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<h3>
Marge Bot pushed to branch master
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>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/commit/4e8a71c1138b587dfbab8a1823b3f7fa6f0166bd">4e8a71c1</a></strong>
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<span>by Richard Eisenberg</span>
<i>at 2020-03-18T14:07:19Z</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add release note about fix to #16502.
We thought we needed to update the manual, but the fix for #16502
actually brings the implementation in line with the manual. So we
just alert users of how to update their code.
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<h4>1 changed file:</h4>
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<a href="#b421960e854c6cfa1e4ca830bd6a5fc6eef91168">
docs/users_guide/8.12.1-notes.rst
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<h4>Changes:</h4>
<li id="b421960e854c6cfa1e4ca830bd6a5fc6eef91168">
<a href="https://gitlab.haskell.org/ghc/ghc/commit/4e8a71c1138b587dfbab8a1823b3f7fa6f0166bd#b421960e854c6cfa1e4ca830bd6a5fc6eef91168"><strong>docs/users_guide/8.12.1-notes.rst</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">@@ -24,6 +24,23 @@ Language</td>
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<pre style="margin: 0;"> <span id="LC24" class="line" lang="plaintext"> would have accepted ``x``, but its type would have involved the mysterious ``Any``</span>
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<pre style="margin: 0;"> <span id="LC25" class="line" lang="plaintext"> internal type family. Now, GHC rejects, explaining the situation.</span>
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<pre style="margin: 0;"> <span id="LC26" class="line" lang="plaintext"></span>
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<pre style="margin: 0;">+<span id="LC27" class="line" lang="plaintext">* GHC now more faithfully implements the instance-lookup scheme described with</span>
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<pre style="margin: 0;">+<span id="LC28" class="line" lang="plaintext"> :extension:`QuantifiedConstraints`. Previous bugs meant that programs like this</span>
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<pre style="margin: 0;">+<span id="LC29" class="line" lang="plaintext"> were accepted::</span>
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<pre style="margin: 0;">+<span id="LC30" class="line" lang="plaintext"></span>
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<pre style="margin: 0;">+<span id="LC31" class="line" lang="plaintext"> data T (c :: Type -> Constraint)</span>
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<pre style="margin: 0;">+<span id="LC32" class="line" lang="plaintext"> instance (forall h. c h => Functor h) => Functor (T c)</span>
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<pre style="margin: 0;">+<span id="LC33" class="line" lang="plaintext"> instance (forall h. c h => Applicative h) => Applicative (T c)</span>
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<pre style="margin: 0;">+<span id="LC34" class="line" lang="plaintext"></span>
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<pre style="margin: 0;">+<span id="LC35" class="line" lang="plaintext"> Note that in the instance declaration for ``Applicative (T c)``, we cannot prove</span>
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<pre style="margin: 0;">+<span id="LC36" class="line" lang="plaintext"> ``Functor (T c)``, because the quantified constraint shadows the global instance.</span>
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<pre style="margin: 0;">+<span id="LC37" class="line" lang="plaintext"> There is an easy workaround, though: just include ``Functor (T c)`` as an assumption. ::</span>
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<pre style="margin: 0;">+<span id="LC38" class="line" lang="plaintext"></span>
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<pre style="margin: 0;">+<span id="LC39" class="line" lang="plaintext"> instance (forall h. c h => Applicative h, Functor (T c)) => Applicative (T c)</span>
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<pre style="margin: 0;">+<span id="LC40" class="line" lang="plaintext"></span>
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<pre style="margin: 0;">+<span id="LC41" class="line" lang="plaintext"> There is a chance we will tweak the lookup scheme in the future, to make this</span>
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<pre style="margin: 0;">+<span id="LC42" class="line" lang="plaintext"> workaround unnecessary.</span>
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<pre style="margin: 0;">+<span id="LC43" class="line" lang="plaintext"> </span>
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<pre style="margin: 0;"> <span id="LC44" class="line" lang="plaintext">Compiler</span>
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<pre style="margin: 0;"> <span id="LC45" class="line" lang="plaintext">~~~~~~~~</span>
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<pre style="margin: 0;"> <span id="LC46" class="line" lang="plaintext"></span>
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