On Sun, Mar 18, 2007 at 05:47:21PM +0000, C Rodrigues wrote:
Type synonyms aren't applied as I would expect during kind checking. What's going on here?
type WithList a b = b [a] type FooPair a b = (b, a -> b)
-- error: `WithList' is applied to too many type arguments ints1 :: WithList Int FooPair [Int] ints1 = ([1], id)
That's caused by kind defaulting, as bulat said.
-- error: `FooPair' is not applied to enough type arguments ints2 :: (WithList Int FooPair) [Int] ints2 = ([1], id)
Type synonyms must be fully applied, i.e. you must always have something like: (FooPair a Int) in types, not just FooPair, (FooPair a) or (FooPair Char). The reason is that otherwise you get type-level lambdas, and type inference becomes undecidable. Thanks Ian