Proposal: Add foldMapWithKey to Data.Map and Data.IntMap

[Edward Kmett]

If you think about it, traverseWithKey already mimics the bracketing of
IntMap internally.

On Sun, Dec 30, 2012 at 5:42 AM, Milan Straka <fox at ucw.cz> wrote:

> Hi all,
>
> > -----Original message-----
> > From: Edward Kmett <ekmett at gmail.com>
> > Sent: 28 Dec 2012, 18:36
> >
> > I'd like to add foldMapWithKey to Data.Map and Data.IntMap.
> >
> > Right now you can foldrWithKey and foldlWithKey on Data.Map and
> > Data.IntMap, but you cannot foldMapWithKey.
> >
> > You can fake this by using fold . mapWithKey but this requires converting
> > the entire structure.
> >
> > You can also implement it with traverseWithKey and the use of the Const
> Monoid
> > in a manner similar to foldMapDefault.
>
> One can implement foldMapWithKey as
>
> foldMapWithKey f = foldlWithKey (\a k b -> a `mappend` f k b) mempty
>
> This is a valid definition which does not create intermediate structure.
> The difference to the proposed implementation is, as Edward mentions:
>
> > When you have a monoid that can take advantage of the balanced nature of
> > the tree, it'd be nice to be able to not lose the original near balanced
> > parenthesization of the source tree.
>
> Both definitions differ in the bracketing of the mappend-s.
> This should be mentioned in the documentation of the foldMapWithKey.
> However, while the bracketing of foldlWithKey and foldrWithKey is well
> defined, the bracketing of foldMapWithKey depends on the internal
> structure of the container. But it is probably safe to assume that the
> bracketing
> will be "balanced" in some way for any implementation of the data
> structure.
>
> Cheers,
> Milan
>
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