A constraint in a data type definition does not do much. It only
restricts the type of the data constructor(s), e.g. here FooType :: Foo
a => a -> FooType a.
Functions that use the data type FooType a cannot take any advantage of
the constraint.
There has been a long discussion on this issue on
Hello,
The following piece of code was rejected by Hugs:
> class Foo a where
> write :: a -> String
> write _ = "I'm a foo!"
>
> data Foo a => FooType a = FooType a
>
> writeFoo :: FooType a -> String
> writeFoo (FooType a) = write a
ERROR "Test.hs" (line xx): Cannot justify constraints in
