Re: inferred type doesn't type-check (using type families)

Am Dienstag 03 November 2009 19:28:55 schrieb Roland Zumkeller:

Hi,

Compiling

...

class WithT a where type T a

f :: T a -> a -> T a f = undefined

g x = f x 42

with -XTypeFamilies -fwarn-missing-signatures gives:

Inferred type: g :: forall a. (Num a) => T a -> T a

Adding

...

g :: Num a => T a -> T a

results in:

Couldn't match expected type `T a' against inferred type `T a1' In the first argument of `f', namely `x'

Is the inferred type not the right one? Is g typeable?

The type function T isn't injective (or, it isn't guaranteed to be), so there's no way to determine which type a to use for 42.

Best,

Roland