Neil, I think this idea is better than what I had suggested, but as it stands it doesn't typecheck. Did you mean something like this? leaves :: Tree -> [Int] leaves = f [] where f rest (Leaf n) = n : rest f rest (Branch l r) = f (f rest r) l -Chad ------------------------------- (from Neil Mitchell) Hi Tom

data Tree = Branch Tree Tree | Leaf Int deriving (Eq, Ord)

leaves :: Tree -> Set Int leaves (Leaf n) = singleton n leaves (Branch left right) = union (leaves left) (leaves right)

The standard method for a traversal over leaves with accumulation is: leaves :: Tree -> Set Int leaves x = f [] where f (Leaf n) rest = n : rest f (Branch l r) rest = f l (f r rest) This makes the construction of the list quite cheap. Then you can do the fromList trick, and that might give you a speed up. Thanks Neil