[Haskell-cafe] Trying to reduce memory costs of String duplicates

Hi all, I'm reading in a data of 216k records into a map of Key, Values Pairs, the values being strings. As it happens out of 216k String values there really are only about 6.6k distinct string values, so I could save a lot of RAM if I was able to "insert" only actually *new* string values into the map and use references to (string) values that already are in memory instead. Is there a container that would, if I wanted to insert an element, return a pair of either the previously inserted, equal value and the container unchanged, or the new, previously unknown value and the new container amended by that element? Günther