Typing problems with polymorphic recursion and typeclasses

[Derek Elkins]

[...]
> 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