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

Tue Jun 26 22:52:04 UTC 2018

Hello,
thanks for the revisions---I read through the new version, and I think I
almost understand the plan.   I would say that for this proposal it makes
more sense to just deal with explicit specificities at the value level, and
never have the curly braces in type constructors.   Since we don't have an
explicit kind application at the type level (e.g., we can't write `Proxy
@Type`), I don't think it really makes sense to add the braces to type
constructors.   If we ever implemented THAT feature, then we can discuss
which declarations should have explicit and which should have implicit
parameters.  In the current set of examples, I find it odd that `T2` does
not have braces, but `T3` does.

-Iavor

On Wed, Jun 20, 2018 at 2:15 PM Richard Eisenberg <rae at cs.brynmawr.edu>
wrote:

> I have posted the new proposal, at
> https://github.com/ghc-proposals/ghc-proposals/pull/148
>
> I've also updated #99 to clarify where the new syntax is allowed.
>
> For Iavor's examples:
>
> data T1 a = C1 a
>
>
> type T1 :: Type -> Type
> C1 :: forall a. a -> T1 a
>
> data T2 (a :: k) = C2 { f2 :: Proxy a }
>
>
> type T2 :: forall k. k -> Type
> C2 :: forall k (a :: k). Proxy a -> T2 a
> f2 :: forall k (a :: k). T2 a -> Proxy a
>
> data T3 a where C3 :: forall k (a::k). Proxy a -> T3 a
>
>
> type T3 :: forall {k}. k -> Type
> C3 :: forall k (a :: k). Proxy a -> T3 a
>
> data T4 a where C4 :: forall {k} (a::k). Proxy a -> T3 a
>
>
> type T4 :: forall {k}. k -> Type
> C4 :: forall {k} (a :: k). Proxy a -> T3 a
>
> data T5 k a where C5 :: forall k (a::k). Proxy a -> T5 k a
>
>
> Rejected, as k is used dependently but is not lexically dependent. (This
> is no change.) It we have
>
> > data T5 k (a :: k) where C5 :: forall k (a :: k). Proxy a -> T5 k a
>
> we would get
>
> type T5 :: forall k -> k -> Type
> 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
>
>
> Rejected, like T6. If we revise to:
>
> > data T6 k (a :: k) where C6 :: forall {k} (a::k). Proxy a -> T6 k a
>
> we get
>
> type T6 :: forall k -> k -> Type
> C6 :: forall {k} (a::k). Proxy a -> T6 k a
>
> I've updated the proposal itself to include these examples.
>
> Does this help to clarify?
>
> Richard
>
> On May 25, 2018, at 1:56 PM, Iavor Diatchki <iavor.diatchki at gmail.com>
> wrote:
>
> 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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>
>
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