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

[Edsko de Vries]

Hey,

> This modified example helps a lot!  It makes it clear that practical
> expressions can also have multiple types. Thank you for taking the
> time to  explain this.

You're welcome :) It is a confusing issue; I only really understood it
when I wrote a typechecker myself :) 

> In fact, how about changing Clean to disallow the use of a type
> variable name in an inner type declaration if the same name appears in
> an outer type  declaration? This would prevent people from doing silly
> things.

That may be a good temporary fix. A better solution would be to allow
for scoped type variables, so that they'll do what the programmer
expects. In fact, I have a uniqueness type system that does have scoped
type variables; see

http://www.cs.tcd.ie/~devriese/pub/tfp07-paper.pdf

Edsko