Proposal: Non-allocating way to iterate over a Data.Map: traverseWithKey_
Andreas Abel
andreas.abel at ifi.lmu.de
Thu Jul 4 22:41:45 CEST 2013
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 at ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/
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