Dear Steering Committee

Vlad proposes to amend proposal #425 to permit more wildcard binder forms in type declarations:

You may find it easiest to look at the rich diff

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@gmail.com> wrote:
Dear Steering 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

You may find it easiest to look at the rich diff.

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@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
_______________________________________________
ghc-steering-committee mailing list
ghc-steering-committee@haskell.org
https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee