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