[...]
Let us keep the function h without the explicit type declaration. Let us apply what I thought as an equivalent transformation:
h = \b -> f (g (h b))
Hugs now reports ERROR "/tmp/b.hs":12 - Unresolved top-level overloading *** Binding : h *** Outstanding context : X b and GHCi reports /tmp/b.hs:12: Ambiguous type variable(s) `a' in the constraint `X a' arising from use of `f' at /tmp/b.hs:12 In a lambda abstraction: f (g (h b))
It is interesting that the error message is exactly the same as when we used the explicit type declaration for the old "h b=..." definition.
Something seems to be going on...
Isn't this just the monomorphism restriction. In GHCi, Prelude> let k = (<) <interactive>:1: Ambiguous type variable(s) `a' in the constraint `Ord a' arising from use of `<' at <interactive>:1 In the definition of `k': (<) as opposed to Prelude> let k a b = a < b Prelude> and further, Prelude> let k = \ a b -> a < b <interactive>:1: Ambiguous type variable(s) `a' in the constraint `Ord a' arising from use of `<' at <interactive>:1 In a lambda abstraction: a < b