes the implementation details of this eta reduction happen.</span></span>
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<pre style="margin: 0;"> <span id="LC453" 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="LC454" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC455" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">instance</span> <span class="kt" style="color: #458; font-weight: 600;">Eq</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">CoAxiom</span> <span class="n" style="color: #333;">br</span><span class="p">)</span> <span class="kr" style="font-weight: 600;">where</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">@@ -118,6 +118,7 @@ data FamInst -- See Note [FamInsts and CoAxioms]</td>
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<pre style="margin: 0;"> <span id="LC118" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC119" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">fi_tys</span> <span class="o" style="font-weight: 600;">::</span> <span class="p">[</span><span class="kt" style="color: #458; font-weight: 600;">Type</span><span class="p">]</span> <span class="c1" style="color: #998; font-style: italic;">-- The LHS type patterns</span></span>
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<pre style="margin: 0;"> <span id="LC120" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- May be eta-reduced; see Note [Eta reduction for data families]</span></span>
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<pre style="margin: 0;">+<span id="LC121" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- in GHC.Core.Coercion.Axiom</span></span>
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<pre style="margin: 0;"> <span id="LC122" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC123" class="line" lang="haskell"> <span class="p">,</span> <span class="n" style="color: #333;">fi_rhs</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">Type</span> <span class="c1" style="color: #998; font-style: italic;">-- the RHS, with its freshened vars</span></span>
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<pre style="margin: 0;"> <span id="LC124" class="line" lang="haskell"> <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">@@ -132,7 +133,8 @@ Note [Arity of data families]</td>
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<pre style="margin: 0;"> <span id="LC133" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Data family instances might legitimately be over- or under-saturated.</span></span>
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<pre style="margin: 0;"> <span id="LC134" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC135" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Under-saturation has two potential causes:</span></span>
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<pre style="margin: 0;">-<span id="LC135" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> U1) Eta reduction. See Note [Eta reduction for data families].</span></span>
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<pre style="margin: 0;">+<span id="LC136" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> U1) Eta reduction. See Note [Eta reduction for data families] in</span></span>
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<pre style="margin: 0;">+<span id="LC137" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> GHC.Core.Coercion.Axiom.</span></span>
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<pre style="margin: 0;"> <span id="LC138" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> U2) When the user has specified a return kind instead of written out patterns.</span></span>
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<pre style="margin: 0;"> <span id="LC139" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Example:</span></span>
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<pre style="margin: 0;"> <span id="LC140" class="line" lang="haskell"></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">@@ -160,8 +162,8 @@ Over-saturation is also possible:</td>
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<pre style="margin: 0;"> <span id="LC162" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC163" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> However, we require that any over-saturation is eta-reducible. That is,</span></span>
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<pre style="margin: 0;"> <span id="LC164" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> we require that any extra patterns be bare unrepeated type variables;</span></span>
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<pre style="margin: 0;">-<span id="LC163" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> see Note [Eta reduction for data families]<span class="idiff left right">. Accordingly, the FamInst</span></span></span>
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<pre style="margin: 0;">-<span id="LC164" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> is never over-saturated.</span></span>
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<pre style="margin: 0;">+<span id="LC165" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> see Note [Eta reduction for data families]<span class="idiff left right"> in GHC.Core.Coercion.Axiom.</span></span></span>
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<pre style="margin: 0;">+<span id="LC166" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> <span class="idiff left right">Accordingly, the FamInst </span>is never over-saturated.</span></span>
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<pre style="margin: 0;"> <span id="LC167" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC168" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Why can we allow such flexibility for data families but not for type families?</span></span>
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<pre style="margin: 0;"> <span id="LC169" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Because data families can be decomposed -- that is, they are generative and</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">@@ -335,7 +337,7 @@ Then we get a data type for each instance, and an axiom:</td>
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<pre style="margin: 0;"> <span id="LC337" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> axiom ax8 a :: T Bool [a] ~ TBoolList a</span></span>
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<pre style="margin: 0;"> <span id="LC338" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC339" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">These two axioms for T, one with one pattern, one with two;</span></span>
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<pre style="margin: 0;">-<span id="LC338" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">see Note [Eta reduction for data families]</span></span>
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<pre style="margin: 0;">+<span id="LC340" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">see Note [Eta reduction for data families]<span class="idiff left right"> in GHC.Core.Coercion.Axiom</span></span></span>
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<pre style="margin: 0;"> <span id="LC341" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC342" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [FamInstEnv determinism]</span></span>
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<pre style="margin: 0;"> <span id="LC343" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -479,7 +481,7 @@ irrelevant (clause 1 of compatible) or benign (clause 2 of compatible).</td>
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481
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<pre style="margin: 0;"> <span id="LC481" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [Compatibility of eta-reduced axioms]</span></span>
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482
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<pre style="margin: 0;"> <span id="LC482" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>
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483
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<pre style="margin: 0;"> <span id="LC483" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">In newtype instances of data families we eta-reduce the axioms,</span></span>
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<pre style="margin: 0;">-<span id="LC482" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">FamInstEnv</span>. This means that</span></span>
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484
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<pre style="margin: 0;">+<span id="LC484" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">Coercion.Axiom</span>. This means that</span></span>
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<pre style="margin: 0;"> <span id="LC485" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">we sometimes need to test compatibility of two axioms that were eta-reduced to</span></span>
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<pre style="margin: 0;"> <span id="LC486" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">different degrees, e.g.:</span></span>
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<pre style="margin: 0;"> <span id="LC487" class="line" lang="haskell"></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">@@ -1057,7 +1059,7 @@ We handle data families and type families separately here:</td>
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1057
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1059
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<pre style="margin: 0;"> <span id="LC1059" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> * For data family instances, though, we need to re-split for each</span></span>
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1058
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<pre style="margin: 0;"> <span id="LC1060" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> instance, because the breakdown might be different for each</span></span>
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1059
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<pre style="margin: 0;"> <span id="LC1061" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> instance. Why? Because of eta reduction; see</span></span>
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<pre style="margin: 0;">-<span id="LC1060" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Note [Eta reduction for data families].</span></span>
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<pre style="margin: 0;">+<span id="LC1062" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Note [Eta reduction for data families]<span class="idiff left right"> in GHC.Core.Coercion.Axiom</span>.</span></span>
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1061
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1063
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<pre style="margin: 0;"> <span id="LC1063" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span>
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1062
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1064
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<pre style="margin: 0;"> <span id="LC1064" class="line" lang="haskell"></span>
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1065
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<pre style="margin: 0;"> <span id="LC1065" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- checks if one LHS is dominated by a list of other branches</span></span>
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0bae7ba256acaf48aa7c1d586fdad8e9fc43b4cd#4aad0050db1a8a20db8bbca149111de99cb299c9"><strong>compiler/GHC/Core/TyCon.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">@@ -240,7 +240,7 @@ See also Note [Wrappers for data instance tycons] in GHC.Types.Id.Make</td>
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<pre style="margin: 0;"> <span id="LC240" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> DataFamInstTyCon T [Int] ax_ti</span></span>
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241
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<pre style="margin: 0;"> <span id="LC241" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC242" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* The axiom ax_ti may be eta-reduced; see</span></span>
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<pre style="margin: 0;">-<span id="LC243" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">FamInstEnv</span></span></span>
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<pre style="margin: 0;">+<span id="LC243" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">Coercion.Axiom</span></span></span>
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244
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<pre style="margin: 0;"> <span id="LC244" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC245" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* Data family instances may have a different arity than the data family.</span></span>
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<pre style="margin: 0;"> <span id="LC246" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> See Note [Arity of data families] in GHC.Core.FamInstEnv</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">@@ -1100,8 +1100,9 @@ data AlgTyConFlav</td>
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1100
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1100
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<pre style="margin: 0;"> <span id="LC1100" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- and R:T is the representation TyCon (ie this one)</span></span>
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1101
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<pre style="margin: 0;"> <span id="LC1101" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- and a,b,c are the tyConTyVars of this TyCon</span></span>
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<pre style="margin: 0;"> <span id="LC1102" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">--</span></span>
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<pre style="margin: 0;">-<span id="LC1103" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- BUT may be eta-reduced; see FamInstEnv</span></span>
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<pre style="margin: 0;">-<span id="LC1104" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Note [Eta reduction for data families]</span></span>
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<pre style="margin: 0;">+<span id="LC1103" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- BUT may be eta-reduced; see</span></span>
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<pre style="margin: 0;">+<span id="LC1104" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Note [Eta reduction for data families] in</span></span>
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<pre style="margin: 0;">+<span id="LC1105" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- GHC.Core.Coercion.Axiom</span></span>
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1106
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<pre style="margin: 0;"> <span id="LC1106" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC1107" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Cached fields of the CoAxiom, but adjusted to</span></span>
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1107
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<pre style="margin: 0;"> <span id="LC1108" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- use the tyConTyVars of this TyCon</span></span>
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<li id="e7f3c171c8b724084e7dad0d8b951d66cc9c0751">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0bae7ba256acaf48aa7c1d586fdad8e9fc43b4cd#e7f3c171c8b724084e7dad0d8b951d66cc9c0751"><strong>compiler/typecheck/TcDeriv.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">@@ -1312,7 +1312,7 @@ write it out</td>
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1312
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<pre style="margin: 0;"> <span id="LC1312" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> return x = MkT [x]</span></span>
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<pre style="margin: 0;"> <span id="LC1313" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> ... etc ...</span></span>
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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="cm" style="color: #998; font-style: italic;">See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">FamInstEnv</span></span></span>
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<pre style="margin: 0;">+<span id="LC1315" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">Coercion.Axiom</span></span></span>
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<pre style="margin: 0;"> <span id="LC1316" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC1317" 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="LC1318" 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">@@ -667,7 +667,7 @@ tcDataFamInstDecl mb_clsinfo</td>
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<pre style="margin: 0;"> <span id="LC667" class="line" lang="haskell"> <span class="n" style="color: #333;">new_or_data</span></span>
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<pre style="margin: 0;"> <span id="LC668" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC669" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Eta-reduce the axiom if possible</span></span>
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<pre style="margin: 0;">-<span id="LC670" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Quite tricky: see Note [<span class="idiff left right">Eta-</span>reduction for data families]</span></span>
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<pre style="margin: 0;">+<span id="LC670" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Quite tricky: see Note [<span class="idiff left right">Implementing eta </span>reduction for data families]</span></span>
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<pre style="margin: 0;"> <span id="LC671" class="line" lang="haskell"> <span class="p">;</span> <span class="kr" style="font-weight: 600;">let</span> <span class="p">(</span><span class="n" style="color: #333;">eta_pats</span><span class="p">,</span> <span class="n" style="color: #333;">eta_tcbs</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">eta_reduce</span> <span class="n" style="color: #333;">fam_tc</span> <span class="n" style="color: #333;">pats</span></span>
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<pre style="margin: 0;"> <span id="LC672" class="line" lang="haskell"> <span class="n" style="color: #333;">eta_tvs</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">map</span> <span class="n" style="color: #333;">binderVar</span> <span class="n" style="color: #333;">eta_tcbs</span></span>
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<pre style="margin: 0;"> <span id="LC673" class="line" lang="haskell"> <span class="n" style="color: #333;">post_eta_qtvs</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">filterOut</span> <span class="p">(`</span><span class="n" style="color: #333;">elem</span><span class="p">`</span> <span class="n" style="color: #333;">eta_tvs</span><span class="p">)</span> <span class="n" style="color: #333;">qtvs</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">@@ -761,7 +761,7 @@ tcDataFamInstDecl mb_clsinfo</td>
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<pre style="margin: 0;"> <span id="LC761" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">return</span> <span class="p">(</span><span class="n" style="color: #333;">fam_inst</span><span class="p">,</span> <span class="n" style="color: #333;">m_deriv_info</span><span class="p">)</span> <span class="p">}</span></span>
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<pre style="margin: 0;"> <span id="LC762" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;"> <span id="LC763" class="line" lang="haskell"> <span class="n" style="color: #333;">eta_reduce</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">TyCon</span> <span class="o" style="font-weight: 600;">-></span> <span class="p">[</span><span class="kt" style="color: #458; font-weight: 600;">Type</span><span class="p">]</span> <span class="o" style="font-weight: 600;">-></span> <span class="p">([</span><span class="kt" style="color: #458; font-weight: 600;">Type</span><span class="p">],</span> <span class="p">[</span><span class="kt" style="color: #458; font-weight: 600;">TyConBinder</span><span class="p">])</span></span>
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<pre style="margin: 0;">-<span id="LC764" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">FamInstEnv</span></span></span>
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<pre style="margin: 0;">+<span id="LC764" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">Coercion.Axiom</span></span></span>
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<pre style="margin: 0;"> <span id="LC765" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Splits the incoming patterns into two: the [TyVar]</span></span>
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<pre style="margin: 0;"> <span id="LC766" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- are the patterns that can be eta-reduced away.</span></span>
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<pre style="margin: 0;"> <span id="LC767" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- e.g. T [a] Int a d c ==> (T [a] Int a, [d,c])</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">@@ -887,8 +887,8 @@ we actually have a place to put the regeneralised variables.</td>
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<pre style="margin: 0;"> <span id="LC887" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Thus: skolemise away. cf. Inst.deeplySkolemise and TcUnify.tcSkolemise</span></span>
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<pre style="margin: 0;"> <span id="LC888" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Examples in indexed-types/should_compile/T12369</span></span>
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<pre style="margin: 0;"> <span id="LC889" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC890" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [<span class="idiff left right">Eta-</span>reduction for data families]</span></span>
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<pre style="margin: 0;">-<span id="LC891" 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="LC890" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [<span class="idiff left right">Implementing eta </span>reduction for data families]</span></span>
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<pre style="margin: 0;">+<span id="LC891" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~<span class="idiff left right">~~~~~~~~~~~~~</span></span></span>
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<pre style="margin: 0;"> <span id="LC892" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Consider</span></span>
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<pre style="margin: 0;"> <span id="LC893" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> data D :: * -> * -> * -> * -> *</span></span>
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<pre style="margin: 0;"> <span id="LC894" class="line" lang="haskell"></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">@@ -906,7 +906,10 @@ and an axiom to connect them</td>
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<pre style="margin: 0;"> <span id="LC906" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC907" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">except that we'll eta-reduce the axiom to</span></span>
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<pre style="margin: 0;"> <span id="LC908" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> axiom AxDrep forall a b. D [(a,b]] = Drep a b</span></span>
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<pre style="margin: 0;">-<span id="LC909" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">There are several fiddly subtleties lurking here</span></span>
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<pre style="margin: 0;">+<span id="LC909" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC910" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">This is described at some length in Note [Eta reduction for data families]</span></span>
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<pre style="margin: 0;">+<span id="LC911" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">in GHC.Core.Coercion.Axiom. There are several fiddly subtleties lurking here,</span></span>
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<pre style="margin: 0;">+<span id="LC912" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">however, so this Note aims to describe these subtleties:</span></span>
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<pre style="margin: 0;"> <span id="LC913" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC914" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* The representation tycon Drep is parameterised over the free</span></span>
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<pre style="margin: 0;"> <span id="LC915" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> variables of the pattern, in no particular order. So there is no</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">@@ -2046,7 +2046,7 @@ reifyFamilyInstance is_poly_tvs (FamInst { fi_flavor = flavor</td>
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<pre style="margin: 0;"> <span id="LC2046" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">DataFamilyInst</span> <span class="n" style="color: #333;">rep_tc</span> <span class="o" style="font-weight: 600;">-></span></span>
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<pre style="margin: 0;"> <span id="LC2047" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">do</span> <span class="p">{</span> <span class="kr" style="font-weight: 600;">let</span> <span class="c1" style="color: #998; font-style: italic;">-- eta-expand lhs types, because sometimes data/newtype</span></span>
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<pre style="margin: 0;"> <span id="LC2048" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- instances are eta-reduced; See #9692</span></span>
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<pre style="margin: 0;">-<span id="LC2049" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">FamInstEnv</span></span></span>
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<pre style="margin: 0;">+<span id="LC2049" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- See Note [Eta reduction for data families] in GHC.Core.<span class="idiff left right">Coercion.Axiom</span></span></span>
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<pre style="margin: 0;"> <span id="LC2050" class="line" lang="haskell"> <span class="p">(</span><span class="n" style="color: #333;">ee_tvs</span><span class="p">,</span> <span class="n" style="color: #333;">ee_lhs</span><span class="p">,</span> <span class="kr" style="font-weight: 600;">_</span><span class="p">)</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">etaExpandCoAxBranch</span> <span class="n" style="color: #333;">branch</span></span>
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<pre style="margin: 0;"> <span id="LC2051" class="line" lang="haskell"> <span class="n" style="color: #333;">fam'</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">reifyName</span> <span class="n" style="color: #333;">fam</span></span>
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<pre style="margin: 0;"> <span id="LC2052" class="line" lang="haskell"> <span class="n" style="color: #333;">dataCons</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">tyConDataCons</span> <span class="n" style="color: #333;">rep_tc</span></span>
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