Hi Manfred,
I may be wrong but having to use Map.toList looks pretty inefficient.
Question: I'd like to know if there is a more efficient way to do it?
I do not know if it is really more efficient (it has to consider each entry, just like converting to list does), but Data.Map is an instance of Data.Traversable, which has the traverse function, see: http://www.haskell.org/ghc/docs/latest/html/libraries/base-4.3.1.0/Data-Trav... e.g. Prelude> :m Data.Map Data.Traversable Prelude Data.Map Data.Traversable> let mymap = insert 1 "bubu" $ insert 2 "baba" $ empty Prelude Data.Map Data.Traversable> traverse print mymap "bubu" "baba" fromList [(1,()),(2,())] Prelude Data.Map Data.Traversable> Is this what you meant? You probably want to ingnore the result value of type Prelude Data.Map Data.Traversable> :t (traverse print mymap) (traverse print mymap) :: (Num t, Ord t) => IO (Map t ()) regards Matthias