On 1 Jan 2009, at 09:36, Max.cs wrote:
thanks!
suppose we have
data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show
and how I could define a function foo :: a -> Tree a that
foo a = Leaf a where a is not a type of Tree foo b = b where b is one of the type of Tree (Leaf or Branch) ?
The following code seems not working......
foo (Leaf a) = a foo a = Leaf a
saying 'Couldn't match expected type `a' against inferred type `Btree a' Hi again Max, I'm assuming this is continuing from the concatT example, and that you're struggling with first function you must pass to foldTree. Remember the type of the function – it's not a -> Tree a, but Tree a - Tree a, because your leaves in the parent tree all contain trees to glue on at that point.
So, the function you want, is the function which looks at the parameter it's given, goes "oh, that's interesting", does nothing to it, and hands it back to replace the Leaf. I recommend searching hoogle for functions of type a -> a, the function you're looking for is built in. Bob