Andriy Palamarchuk wrote:

I'm currently improving documentation for Data.Map. mapKeys documentation says:

"The size of the result may be smaller if f maps two or more distinct keys to the same new key. In this case the value at the smallest of these keys is retained."

However the implementation retains the value of the *largest* key:

Prelude Data.Map> let map = fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")] Prelude Data.Map> mapKeys (\ _ -> 1) map {1:="c"} Prelude Data.Map> mapKeys (\ _ -> 3) map {3:="c"}

See http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Map.html#v%3...

Should I fix the documentation to confirm to the actual function behavior?

Yes, as a fix I would say change the documentation to be consistent with the implementation, just in case somebody is actually using this function in code that already "works". But I suspect that in reality nobody is using it (except yourself possibly:-) as this function makes no sense to me. Personally, I think this function should be deprecated, which is what I did in the AVL based Data.Map clone.. http://darcs.haskell.org/packages/collections-ghc6.6/Data/Map/AVL.hs Users should be made to specify the behaviour they want using mapKeysWith (or use mapKeysOneToOne or mapKeysMonotonic). But now that I look at it I see my (deprecated) "clone" of this function is consistent with the originals documentation, but not it's implementation. I guess I'd better fix that :-( Regards -- Adrian Hey