Folds over Data.Map
greg at gregorycollins.net
Mon Aug 23 05:40:50 EDT 2010
Johan Tibell <johan.tibell at gmail.com> writes:
> It's not true that
> fold f z == foldr f z . elems
> in general. It only holds if `z` is an identity for `f` as `z` is used
> at every leaf in the tree.
-- | /O(n)/. Post-order fold.
foldr :: (k -> a -> b -> b) -> b -> Map k a -> b
foldr _ z Tip = z
foldr f z (Bin _ kx x l r) = foldr f (f kx x (foldr f z r)) l
Note the third parameter to the recursive foldr call -- the "z" you see
at the Tips is not the same "z" that was originally passed in.
Gregory Collins <greg at gregorycollins.net>
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