[Haskell-cafe] Vanishing polymorphism

[David House]

On 08/05/07, Matthew Sackman <matthew at wellquite.org> wrote:
> > :t let f r s = (return negate) >>= (\(fn::forall n . (Num n) => n -> n) -> return (fn r, fn s)) in f
>
> <interactive>:1:35:
>     Couldn't match expected type `a -> a'
>            against inferred type `forall n. (Num n) => n -> n'
>     In the pattern: fn :: forall n. (Num n) => n -> n
>     In a lambda abstraction:
>         \ (fn :: forall n. (Num n) => n -> n) -> return (fn r, fn s)
>     In the second argument of `(>>=)', namely
>         `(\ (fn :: forall n. (Num n) => n -> n) -> return (fn r, fn s))'
>
> I.e. why does the polymorphism get destroyed?

Here fn is bound by a lambda abstraction, and is therefore
monomorphic. I can't find anything in the Report about that, but that
is how it works. It might be how a H-M type system works in general,
I'm not sure.

-- 
-David House, dmhouse at gmail.com