type specs not making it in to functions

[Hal Daume III]

consider the following definition:

> class C a where foo :: a -> Int
> instance C Bool where foo _ = 5

I can then say:

> bar :: C a => a -> Int
> bar (x :: a) = foo (undefined :: a)

But not:

> bar :: C a => a -> Int
> bar x = foo (undefined :: a)

because it tries to use a new scope for the type variable a and doesn't
unify it with the one in the type spec.  Why not?

On a more general note, suppose I have a class and there are "constant
members" of the class.  For instance, something like:

> class D a where constMember :: Int
> instance D Int where constMember = 8

It seems ehre that there's no way to extract constMember for a
/particular/ class, since you can't tell it what "a" is supposed to
be.  So, instead, I do:

> class D a where constMember :: a -> Int -- should be independent of arg
> instance D Int where constMember _ = 8

(which brings me to my use of "undefined::a").  But is there a
better/preferred way to do this?

 - Hal

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume