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<h3>
Simon Peyton Jones pushed to branch wip/T17656
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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<h4>
Commits:
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<ul>
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/fb595feaf83bd616cfa7b8f1477f96081bd9c381">fb595fea</a></strong>
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<span>by Simon Peyton Jones</span>
<i>at 2020-12-10T14:27:19+00:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Adjust error message
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/038ea2025161516387d06274f23a27ec336a9a8c">038ea202</a></strong>
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<span>by Simon Peyton Jones</span>
<i>at 2020-12-11T16:50:22+00:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">wibbles
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<h4>2 changed files:</h4>
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<li class="file-stats">
<a href="#74295edeac870ee7f2f3484ebaffab5c948b735a">
compiler/GHC/Tc/Utils/Unify.hs
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<li class="file-stats">
<a href="#b2b1e851e999ce5a79bd64d2cac7b5ff163689cd">
testsuite/tests/polykinds/T7594.stderr
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<h4>Changes:</h4>
<li id="74295edeac870ee7f2f3484ebaffab5c948b735a">
<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/da9cab26faedb886de0b563f293a15b8bd9b6c21...038ea2025161516387d06274f23a27ec336a9a8c#74295edeac870ee7f2f3484ebaffab5c948b735a"><strong>compiler/GHC/Tc/Utils/Unify.hs</strong></a>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1466" 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="1466" 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">@@ -1466,6 +1466,7 @@ uUnfilledVar2 origin t_or_k swapped tv1 ty2</td>
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1466
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<pre style="margin: 0;"> <span id="LC1466" class="line" lang="haskell"> <span class="n" style="color: #333;">defer</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">unSwap</span> <span class="n" style="color: #333;">swapped</span> <span class="p">(</span><span class="n" style="color: #333;">uType_defer</span> <span class="n" style="color: #333;">t_or_k</span> <span class="n" style="color: #333;">origin</span><span class="p">)</span> <span class="n" style="color: #333;">ty1</span> <span class="n" style="color: #333;">ty2</span></span>
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<td class="diff-line-num old_line" data-linenumber="1467" 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">
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1467
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<pre style="margin: 0;"> <span id="LC1467" class="line" lang="haskell"></span>
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<td class="diff-line-num old_line" data-linenumber="1468" 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">
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<pre style="margin: 0;"> <span id="LC1468" class="line" lang="haskell"><span class="n" style="color: #333;">canSolveByUnification</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">MetaInfo</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">TcType</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">Bool</span></span>
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<td class="diff-line-num new new_line" data-linenumber="1469" 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="LC1469" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- See Note [When unification can happen]</span></span>
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1469
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<td class="diff-line-num new_line" data-linenumber="1470" 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">
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<pre style="margin: 0;"> <span id="LC1470" class="line" lang="haskell"><span class="n" style="color: #333;">canSolveByUnification</span> <span class="n" style="color: #333;">info</span> <span class="n" style="color: #333;">xi</span></span>
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<td class="diff-line-num new_line" data-linenumber="1471" 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">
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<pre style="margin: 0;"> <span id="LC1471" 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;">info</span> <span class="kr" style="font-weight: 600;">of</span></span>
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1471
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<td class="diff-line-num new_line" data-linenumber="1472" 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">
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<pre style="margin: 0;"> <span id="LC1472" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">CycleBreakerTv</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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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="1523" 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="1524" 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">@@ -1523,8 +1524,76 @@ lhsPriority tv</td>
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<pre style="margin: 0;"> <span id="LC1524" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">TauTv</span> <span class="o" style="font-weight: 600;">-></span> <span class="mi" style="color: #099;">2</span></span>
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<pre style="margin: 0;"> <span id="LC1525" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">RuntimeUnkTv</span> <span class="o" style="font-weight: 600;">-></span> <span class="mi" style="color: #099;">3</span></span>
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<pre style="margin: 0;"> <span id="LC1526" class="line" lang="haskell"></span>
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<td class="diff-line-num new_line old" data-linenumber="1527" 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="LC1526" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [TyVar/TyVar orientation]</span></span>
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<pre style="margin: 0;">-<span id="LC1527" 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="LC1527" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [When unification can happen]</span></span>
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<pre style="margin: 0;">+<span id="LC1528" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>
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<td class="diff-line-num new new_line" data-linenumber="1529" 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="LC1529" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Question: given a homogeneous equality (alpha ~# ty), where</span></span>
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<pre style="margin: 0;">+<span id="LC1530" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">"homogenous" means both sides have the same kind, when is it OK to</span></span>
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<pre style="margin: 0;">+<span id="LC1531" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">unify alpha := ty?</span></span>
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<pre style="margin: 0;">+<span id="LC1532" class="line" lang="haskell"></span>
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<td class="diff-line-num new new_line" data-linenumber="1533" 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="LC1533" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">There are three reasons not to unify:</span></span>
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1534
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<pre style="margin: 0;">+<span id="LC1534" class="line" lang="haskell"></span>
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1535
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<pre style="margin: 0;">+<span id="LC1535" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(SKOL-ESC) Skolem-escape</span></span>
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1536
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<pre style="margin: 0;">+<span id="LC1536" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Consider the constraint</span></span>
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1537
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<pre style="margin: 0;">+<span id="LC1537" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> forall[2] a[2]. alpha[1] ~ Maybe a[2]</span></span>
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1538
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<pre style="margin: 0;">+<span id="LC1538" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> If we unify alpha := Maybe a, the skolem 'a' may escape its scope.</span></span>
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1539
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<pre style="margin: 0;">+<span id="LC1539" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> The level alpha[1] says that alpha may be used outside this constraint,</span></span>
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1540
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<pre style="margin: 0;">+<span id="LC1540" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> where 'a' is not in scope at all. So we must not unify.</span></span>
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1541
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<pre style="margin: 0;">+<span id="LC1541" class="line" lang="haskell"></span>
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1542
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<pre style="margin: 0;">+<span id="LC1542" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Bottom line: when looking at a constraint alpha[n] := ty, do not unify</span></span>
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1543
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<pre style="margin: 0;">+<span id="LC1543" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> if any free varaible of 'ty' has level deeper (greater) than n</span></span>
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1544
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<pre style="margin: 0;">+<span id="LC1544" class="line" lang="haskell"></span>
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1545
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<pre style="margin: 0;">+<span id="LC1545" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(GIVEN-EQ) Given equalities</span></span>
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1546
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<pre style="margin: 0;">+<span id="LC1546" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Consider the constraint</span></span>
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1547
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<pre style="margin: 0;">+<span id="LC1547" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> forall[2] a[2]. b[1] ~ Int => alpha[1] ~ Int</span></span>
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1548
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<pre style="margin: 0;">+<span id="LC1548" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> There is no (SKOL-ESC) problem with unifying alpha := Int, but it might</span></span>
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1549
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<pre style="margin: 0;">+<span id="LC1549" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> not be the principal solution. Perhpas the "right" solution is alpha := b.</span></span>
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1550
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<pre style="margin: 0;">+<span id="LC1550" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> We simply can't tell. See "OutsideIn(X): modular type inference with local</span></span>
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1551
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<pre style="margin: 0;">+<span id="LC1551" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> assumptions", section 2.2.</span></span>
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1552
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<pre style="margin: 0;">+<span id="LC1552" class="line" lang="haskell"></span>
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1553
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<pre style="margin: 0;">+<span id="LC1553" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> Bottom line: at amibient level 'l', when looking at a constraint</span></span>
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1554
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<pre style="margin: 0;">+<span id="LC1554" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> alpha[n] ~ ty, do not unify alpha := ty if there are any given equalities</span></span>
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1555
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<pre style="margin: 0;">+<span id="LC1555" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> between levels 'n' and 'l'.</span></span>
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1556
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<pre style="margin: 0;">+<span id="LC1556" class="line" lang="haskell"></span>
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1557
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<pre style="margin: 0;">+<span id="LC1557" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">(TYVAR-TV) Unifying TyVarTvs</span></span>
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1558
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<pre style="margin: 0;">+<span id="LC1558" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> When considering alpha{tyv} ~ ty, if alpha{tyv} is a TyVarTv it can</span></span>
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1559
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<pre style="margin: 0;">+<span id="LC1559" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> only unify with a type variable, not with a structured type. So if</span></span>
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1560
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<pre style="margin: 0;">+<span id="LC1560" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> 'ty' is a structured type, such as (Maybe x), don't unify.</span></span>
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1561
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<pre style="margin: 0;">+<span id="LC1561" class="line" lang="haskell"></span>
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1562
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<pre style="margin: 0;">+<span id="LC1562" class="line" lang="haskell"></span>
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1563
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<pre style="margin: 0;">+<span id="LC1563" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Needless to say, all three have wrinkles:</span></span>
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1564
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<pre style="margin: 0;">+<span id="LC1564" class="line" lang="haskell"></span>
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1565
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<pre style="margin: 0;">+<span id="LC1565" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* (SKOL-ESC) Promotion. Given alpha[n] ~ ty, what if beta[k] is free</span></span>
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1566
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<pre style="margin: 0;">+<span id="LC1566" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> in 'ty', where beta is a unification variable, and k>n? 'beta'</span></span>
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1567
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<pre style="margin: 0;">+<span id="LC1567" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> stands for a monotype, and since it is part of a level-n type</span></span>
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1568
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<pre style="margin: 0;">+<span id="LC1568" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> (equal to alpha[n]), we must /promote/ beta to level n. Just make</span></span>
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1569
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<pre style="margin: 0;">+<span id="LC1569" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> up a fresh gamma[n], and unify beta[k] := gamma[n].</span></span>
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1570
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<pre style="margin: 0;">+<span id="LC1570" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC1571" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* (TYVAR-TV) Unification variables. Suppose alpha[tyv,n] is a level-n</span></span>
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1572
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<pre style="margin: 0;">+<span id="LC1572" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> TyVarTv (see Note [Signature skolems] in GHC.Tc.Types.TcType)? Now</span></span>
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1573
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<pre style="margin: 0;">+<span id="LC1573" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> consider alpha[tyv,n] ~ Bool. We don't want to unify because that</span></span>
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1574
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<pre style="margin: 0;">+<span id="LC1574" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> would break the TyVarTv invariant.</span></span>
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1575
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<pre style="margin: 0;">+<span id="LC1575" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC1576" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> What about alpha[tyv,n] ~ beta[tau,n], where beta is an ordinary</span></span>
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1577
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<pre style="margin: 0;">+<span id="LC1577" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> TauTv? Again, don't unify, because beta might later be unified</span></span>
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1578
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<pre style="margin: 0;">+<span id="LC1578" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> with, say Bool. (If levels permit, we reverse the orientation here;</span></span>
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1579
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<pre style="margin: 0;">+<span id="LC1579" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> see Note [TyVar/TyVar orientation].)</span></span>
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1580
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<pre style="margin: 0;">+<span id="LC1580" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC1581" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">* (GIVEN-EQ) Given equalites. When considering (alpha[n] ~ ty), how</span></span>
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1582
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<pre style="margin: 0;">+<span id="LC1582" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> do we know whether there are any given equalities between level n</span></span>
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1583
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<pre style="margin: 0;">+<span id="LC1583" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> and the ambient level. We answer in two ways:</span></span>
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1584
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<pre style="margin: 0;">+<span id="LC1584" class="line" lang="haskell"></span>
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1585
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<pre style="margin: 0;">+<span id="LC1585" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> * In the eager unifier, we only unify if l=n. If not, we say that</span></span>
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1586
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<pre style="margin: 0;">+<span id="LC1586" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> alpha is "untouchable", and defer to the constraint solver.</span></span>
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1587
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<pre style="margin: 0;">+<span id="LC1587" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> This check is made in GHC.Tc.Utils.uUnifilledVar2, in the guard</span></span>
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1588
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<pre style="margin: 0;">+<span id="LC1588" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> isTouchableMetaTyVar.</span></span>
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1589
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<pre style="margin: 0;">+<span id="LC1589" class="line" lang="haskell"></span>
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1590
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<pre style="margin: 0;">+<span id="LC1590" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> * In the constraint solver, we track where Given equalities occur</span></span>
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1591
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<pre style="margin: 0;">+<span id="LC1591" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> and use that to guard unification in GHC.Tc.Solver.Canonical.unifyTest</span></span>
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1592
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<pre style="margin: 0;">+<span id="LC1592" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;"> More details in Note [Tracking given equalities in the solver]</span></span>
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1593
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<pre style="margin: 0;">+<span id="LC1593" class="line" lang="haskell"></span>
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1594
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<pre style="margin: 0;">+<span id="LC1594" class="line" lang="haskell"></span>
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1595
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<pre style="margin: 0;">+<span id="LC1595" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [TyVar/TyVar orientation]</span></span>
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1596
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<pre style="margin: 0;">+<span id="LC1596" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~</span></span>
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1528
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1597
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<pre style="margin: 0;"> <span id="LC1597" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Given (a ~ b), should we orient the CEqCan as (a~b) or (b~a)?</span></span>
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1529
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1598
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<pre style="margin: 0;"> <span id="LC1598" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">This is a surprisingly tricky question! This is invariant (TyEq:TV).</span></span>
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1530
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1599
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<pre style="margin: 0;"> <span id="LC1599" 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">@@ -1649,11 +1718,9 @@ But, to my surprise, it didn't seem to make any significant difference</td>
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1649
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1718
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<pre style="margin: 0;"> <span id="LC1718" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">to the compiler's performance, so I didn't take it any further. Still</span></span>
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1650
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1719
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<pre style="margin: 0;"> <span id="LC1719" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">it seemed too nice to discard altogether, so I'm leaving these</span></span>
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1651
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1720
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<pre style="margin: 0;"> <span id="LC1720" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">notes. SLPJ Jan 18.</span></span>
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1652
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<pre style="margin: 0;">-<span id="LC1652" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">-}</span></span>
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1653
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1721
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<pre style="margin: 0;"> <span id="LC1721" class="line" lang="haskell"></span>
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1654
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<pre style="margin: 0;">-<span id="LC1654" class="line" lang="haskell"></span>
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<pre style="margin: 0;">-<span id="LC1655" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">{- Note [Prevent unification with type families]</span></span>
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<pre style="margin: 0;">-<span id="LC1656" 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="LC1722" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">Note [Prevent unification with type families]</span></span>
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<pre style="margin: 0;">+<span id="LC1723" 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="LC1724" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">We prevent unification with type families because of an uneasy compromise.</span></span>
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<pre style="margin: 0;"> <span id="LC1725" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">It's perfectly sound to unify with type families, and it even improves the</span></span>
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<pre style="margin: 0;"> <span id="LC1726" class="line" lang="haskell"><span class="cm" style="color: #998; font-style: italic;">error messages in the testsuite. It also modestly improves performance, at</span></span>
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<a href="https://gitlab.haskell.org/ghc/ghc/-/compare/da9cab26faedb886de0b563f293a15b8bd9b6c21...038ea2025161516387d06274f23a27ec336a9a8c#b2b1e851e999ce5a79bd64d2cac7b5ff163689cd"><strong>testsuite/tests/polykinds/T7594.stderr</strong></a>
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<pre style="margin: 0;"> <span id="LC1" class="line" lang="plaintext"></span>
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<pre style="margin: 0;"> <span id="LC2" class="line" lang="plaintext">T7594.hs:37:12: error:</span>
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<pre style="margin: 0;">-<span id="LC3" class="line" lang="plaintext"> • Couldn't match type ‘b0’ with ‘IO ()’</span>
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<pre style="margin: 0;">-<span id="LC4" class="line" lang="plaintext"> Expected: a -> b0</span>
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<pre style="margin: 0;">+<span id="LC3" class="line" lang="plaintext"> • Could not deduce: b ~ IO ()</span>
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<pre style="margin: 0;">+<span id="LC4" class="line" lang="plaintext"> from the context: (:&:) c0 Real a</span>
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<pre style="margin: 0;">+<span id="LC5" class="line" lang="plaintext"> bound by a type expected by the context:</span>
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<pre style="margin: 0;">+<span id="LC6" class="line" lang="plaintext"> forall a. (:&:) c0 Real a => a -> b</span>
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<pre style="margin: 0;">+<span id="LC7" class="line" lang="plaintext"> at T7594.hs:37:12-16</span>
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<pre style="margin: 0;">+<span id="LC8" class="line" lang="plaintext"> Expected: a -> b</span>
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<pre style="margin: 0;"> <span id="LC9" class="line" lang="plaintext"> Actual: a -> IO ()</span>
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<pre style="margin: 0;">-<span id="LC6" class="line" lang="plaintext"> ‘b0’ is untouchable</span>
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<pre style="margin: 0;">-<span id="LC7" class="line" lang="plaintext"> inside the constraints: (:&:) c0 Real a</span>
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<pre style="margin: 0;">-<span id="LC8" class="line" lang="plaintext"> bound by a type expected by the context:</span>
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<pre style="margin: 0;">-<span id="LC9" class="line" lang="plaintext"> forall a. (:&:) c0 Real a => a -> b0</span>
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<pre style="margin: 0;">-<span id="LC10" class="line" lang="plaintext"> at T7594.hs:37:12-16</span>
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<pre style="margin: 0;">+<span id="LC10" class="line" lang="plaintext"> ‘b’ is a rigid type variable bound by</span>
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<pre style="margin: 0;">+<span id="LC11" class="line" lang="plaintext"> the inferred type of bar2 :: b</span>
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<pre style="margin: 0;">+<span id="LC12" class="line" lang="plaintext"> at T7594.hs:37:1-19</span>
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<pre style="margin: 0;">+<span id="LC13" class="line" lang="plaintext"> Possible fix: add a type signature for ‘bar2’</span>
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<pre style="margin: 0;"> <span id="LC14" class="line" lang="plaintext"> • In the first argument of ‘app’, namely ‘print’</span>
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<pre style="margin: 0;"> <span id="LC15" class="line" lang="plaintext"> In the expression: app print q2</span>
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<pre style="margin: 0;"> <span id="LC16" class="line" lang="plaintext"> In an equation for ‘bar2’: bar2 = app print q2</span>
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<pre style="margin: 0;">-<span id="LC14" class="line" lang="plaintext"> • Relevant bindings include bar2 :: b<span class="idiff left right">0</span> (bound at T7594.hs:37:1)</span></pre>
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<pre style="margin: 0;">+<span id="LC17" class="line" lang="plaintext"> • Relevant bindings include bar2 :: b (bound at T7594.hs:37:1)</span></pre>
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