[clean-list] a local function typing question in function composition

[Edsko de Vries]

On Thu, Apr 12, 2007 at 10:07:05AM -0400, Philip Matthews wrote:
> 
> On 12-Apr-07, at 09:57 , Edsko de Vries wrote:
> 
> >Hey,
> >
> >>Does this mean that it is illegal in Clean to use type variables
> >>from an outer function definition in an inner function  definition?
> >
> >No, not really. It's just that all type variables are implicitly
> >quantified by a "forall" (just like in Haskell, I might add). So, for
> >example, the following code is perfectly fine, albeit a bit contrived:
> >
> >module t
> >
> >f :: [a] -> [a]
> >f xs = ys
> >    where
> >        ys :: [a]
> >        ys = []
> >
> >Start = f [1,2,3]
> >
> >Again, the type annotation on "ys" says that "ys" has type "[a]"  
> >for all a (and
> >it does, because the empty list has that type).
> 
> Would I be correct in saying that type variable "a" in "f:[a]->{a]"  
> is completely distinct
> from type variable "a" in "ys::[a]", even though they have the same  
> name?

Yes, that is correct, because they are both bound by a forall. What that
definition really says is

f :: forall a. [a] -> [a]
f xs = ys
    where
        ys :: forall a. [a]
        ys = []

Edsko