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

[Reto Kramer]

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