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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Replying by email because I’m on a train.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Huh. Put otherwise, your point is this. Suppose we<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">have the following kind for `(->)`:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> (->) :: forall v1 v2 r1 r2. TYPE v1 r1 -> TYPE v2 r2 -> Type<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">To coerce from<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> (C a -> Int) to (a -> Int)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">we'd have to cough up a coercion `g`:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US">g : (->) Vanilla Vanilla Ptr Ptr (C a) Int<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> ~R (->) Constraint Vanilla Ptr Ptr a Int<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">And now (Nth 1 g :: Vanilla ~R Constraint). Nothing about<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">`KindCo` there; it's just that `(->)` takes some kind<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">arguments.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">But that can only happen if `(->)` has suitable roles.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">What if it doesn't?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">What if we just had an axiom<o:p></o:p></span></p>
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<p class="Code"><span style="mso-fareast-language:EN-US">axArrow v :: (->) Vanilla v
<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> ~R (->) Constraint v<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">or something like that. Then we get<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US">[W] g : (->) Vanilla Vanilla Ptr Ptr (C a) Int<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> ~R (->) Constraint Vanilla Ptr Ptr a Int<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">We decompose partly and solve thus<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US">g = (axArrow Vanilla) <Ptr> <Ptr> axC <Int><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></a></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"> noreply@phabricator.haskell.org [mailto:noreply@phabricator.haskell.org]
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<b>Sent:</b> 31 January 2017 12:51<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Subject:</b> [Differential] [Commented On] D2038: [WIP] TysPrim: Generalize kind of (->)<o:p></o:p></span></p>
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goldfire added a comment. <o:p></o:p></p>
<p class="MsoNormal"><a href="https://phabricator.haskell.org/D2038"><b><span style="color:#464C5C;background:#F7F7F9;text-decoration:none">View Revision</span></b></a><o:p></o:p></p>
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<span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<span style="font-size:10.0pt;font-family:"Calibri",sans-serif;color:#6B748C">In <a href="https://phabricator.haskell.org/D2038#89360">
<b><span style="color:black;border:none windowtext 1.0pt;padding:0cm;background:#E7E7E7;text-decoration:none">D2038#89360</span></b></a>,
<a href="https://phabricator.haskell.org/p/simonpj/"><b><span style="color:#19558D;border:none windowtext 1.0pt;padding:0cm;background:#F1F7FF">@simonpj</span></b></a> wrote:<o:p></o:p></span></p>
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<p style="background:#F8F9FC"><i><span style="color:#6B748C">To avoid being able to extract
</span></i><tt><i><span style="font-size:10.0pt;color:#6B748C;background:#EBEBEB">ContraintRep ~R LiftedPtrRep</span></i></tt><i><span style="color:#6B748C"> we decided to weaken one of the coercion constructors, the one that gets a kind coercion from a type
coercion. We don't need it, and it's awkward here.<o:p></o:p></span></i></p>
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<p>The problem is that we need it with this patch. I was able to weaken this coercion constructor (<tt><span style="font-size:10.0pt;background:#EBEBEB">KindCo</span></tt>) in my patch
<a href="https://phabricator.haskell.org/D3023"><b><span style="color:black;border:none windowtext 1.0pt;padding:0cm;background:#E7E7E7;text-decoration:none">D3023</span></b></a>, but this patch uses it in a fundamental way that we can't get around. To wit:<o:p></o:p></p>
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<pre style="line-height:11.25pt"><span style="font-size:8.5pt;font-family:Consolas">class C a where<o:p></o:p></span></pre>
<pre style="line-height:11.25pt"><span style="font-size:8.5pt;font-family:Consolas"> meth :: a<o:p></o:p></span></pre>
<pre style="line-height:11.25pt"><span style="font-size:8.5pt;font-family:Consolas"><o:p> </o:p></span></pre>
<pre style="line-height:11.25pt"><span style="font-size:8.5pt;font-family:Consolas">axC :: (C a :: Constraint) ~R (a :: Type)<o:p></o:p></span></pre>
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<p>Now, we wish to cast <tt><span style="font-size:10.0pt;background:#EBEBEB">C a -> a</span></tt> to
<tt><span style="font-size:10.0pt;background:#EBEBEB">a -> a.</span></tt>. This cast will look like
<tt><span style="font-size:10.0pt;background:#EBEBEB">(->) ?? <LiftedRep> axC <a></span></tt>. What goes in the
<tt><span style="font-size:10.0pt;background:#EBEBEB">??</span></tt>? It's got to be something involving
<tt><span style="font-size:10.0pt;background:#EBEBEB">KindCo axC</span></tt>, which is disallowed as per our earlier decision. Therein lies the problem.<o:p></o:p></p>
<p>As for <tt><span style="font-size:10.0pt;background:#EBEBEB">reify</span></tt>: Yes, I'm agreed with that email. But is that implemented yet? Is a design settled on? I don't see a ghc-proposal. Are we wiling to take a dependency on that work in order to
get this done?<o:p></o:p></p>
<p>To be clear, my chief worry isn't that these problems cannot be solved by any means -- I'm just worried about the timing of this all and our desire to get 8.2 out the door.<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif">REPOSITORY</span></strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif">rGHC Glasgow Haskell Compiler<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif">REVISION DETAIL</span></strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><a href="https://phabricator.haskell.org/D2038">https://phabricator.haskell.org/D2038</a><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif">EMAIL PREFERENCES</span></strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><a href="https://phabricator.haskell.org/settings/panel/emailpreferences/">https://phabricator.haskell.org/settings/panel/emailpreferences/</a><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif">To:
</span></strong><span style="font-size:10.0pt;font-family:"Calibri",sans-serif">bgamari, goldfire, austin<br>
<strong><span style="font-family:"Calibri",sans-serif">Cc: </span></strong>simonpj, RyanGlScott, thomie<o:p></o:p></span></p>
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