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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Alejandro: excellent point. I mis-spoke before. In my proposal we WILL allow types like (Tree (forall a. a->a)).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I’m trying to get round to writing a proposal (would someone else like to write it – it should be short), but the idea is this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">When you have -XImpredicativeTypes<o:p></o:p></span></b></p>
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<![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">You can write a polytype in a visible type argument; eg. f @(forall a. a->a)<o:p></o:p></span></b></p>
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</span></span></span><![endif]><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">You can write a polytype as an argument of a type in a signature e.g. f :: [forall a. a->a] -> Int<o:p></o:p></span></b></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></b></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">And that’s all. A unification variable STILL CANNOT be unified with a polytype. The only way you can call a polymorphic function at
a polytype is to use Visible Type Application.<o:p></o:p></span></b></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></b></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">So using impredicative types might be tiresome. E.g.<o:p></o:p></span></b></p>
<p class="Code" style="margin-left:54.0pt"><b><span style="mso-fareast-language:EN-US"> type SID = forall a. a->a<o:p></o:p></span></b></p>
<p class="Code" style="margin-left:54.0pt"><b><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></b></p>
<p class="Code" style="margin-left:54.0pt"><b><span style="mso-fareast-language:EN-US"> xs :: [forall a. a->a]<o:p></o:p></span></b></p>
<p class="Code" style="margin-left:54.0pt"><b><span style="mso-fareast-language:EN-US"> xs = (:) @SID id ( (:) @SID id ([] @ SID))<o:p></o:p></span></b></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></b></p>
<p class="MsoNormal" style="margin-left:18.0pt"><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">In short, if you call a function at a polytype, you must use VTA. Simple, easy, predictable; and doubtless annoying. But possible</span></b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Alejandro Serrano Mena [mailto:trupill@gmail.com]
<br>
<b>Sent:</b> 26 September 2016 08:13<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> ghc-users@haskell.org; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Getting rid of -XImpredicativeTypes<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
What would be the story for the types of the arguments. Would I be allowed to write the following?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
> f (lst :: [forall a. a -> a]) = head @(forall a. a -> a) lst 3<o:p></o:p></p>
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Regards,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Alejandro<o:p></o:p></p>
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<o:p> </o:p></p>
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2016-09-25 20:05 GMT+02:00 Simon Peyton Jones via ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>>:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Friends<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">GHC has a flag -XImpredicativeTypes that makes a half-hearted attempt to support impredicative polymorphism. But it is vestigial…. if it works, it’s really a fluke. We don’t really
have a systematic story here at all.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I propose, therefore, to remove it entirely. That is, if you use -XImpredicativeTypes, you’ll get a warning that it does nothing (ie. complete no-op) and you should remove it.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Before I pull the trigger, does anyone think they are using it in a mission-critical way?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Now that we have Visible Type Application there is a workaround: if you want to call a polymorphic function at a polymorphic type, you can explicitly apply it to that type. For
example:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p>{-# LANGUAGE ImpredicativeTypes, TypeApplications, RankNTypes #-}<o:p></o:p></p>
<p>module Vta where<o:p></o:p></p>
<p> f x = id @(forall a. a->a) id @Int x<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">You can also leave out the @Int part of course.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Currently we have to use -XImpredicativeTypes to allow the @(forall a. a->a). Is that sensible? Or should we allow it regardless? I rather think the latter… if you have Visible
Type Application (i.e. -XTypeApplications) then applying to a polytype is nothing special. So I propose to lift that restriction.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I should go through the GHC Proposals Process for this, but I’m on a plane, so I’m going to at least start with an email.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#888888"> <o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#888888">Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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