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<div class="moz-cite-prefix">Elsewhere in the thread, you said<br>
<br>
<blockquote type="cite">
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style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:36.0pt"><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>
<span
style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Yes,
they’d disappear.</span></blockquote>
<br>
but here you're talking about 'xs :: [forall a . a->a]' being
possible with VTA - is the idea that such types will be possible
but only with both explicit signatures and VTA?<br>
<br>
On 30/09/2016 16:29, Simon Peyton Jones via ghc-devs wrote:<br>
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<div class="WordSection1">
<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>
<p class="MsoListParagraph"
style="margin-left:54.0pt;text-indent:-18.0pt;mso-list:l0
level1 lfo1">
<!--[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>
<p class="MsoListParagraph"
style="margin-left:54.0pt;text-indent:-18.0pt;mso-list:l0
level1 lfo1">
<!--[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 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>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm
0cm 0cm 4.0pt">
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="EN-US">From:</span></b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif"
lang="EN-US"> Alejandro Serrano Mena
[<a class="moz-txt-link-freetext" href="mailto:trupill@gmail.com">mailto:trupill@gmail.com</a>]
<br>
<b>Sent:</b> 26 September 2016 08:13<br>
<b>To:</b> Simon Peyton Jones
<a class="moz-txt-link-rfc2396E" href="mailto:simonpj@microsoft.com"><simonpj@microsoft.com></a><br>
<b>Cc:</b> <a class="moz-txt-link-abbreviated" href="mailto:ghc-users@haskell.org">ghc-users@haskell.org</a>; <a class="moz-txt-link-abbreviated" href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Getting rid of
-XImpredicativeTypes<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<div>
<div>
<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>
</div>
<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>
</div>
<p class="MsoNormal"
style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">Regards,<o:p></o:p></p>
</div>
<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>
</div>
<div>
<p class="MsoNormal"
style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm"><o:p> </o:p></p>
<div>
<p class="MsoNormal"
style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">2016-09-25
20:05 GMT+02:00 Simon Peyton Jones via ghc-devs <<a
moz-do-not-send="true"
href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>>:<o:p></o:p></p>
<blockquote style="border:none;border-left:solid #CCCCCC
1.0pt;padding:0cm 0cm 0cm
6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<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>
</div>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
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<p class="MsoNormal"><o:p> </o:p></p>
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