[Haskell-beginners] mapKeysMaybe

[Lyndon Maydwell]

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 <maydwell at gmail.com> wrote:

> Oops disregard that. The mapMaybeWithKey doesn't quite do what I expected.
>
> On Fri, Nov 6, 2015 at 6:43 PM, Lyndon Maydwell <maydwell at gmail.com>
> 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 <dennis.raddle at gmail.com>
>> 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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>>> Beginners at haskell.org
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>>>
>>>
>>
>
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