Dear Committee,
Jakob Brünker has proposed #607, an amendment to proposals #281 (visible forall) and #378 (Design of DH).
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