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<p class="MsoNormal">Friends<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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"><o:p> </o:p></p>
<p class="MsoNormal">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"><o:p> </o:p></p>
<p class="MsoNormal">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"><o:p> </o:p></p>
<p class="MsoNormal">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"><o:p> </o:p></p>
<p class="Code">{-# LANGUAGE ImpredicativeTypes, TypeApplications, RankNTypes #-}<o:p></o:p></p>
<p class="Code">module Vta where<o:p></o:p></p>
<p class="Code"> f x = id @(forall a. a->a) id @Int x<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">You can also leave out the @Int part of course.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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"><o:p> </o:p></p>
<p class="MsoNormal">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"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
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