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<p class="MsoPlainText">I'm supportive too. But, as I say on the proposal thread, before this lands we should decide about
<a href="https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0026-explicit-specificity.rst">
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0026-explicit-specificity.rst</a>.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I came across another pretty convincing use-case for explicit specificity, in new draft paper "Higher-order Type-level Programming in Haskell". Will publish a link to it shortly. But in brief we have a function.<o:p></o:p></p>
<p class="Code">hMap :: forall {m :: Matchability}<o:p></o:p></p>
<p class="Code"> (c :: * -> Constraint)<o:p></o:p></p>
<p class="Code"> (f :: * ->m *) as.<o:p></o:p></p>
<p class="Code"> All as c => (forall a. c a => a -> f a) -> HList as -> HList (Map f as)<o:p></o:p></p>
<p class="MsoPlainText" style="margin-left:36.0pt"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal">Where the ‘m’ parameter is entirely forced by the ‘f’ parameter. So we’d end up writing<o:p></o:p></p>
<p class="Code">Hmap @_ @Db @DbUser<o:p></o:p></p>
<p class="MsoNormal">a lot. That “@_” is a bit silly<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Simon<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText"><span lang="EN-US" style="mso-fareast-language:EN-GB">-----Original Message-----<br>
From: ghc-steering-committee <ghc-steering-committee-bounces@haskell.org> On Behalf Of Eric Seidel<br>
Sent: 02 March 2019 21:42<br>
To: ghc-steering-committee@haskell.org<br>
Subject: [ghc-steering-committee] Proposal #209: Levity polymorphic lift. Recommendation: accept</span></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Hi everyone,<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">This proposal[1] makes the `lift` and `liftTyped` methods of the `Lift` class levity-polymorphic, which allows us to write proper `Lift` instances for unlifted types. It would also allow GHC to remove the special logic that currently
supports lifting records with unlifted fields.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">The main downside is the potential for breakage since `lift @Foo` would now fix the RuntimeRep parameter rather than the `a`. This is unfortunate, but I doubt it will show up much. It also unfortunately requires making both `lift` and
`liftTyped` methods, when we had just decided to extract `lift` from the class. <o:p>
</o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">I recommend accepting the proposal.<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">Thanks!<o:p></o:p></p>
<p class="MsoPlainText">Eric<o:p></o:p></p>
<p class="MsoPlainText"><o:p> </o:p></p>
<p class="MsoPlainText">[1]: <a href="https://github.com/harpocrates/ghc-proposals/blob/levity-polymorphic-lift/proposals/0000-levity-polymorphic-lift.rst">
<span style="color:windowtext;text-decoration:none">https://github.com/harpocrates/ghc-proposals/blob/levity-polymorphic-lift/proposals/0000-levity-polymorphic-lift.rst</span></a><o:p></o:p></p>
<p class="MsoPlainText">_______________________________________________<o:p></o:p></p>
<p class="MsoPlainText">ghc-steering-committee mailing list<o:p></o:p></p>
<p class="MsoPlainText"><a href="mailto:ghc-steering-committee@haskell.org"><span style="color:windowtext;text-decoration:none">ghc-steering-committee@haskell.org</span></a><o:p></o:p></p>
<p class="MsoPlainText"><a href="https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee"><span style="color:windowtext;text-decoration:none">https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee</span></a><o:p></o:p></p>
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