Here's a more correct solution, unfortunately it's not as nice. {-# LANGUAGE TupleSections #-} import Data.Map hiding (mapMaybe) import Data.Maybe mkm :: Ord k => (t -> Maybe k) -> Map t a -> Map k a mkm f = fromList . mapMaybe (\(k,v) -> fmap (,v) (f k)) . toList On Fri, Nov 6, 2015 at 6:46 PM, Lyndon Maydwell wrote:

Oops disregard that. The mapMaybeWithKey doesn't quite do what I expected.

On Fri, Nov 6, 2015 at 6:43 PM, Lyndon Maydwell wrote:

...

Well, this might not be the most efficient, but:

mapKeysMaybe f = Data.Map.mapMaybeWithKey (const . f)

What do you reckon?

On Fri, Nov 6, 2015 at 6:29 PM, Dennis Raddle wrote:

...

Can someone give me a clever way to implement mapKeysMaybe, defined as follows?

Ord k => (k -> Maybe k) -> Map k a -> Map k a

where the transformed keys that evaluate to Just are the only ones kept?

I am thinking this will probably help me understand a new type, like traversable or something.

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