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

[Iavor Diatchki]

Hello,

well, I thought that Richard was going to write a new proposal based on the
feedback here, but it sounds like he is planning to revise #99, and then
write a separate new one.    I guess we should discuss the proposal again
once the changes are in.   I would encourage Richard to add some text and
examples to clarify exactly what's in the proposal and what's not, and how
things are supposed to work.  Here are some examples, for which it would be
illuminating (to me) to see the types/kinds of all names introduced.

data T1 a = C1 a
data T2 (a :: k) = C2 { f2 :: Proxy a }
data T3 a where C3 :: forall k (a::k). Proxy a -> T3 a
data T4 a where C4 :: forall {k} (a::k). Proxy a -> T3 a
data T5 k a where C5 :: forall k (a::k). Proxy a -> T5 k a
data T6 k a where C6 :: forall {k} (a::k). Proxy a -> T6 k a

-Iavor


On Fri, May 25, 2018 at 2:02 AM Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> I’m keen to get #99 into GHC in some form.
>
>
>
> 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.
>
>
>
> based on the discussion so far, it seems that #99 in its current form
> might not be exactly what we want
>
>
>
> Can you summarise the reasons it might not be exactly what we want?
>
>
>
> Simon
>
>
>
> *From:* ghc-steering-committee <ghc-steering-committee-bounces at haskell.org>
> *On Behalf Of *Richard Eisenberg
> *Sent:* 24 May 2018 21:17
> *To:* Iavor Diatchki <iavor.diatchki at gmail.com>
> *Cc:* ghc-steering-committee at haskell.org; Joachim Breitner <
> mail at joachim-breitner.de>
> *Subject:* Re: [ghc-steering-committee] Discussion on proposal #99:
> forall {k}
>
>
>
> 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.
>
>
>
> Richard
>
>
>
> On May 24, 2018, at 1:54 PM, Iavor Diatchki <iavor.diatchki at gmail.com>
> wrote:
>
>
>
> Hello,
>
>
>
> 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?
>
>
>
> -Iavor
>
>
>
>
>
>
>
>
>
> On Sat, May 5, 2018 at 8:48 PM Joachim Breitner <mail at joachim-breitner.de>
> wrote:
>
> 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/
> <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>
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