Folds over Data.Map

[Johan Tibell]

On Mon, Aug 23, 2010 at 11:40 AM, Gregory Collins
<greg at gregorycollins.net>wrote:

> 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.
>
> Hey Johan,
>
>    -- | /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.
>

You're of course right. I can get back to my optimizing. :)
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