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<span style="font-family:"Arial",sans-serif">1) ImpredicativeTypes enables types like `Maybe (forall a. a)`. Do those just disappear, or are they also enabled anyway? (I would guess the former.)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Yes, they’d disappear.<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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<span style="font-family:"Arial",sans-serif">2) There was a sketch drawn up around a year ago (I think) aiming to actually fix ImpredicativeTypes. I don't recall who was working on it, but I think when I mentioned it in the context of something else, you didn't
seem to be aware of it. I guess it's safe to say that nothing ever came of it, at least inasmuch as no one ever showed you their proposal for a properly functioning ImpredicativeTypes?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">It’s just a swamp. I have tried multiple times to fix ImpredicativeTypes, and failed every time. Which is not to say that someone shouldn’t try again, with new
thinking.<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"> Dan Doel [mailto:dan.doel@gmail.com]
<br>
<b>Sent:</b> 26 September 2016 00:54<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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<span style="font-family:"Arial",sans-serif">I don't use the extension, because it's more pleasant to use newtypes with polymorphic contents. But here are some questions:<o:p></o:p></span></p>
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<span style="font-family:"Arial",sans-serif">1) ImpredicativeTypes enables types like `Maybe (forall a. a)`. Do those just disappear, or are they also enabled anyway? (I would guess the former.)<o:p></o:p></span></p>
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<span style="font-family:"Arial",sans-serif">2) There was a sketch drawn up around a year ago (I think) aiming to actually fix ImpredicativeTypes. I don't recall who was working on it, but I think when I mentioned it in the context of something else, you didn't
seem to be aware of it. I guess it's safe to say that nothing ever came of it, at least inasmuch as no one ever showed you their proposal for a properly functioning ImpredicativeTypes?<o:p></o:p></span></p>
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<span style="font-family:"Arial",sans-serif">Anyhow, if it can't be fixed, I think not having the extension is superior to its current state. And really, I think even if fixing it were on the roadmap, it'd be better to get rid of it until it were actually fixed.<o:p></o:p></span></p>
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<span style="font-family:"Arial",sans-serif">-- Dan<o:p></o:p></span></p>
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<o:p> </o:p></p>
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On Sun, Sep 25, 2016 at 2:05 PM, Simon Peyton Jones via ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>> wrote:<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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