Why not make it unlossy and have: trans :: (Ord k, Ord a) => Map k a -> Map a (Set k) On Thu, Jun 16, 2011 at 9:10 AM, Johan Tibell wrote:

On Thu, Jun 16, 2011 at 3:01 PM, Dmitri O.Kondratiev wrote:

...

Hi, Data.Map has many great functions, yet I could not find the one that allows from one map create another map where keys are values and values are keys of the first one. Something like: transMap:: (Ord k, Ord a) => Map k a -> Map a k

Does such function exist?

Note that such a function would be lossy as there might be duplicate values in the map.

Cheers, Johan

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On Thu, Jun 16, 2011 at 5:38 PM, Johan Tibell wrote:

On Thu, Jun 16, 2011 at 3:01 PM, Dmitri O.Kondratiev wrote:

...

Hi, Data.Map has many great functions, yet I could not find the one that allows from one map create another map where keys are values and values are keys of the first one. Something like: transMap:: (Ord k, Ord a) => Map k a -> Map a k

I don't think implementing this function in the library would add much as it cannot be implemented more efficiently with access to the internal representation than it can using the public API. Just write

transMap = M.fromList . map swap . M.toList

and stick it in some utility file.

Johan

Yes, this is a good one. Thanks!

On Thu, Jun 16, 2011 at 04:17:55PM +0200, Francesco Mazzoli wrote:

On 16/06/11 15:01, Dmitri O.Kondratiev wrote:

...

Hi, Data.Map has many great functions, yet I could not find the one that allows from one map create another map where keys are values and values are keys of the first one. Something like: transMap:: (Ord k, Ord a) => Map k a -> Map a k

Does such function exist? Thanks!

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What about something like

transMap :: (Ord k, Ord a) => Map k a -> Map a k transMap = M.fromList . map swap . M.toList

?

Or, if you want to keep duplicates, import qualified Data.Set as S import Control.Arrow (second) transMap = M.fromListWith S.union . map (second S.singleton . swap) . M.toList -Brent