The new Haskell report 1.3 (preliminary) allows type variables of
other kinds than *, e.g. it gives the example (page 51)

        data App f a = A (f a)

where the type variable f has kind *->*, and App has (*->*)->*->*.

My problem is that I haven't seen (in the report) any mention
about the interaction of this with type inference.

Take for instance the declaration:
        something = A ()
what is the type of  something  ?
It could be  App (\x->x) (), but it could also be
App (\x->()) a, for any type a, and a few others.

But these are different types and it matters for type inference which
one we choose, there is no longer a most general type, a principal type.
Basically, these are the usual problems with higher-order unification
(including that it's undecidable).

I'm not arguing against this feature, I'm just complaining that the
report doesn't say anything about the problem of what do with
non-determined higher-order types during type inference.

Stefan Kahrs

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