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<p class="MsoNormal"><span style="font-size:12.0pt">I’m keen to get #99 into GHC in some form.
<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">My motivation (which could be a fourth bullet in the proposal) is that it should be possible for a programmer to write a fully-explicit type signature for anything GHC can infer. But currently we can’t.
For typeRep1 GHC infers the signature shown for typeRep3; but we can’t write it down.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt">based on the discussion so far, it seems that #99 in its current form might not be exactly what we want<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Can you summarise the reasons it might not be exactly what we want?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-steering-committee <ghc-steering-committee-bounces@haskell.org>
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 24 May 2018 21:17<br>
<b>To:</b> Iavor Diatchki <iavor.diatchki@gmail.com><br>
<b>Cc:</b> ghc-steering-committee@haskell.org; Joachim Breitner <mail@joachim-breitner.de><br>
<b>Subject:</b> Re: [ghc-steering-committee] Discussion on proposal #99: forall {k}<o:p></o:p></span></p>
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<p class="MsoNormal">I do plan on turning Joachim's recent suggestion into a separate proposal, and then to modify #99. But the modification would remove only the bit about classes, not the feature overall. I don't have time to do this now, though -- will do
next week.<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal">On May 24, 2018, at 1:54 PM, Iavor Diatchki <<a href="mailto:iavor.diatchki@gmail.com">iavor.diatchki@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hello,<o:p></o:p></p>
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<p class="MsoNormal">based on the discussion so far, it seems that #99 in its current form might not be exactly what we want, so I'd say that we should reject it for the moment. Overall, I agree that it would be nice to come up with a consistent notation
for things that are currently happening in GHC but we can't write, so perhaps we could revisit this with a revised proposal at a later time?<o:p></o:p></p>
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<p class="MsoNormal">-Iavor<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Sat, May 5, 2018 at 8:48 PM Joachim Breitner <<a href="mailto:mail@joachim-breitner.de">mail@joachim-breitner.de</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hi,<br>
<br>
Am Mittwoch, den 02.05.2018, 16:10 -0400 schrieb Richard Eisenberg:<br>
> Joachim, you are always a fount of interesting ideas.<br>
> <br>
> > On May 2, 2018, at 2:51 PM, Joachim Breitner <mail@joachim-breitner<br>
> > .de> wrote:<br>
> > <br>
> > class C k (a : k) where meth :: a<br>
> > meth :: forall {k} a. C k a -> k -> Constraint<br>
> <br>
> I think this is brilliant. But not only for this proposal! Imagine<br>
> this:<br>
> <br>
> class Num a where<br>
> fromInteger :: Integer -> a<br>
> <br>
> fromInteger :: Integer -> forall a. Num a => a<br>
> <br>
> If we do that, then #129 is essentially solved, at no further cost to<br>
> anyone. (Note that in all Haskell98-style code, no one will ever be<br>
> able to notice the changed type of fromInteger.)<br>
> <br>
> This approach also allows for the possibility of reordering<br>
> quantified type variables for Haskell98-style constructors, if anyone<br>
> should want to do it.<br>
> <br>
> And it allows for updated types (including quantified variable<br>
> ordering, etc.) for record selectors.<br>
> <br>
> And it allows (maybe?) for giving good types to GADT record<br>
> selectors:<br>
> <br>
> data X a where<br>
> Foo :: { bar :: Int } -> X Int<br>
> Quux :: { bar :: Bool } -> X Bool<br>
> bar :: X a -> a<br>
> <br>
> GHC currently rejects the declaration for X, but it could be accepted<br>
> if only we could specify the correct type of bar. And now we can. I<br>
> don't particularly want to cook up the typing rules here, but I don't<br>
> think I'm totally crazy.<br>
> <br>
> GADT record selectors aside, the rule for these could be that the<br>
> top-level type signature must be equivalent w.r.t. the subtype<br>
> relation with the original type signature. That is, if the new<br>
> signature is t1 and the old was t2, then t1 <: t2 and t2 <: t1. Easy<br>
> enough to check for. The implementation would probably do a little<br>
> worker/wrapper stunt.<br>
<br>
I smell a new proposal… what does this mean for #99? Will you want to<br>
revise it?<br>
<br>
Cheers,<br>
Joachim<br>
-- <br>
Joachim Breitner<br>
<a href="mailto:mail@joachim-breitner.de" target="_blank">mail@joachim-breitner.de</a><br>
<a href="https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7Ca43c362fd18e4163aeba08d5c1b34be2%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636627898179266033&sdata=1cwPEVKXCcP3DuCHXOFuA%2BbEBniRLjcKI%2BAZji5vR4Q%3D&reserved=0" target="_blank">
http://www.joachim-breitner.de/</a><br>
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