Re: remove a Set of keys from a Map

* Henning Thielemann [2011-12-10 12:33:28+0100]

I have (x :: Set key) and (m :: Map key a) and want to remove all keys and corresponding values from m that are in x. I could solve it efficiently with a

differenceSet :: Map key a -> Set key -> Map key a

or with a

Map.fromKeySet :: a -> Set key -> Map key a

or a

Map.fromKeySet :: (key -> a) -> Set key -> Map key a

but I did not find one of these functions in containers-0.4.2.

You can implement e.g. fromKeySet as fromKeySet a = Map.fromAscList . map (, a) . Set.toAscList In practice, when conversion between Sets and Maps is needed often, I found it convenient (and efficient) to emulate "Set a" by "Map a ()". -- Roman I. Cheplyaka :: http://ro-che.info/