[Haskell-cafe] Class annotation on datatype ineffective in function

The code below does not compile unless the "bar" function is annotated with a suitable constraint on the class of the formal parameter.

module Main where

class (C a) data (C foo) => XY foo = X foo | Y foo

bar :: a -> XY a bar aFoo = X aFoo

main = return ()

I get:

$ ghc Test.hs

Test.hs:8:8: No instance for (C a) arising from use of `X' at Test.hs:8:8-10 Possible fix: add (C a) to the type signature(s) for `foo' In the expression: X a In the definition of `foo': foo a = X a

As suggested, this works:

bar :: (C a) => a -> XY a

Can someone explain to me why the compiler can not infer that "a" (in bar) must be (C a) from the bar result type "XY a" (by way of the "C class" provided for the datatype)? Thanks, - Reto