[Haskell-cafe] type inference question

[minh thu]

2009/10/8 Cristiano Paris <frodo at theshire.org>:
> On Thu, Oct 8, 2009 at 12:48 PM, Lennart Augustsson
> <lennart at augustsson.net> wrote:
>> The reason a gets a single type is the monomorphism restriction (read
>> the report).
>> Using NoMonomorphismRestriction your example with a works fine.
>
> Could you explain why, under NoMonomorphismRestriction, this typechecks:
>
> let a = 1 in (a + (1 :: Int),a + (1 :: Float))
>
> while this not:
>
> foo :: Num a => a -> (Int,Float)
> foo k = (k + (1 :: Int), k + (1.0 :: Float))

I think it is the same thing that my (\f -> f f True) question, i.e.
the polymorphism of k is fixed inside foo (you don't want that). So I
guess using the rank-2 types and the corresponding type annotation to
keep k polymorph should work. In other words, foo is polymorph on k
but k is not polymorph when given to foo in a specific application.

Thu