[Haskell-beginners] a problem with maps

[Dennis Raddle]

Oops that was supposed to be

mapOutList list = M.fromListWith (++) $ map (second (: [])) list

On Fri, Jul 22, 2011 at 6:40 PM, Dennis Raddle <dennis.raddle at gmail.com>wrote:

> In my application I find an issue coming up frequently. I have a map
>
> Ord k => Map k [a]
>
> That is, a list of things may be stored at each key. I sometimes want to
> convert it to
>
> [(k,a)]
>
> where each value is paired with its key, and some keys may repeat in that
> form of the list. No particular order is needed.
>
> so I have the following code. Would appreciate hearing if there are any
> tricks I'm missing
>
> import qualified Map as M
>
> listOutMap :: Ord k => Map k [a] -> [(k,a)]
> listOutMap m = concatMap (\(k,a) -> zip (repeat k) a) (M.toList m)
>
> mapOutList :: Ord k => [(k,a)] -> Mpa k [a]
> mapOutList list = M.fromList $ map (second (: [])) list
>
>
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