[Haskell-cafe] Re: Monad instance for partially applied type constructor?

[Stefan Holdermans]

David,

Ryan Ingram wrote:

>>> Haskell doesn't have true type functions; what you are really saying
>>> is
>>> 
>>> instance Monad (\v -> Vect k (Monomial v))

Daniel Fischer wrote:

> I think there was a theoretical reason why that isn't allowed (making type 
> inference undecidable? I don't remember, I don't recall ...).

Indeed: type inference in the presence of type-level lambdas requires higher-order unification, which is undecidable [1].

Cheers,

  Stefan

[1] Gérard P. Huet: The Undecidability of Unification in Third Order Logic Information and Control 22(3): 257-267 (1973)