[Haskell-cafe] type versus data declarations

Hello, I'm wondering why you can write

data FSet a = Show a => M (a -> Double)

a :: FSet Double a = M $ \x -> 0

and it works, but

type FSet a = Show a => (a -> Double)

a :: FSet Double a _ = 0

fails with

All of the type variables in the constraint `Show a' are already in scope (at least one must be universally quantified here) In the type synonym declaration for `FSet'

If I understood the GHC manual, this should work, as FSet a should be replaced in a, so a type signature would come:

a :: Show a => (a -> Double)

which is OK. Sorry if this is FAQ, but I did some searches and nothing came out. Thanks in advance, Emilio