On Tuesday 24 August 2010 13:09:56, Thomas wrote:
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
Do not use `if length list > 1', if the list is long, that takes long too. Use `if not (null $ drop 1 list)'.
=> 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])]'.
Right. However, the definition of testFold says a can't be *any* type. In the then-branch, the result is (node, is, ns) : a, which is a list, and forces a to be a list too, so the type of testFold cannot be more general than XMLCallback [a] (and that should compile).
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.
In the definition of xmlTreeFold, nothing restricts the type of the accumulator argument, so it can be anything.
Thanks for any insights, Thomas