On Tue, 2 Jul 2013, Ryan Newton wrote:
If there's not an existing way to do this, I suppose this is a minor proposal. When writing monadic code that consumes a container (Set, Map) etc, I often find that I just want what iterators in imperative languages provide: that is, to iterate over the contents of a large container, performing monadic effects along the way, but without allocating.
The Foldable instance for Data.Map almost gives you what you want, but it only exposes the values, not their keys. The "traverseWithKey" function is close too, but it builds a new Map as output:
traverseWithKey :: Applicative t => (k -> a -> t b) -> Map k a -> t (Map k b)
So in practice what I do, and what I assume others do is this:
mapM_ f (toList myMap)
You could also do Map.foldrWithKey (\k a -> f k a >>) (return ()) which does not need an interim Map, or Foldable.sequence_ . Map.mapWithKey f which looks more elegant.