[ghc-steering-committee] Discussion on proposal #99: forall {k}

[Joachim Breitner]

Hi,

Am Mittwoch, den 02.05.2018, 16:10 -0400 schrieb Richard Eisenberg:
> Joachim, you are always a fount of interesting ideas.
> 
> > On May 2, 2018, at 2:51 PM, Joachim Breitner <mail at joachim-breitner
> > .de> wrote:
> > 
> >   class C k (a : k) where meth :: a
> >   meth :: forall {k} a. C k a -> k -> Constraint
> 
> I think this is brilliant. But not only for this proposal! Imagine
> this:
> 
> class Num a where
>   fromInteger :: Integer -> a
> 
> fromInteger :: Integer -> forall a. Num a => a
> 
> If we do that, then #129 is essentially solved, at no further cost to
> anyone. (Note that in all Haskell98-style code, no one will ever be
> able to notice the changed type of fromInteger.)
> 
> This approach also allows for the possibility of reordering
> quantified type variables for Haskell98-style constructors, if anyone
> should want to do it.
> 
> And it allows for updated types (including quantified variable
> ordering, etc.) for record selectors.
> 
> And it allows (maybe?) for giving good types to GADT record
> selectors:
> 
> data X a where
>   Foo :: { bar :: Int } -> X Int
>   Quux :: { bar :: Bool } -> X Bool
> bar :: X a -> a
> 
> GHC currently rejects the declaration for X, but it could be accepted
> if only we could specify the correct type of bar. And now we can. I
> don't particularly want to cook up the typing rules here, but I don't
> think I'm totally crazy.
> 
> GADT record selectors aside, the rule for these could be that the
> top-level type signature must be equivalent w.r.t. the subtype
> relation with the original type signature. That is, if the new
> signature is t1 and the old was t2, then t1 <: t2 and t2 <: t1. Easy
> enough to check for. The implementation would probably do a little
> worker/wrapper stunt.

I smell a new proposal… what does this mean for #99? Will you want to
revise it?

Cheers,
Joachim
-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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