inferred type doesn't type-check (using type families)
daniel.is.fischer at web.de
Tue Nov 3 14:20:00 EST 2009
Am Dienstag 03 November 2009 19:28:55 schrieb Roland Zumkeller:
> > 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
> > 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.
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