[Haskell-beginners] type variables

[Thomas]

Hello!

I have a question about type variables.
The following works:

type XMLCallback a = (XMLTreeNode -> a -> [Int] -> [XMLTreeNode] -> a)

xmlTreeFold :: XMLCallback a -> a -> Maybe XMLTreeNode -> Maybe a
xmlTreeFold _ _ Nothing = Nothing
xmlTreeFold func acc (Just tree) =
   Just (xmlTreeWalkerWithContext func acc tree [] [])

testFold :: XMLCallback [(XMLTreeNode, [Int], [XMLTreeNode])]
testFold node a is ns =
	if (length is) > 1 then ((node, is, ns):a) else a

=> xmlTreeFold testFold [] tree

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'?
Especially since in the declaration of 'xmlTreeFold' the 'XMLCallback a' 
is well received.

Thanks for any insights,
Thomas