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:
So in practice what I do, and what I assume others do is this:
mapM_ f (toList myMap)
And I don't see any fusion rules that would ameliorate this (they seem limited to pure code), so I'm betting I always pay the cost of conversion to a list.
In the case of Data.Map
the proposal is simply a "traverseWithKey_" that returns "t ()". This follows the suffixing convention of mapM/mapM_ etc.
More generally, it would be nice to do an audit of all popular containers with the assumption of very large collections that cant' be frivolously copied.
-Ryan
P.S. Actually, there are a number of places where the standard libraries could do with a more complete set of "_" functions -- e.g. it always chafes to not have "atomicModifyIORef_", which would apply a (a->a) function like a regular modifyIORef.