Hello Roman, On 04.07.2013 19:41, Roman Cheplyaka wrote:
It's not a foldr you would expect. Here's the code:
foldrWithKey :: (k -> a -> b -> b) -> b -> Map k a -> b foldrWithKey f z = go z where go z' Tip = z' go z' (Bin _ kx x l r) = go (f kx x (go z' r)) l
You can see that 'go' is (partially) driven by the tree structure.
This called an in-order traversal of the tree. (The node is handled in-between the sub trees.) foldrWithKey f z = foldr (uncurry f) z . toAscList
A more foldr-y foldr would look like
foldrWithKey :: (k -> a -> b -> b) -> b -> Map k a -> b foldrWithKey f z = go z where go z' Tip = z' go z' (Bin _ kx x l r) = f kx x (go (go z' r) l)
This is a post-order traversal. (The node is handled after the subtrees.) This is a different thing.
Perhaps this should be fixed?
I don't think so. You would change the semantics. (Try to convert a Map into an ascending key-value list with your new definition.) Cheers, Andreas -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel@ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/