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

[Malte Ott]

On 22/03/2024 18:19, Simon Peyton Jones wrote:
>      >From my understanding the biggest argument against this is the
>     change in
>     template-haskell?
> 
> 
> Not specifically.  My reservation is that
> 
>   * it's an unforced change,
>   * with no user demand
>   * but some real user impact (you mention TH)
>   * and some implementation cost (modest but very non-zero)
>   * aiming to anticipate as-yet-unknown future requirements
> 
> That's not a combination I like.  Pain now for possible (but uncertain)
> gain in the future.
> 
> I don't object to making types and terms behave similarly -- indeed I
> have invested lots of time working with Richard, Vlad, Andrei and others
> on proposals and MRs that move in this direction.  I'm just very
> unconvinced about *this *proposal.
> 
> One minor point.  In patterns we allow this:
> f ((,) @Int @[a] x y) = ...
> Here the type arguments are not type variables but full-blown types, and
> of course nested parens etc come "for free".  But this proposal concerns
> data type declarations in which we definitely don't want fulll-blown
> types. So it's more than a "terms and types should be the same"
> discussion.
> 
> Simon

I see. I admit that I don’t feel expert enough to know if this design will turn
out to be optimal.

If forced to voice an opinion I am still slightly in favor of the proposal.
Mainly because we have an implementor here who is motivated to implement this,
thus the implementation cost seems less problematic.

Best,
Malte