Evan Laforge schrieb:
So, I mentioned this a long time ago but didn't get any responses and then I got distracted. So this time I added a ticket and patch and everything: 2580
Here's the text: It's even implemented, but not exported. Without this, there's apparently no way to iterate over a map from high to low, since foldl is also not exported.
Hi, foldl is available via the Foldable instance for Set,Map,IntMap. And if I'm not mistaken, a 'left fold' corresponds to 'iterate from low to high' (try foldlM failing on the first element). Anyway, while there is foldWithKey
toAscList = foldWithKey (\k v m -> (k,v) : m) []
_foldrWithKey_ seems to be missing for Map/IntMap. I don't know if there is a performance penalty using 'reverse . toAscList' (e.g. in monadic traversals which stop after a few elements), but I suppose you were talking about the API anyway. Finally, I'd like to see a view on the association list, something like
viewAssocs :: Map k v -> MapView (k,v) instance Foldable MapView
which would provide 'fold[lr]WithKey[M]' in a standard way. best regards, benedikt