[ghc-steering-committee] Please review #641: Wildcard binders in type declarations

[Simon Peyton Jones]

Dear Steering Committee

Vlad proposes to amend proposal #425
> <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-decl-invis-binders.rst>to
> permit more wildcard binder forms in type declarations:
>
>   https://github.com/ghc-proposals/ghc-proposals/pull/641
>
You may find it easiest to look at the rich diff
<https://github.com/ghc-proposals/ghc-proposals/pull/641/files?short_path=cb2a762#diff-cb2a762676d938436a07317bbd007570b5efdfa00b40763b897ee920694bcbb5>


We have since exchanged some email, and I invite you to look at my latest
post on the ticket, namely:

https://github.com/ghc-proposals/ghc-proposals/pull/641#issuecomment-2091099038

In the light of these exchanges I'd like to modify my previous
recommendation of "reject/park".  I now propose:

   - Add wildcards to *all* binders in type declarations, not just @_.
   - Allow kind annotations for wildcards

These changes just fill out the design of #425 in two obvious ways.  In
particular:

data T1 @_         = MkT1  -- Allowed by #425
data T2 @(_::Nat)  = MkT2  -- Not currently allowed by #425
data T3 _          = MkT3  -- Not currently allowed by #425
data T4 (_ :: Nat) = MkT4  -- Not currently allowed by #425

HoweverI propose that we reject

   - Do all this recursively
   - Make changes to Template Haskell

There is no obvious user need for
  data T2 @((k::Nat)::Nat) = MkT2
New recursive structures just add (modest but palpable) complexity to the
specification and to the implementation.  It is a cost without (so far as I
can see) a user need or tangible benefit.

As to Template Haskell, neither #425 nor the above enhancement forces us to
make a change to TH.  Currently binders in TH data type declarations cannot
have wildcards.  There is no pressing reason to introduce them: one can
always use a type variable instead.  And our General Rules ask us to avoid
making breaking changes without a strong reason to do so.

Perhaps we might enhance TH as part of a subsequent, bigger upheaval of TH
syntax.  But not now.

None of this is a terribly big deal.  Unless anyone objects strenuously I
propose to make this our decision and move on.  It's always possible to do
more later if there is a user need.

If you want to object to this proposal, please do so by the end of Weds 8th
May.  Thanks!

Simon

Simon

On Thu, 21 Mar 2024 at 09:56, Simon Peyton Jones <
simon.peytonjones at gmail.com> wrote:

> Dear Steering Committee
>
> Vlad proposes to amend proposal #425
> <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-decl-invis-binders.rst>to
> permit more wildcard binder forms in type declarations:
>   https://github.com/ghc-proposals/ghc-proposals/pull/641
>
> You may find it easiest to look at the rich diff
> <https://github.com/ghc-proposals/ghc-proposals/pull/641/files?short_path=cb2a762#diff-cb2a762676d938436a07317bbd007570b5efdfa00b40763b897ee920694bcbb5>
> .
>
> This is a pretty small generalisation which would allow
>
> data T (( (a :: k1) :: k2)) = ...
>
> in which the binder has multiple kind signatures and redundant parens.
> The change is *not driven by user need*, but rather solely by *uniformity*:
> these same forms are permitted in function definitions:
>
> f :: forall (a :: k). blah
> f @(((a::k1)::k2))) = ...
>
> is permitted.
>
> It imposes a change on Template Haskell syntax too.
>
> The implementation becomes a bit more complicated; more recursive data
> types, etc.  Nothing hard, but more.
>
> It's not a big deal either way.  Very few people expressed a view on
> GitHub.  My personal view is that the modest (albeit non-zero) gain does
> not justify the definite (albeit modest) pain. I would leave this until
> someone actually wants it.
>
> Vlad argues for future-proofing, but my experience is that an eye to the
> future is sensible when you are making changes anyway; but making unforced
> changes solely for the future risks incurring pain now that, when the
> future comes, turns out to have been a poor investment.  We may have
> correctly anticipated, or we may not.
>
> So my recommendation is to park this until we get a real user demand.
>
> It's a perfectly sensible proposal, but adopting it is a judgement call.
> I'll leave a week for committee responses, and then we can just vote.
>
> Simon
>
> On Thu, 21 Mar 2024 at 08:07, Adam Gundry <adam at well-typed.com> wrote:
>
>> Dear Committee,
>>
>> Vlad proposes to amend proposal #425 to permit more wildcard binder
>> forms in type declarations:
>>
>> https://github.com/ghc-proposals/ghc-proposals/pull/641
>>
>> I'd like to nominate Simon PJ as the shepherd.
>>
>> Please guide us to a conclusion as outlined in
>> https://github.com/ghc-proposals/ghc-proposals#committee-process
>>
>> Cheers,
>>
>> Adam
>>
>>
>> --
>> Adam Gundry, Haskell Consultant
>> Well-Typed LLP, https://www.well-typed.com/
>>
>> Registered in England & Wales, OC335890
>> 27 Old Gloucester Street, London WC1N 3AX, England
>> _______________________________________________
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>>
>
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