[ghc-steering-committee] #607: Amend #281 (visible forall) and #378 (Design of DH) to clarify treatment of term variables in types (rec: accept)

[Richard Eisenberg]

Yes, in support. Thanks for the nice summary.

Richard

> On Aug 29, 2023, at 4:40 AM, Moritz Angermann <moritz.angermann at gmail.com> wrote:
> 
> Looks like an overall improvement to me. I’m in support. 
> 
> On Tue, 29 Aug 2023 at 2:48 PM, Vladislav <vlad.z.4096 at gmail.com <mailto:vlad.z.4096 at gmail.com>> wrote:
> Dear Committee,
> 
> Jakob Brünker has proposed #607, an amendment to proposals #281 (visible forall) and #378 (Design of DH). 
> See the diff here: https://github.com/ghc-proposals/ghc-proposals/pull/607/files <https://github.com/ghc-proposals/ghc-proposals/pull/607/files>
> 
> The amendment concerns type checking of term-level variables appearing in types, for example:
> 
>   f :: forall (a :: Bool) -> ...   -- visible forall, `f` expects a type-level Bool
>   g :: Bool -> ...  -- ordinary arrow, `g` expects a term-level Bool
>   g x = f x
> 
> What should happen to `x`, bound as a term variable but used as a type? #378 "Design for Dependent Types" already has an answer to this question: treat `x` as a skolem. But there are two problems
> 
> 1. #378 does not explain this design decision in sufficient detail. This led Simon to question its inclusion: "Does it have any value, really? Why not just reject?"
> 2. #281 follows this design in one section, saying "treated as a fresh skolem constant"; and rejects it in another section, saying "any uses of terms in types are ill-typed". There is a contradiction.
> 
> The amendment addresses both issues as follows:
> 
> 1. A new example is added to #378, explaining how treating `x` as a skolem is useful, but only if `foreach` is available (a retained quantifier that allows dependent pattern matching)
> 2. Contradiction in #281 is resolved in favor of rejecting any uses of terms in types as ill-typed, saving this feature for a future proposal.
> 
> I recommend to accept. Please share your thoughts either here or directly on GitHub.
> 
> Vlad
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