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

Reto Kramer ml at retokramer.net
Tue Dec 19 01:43:28 EST 2006

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

 > 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)?

- Reto

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