[Haskell-beginners] type variables

[Brent Yorgey]

On Tue, Aug 24, 2010 at 01:09:56PM +0200, Thomas wrote:
> 
> But if I change the type declaration of 'testFold' to:
> testFold :: XMLCallback a
> it will not compile.
> 
> I thought that 'a' is a type /variable/ thus able to hold any type,
> for example, but not limited to '[(XMLTreeNode, [Int],
> [XMLTreeNode])]'. Why do I need to be that much more specific in the
> declaration for 'testFold'?

Daniel has already given a more detailed answer, but just to emphasize
the main point a bit more:

  testFold :: XMLCallback a

means that testFold *should work no matter what a is*.  But as Daniel
has explained, this is not the case; testFold only works for certain
specific types in place of a.  Type variables are not a shortcut used
when one can't be bothered to give the type in more detail; rather,
they are strong assertions that *any type will work*.

-Brent