flip package:MissingH

flipAL :: (Eq key, Eq val) => [(key, val)] -> [(val, [key])]
MissingH Data.List.Utils
Flips an association list. Converts (key1, val), (key2, val) pairs to (val, [key1, key2]).
flipM :: (Ord key, Ord val) => Map key val -> Map val [key]
MissingH Data.Map.Utils
Flips a Map. See flipAL for more on the similar function for lists.
flippedLookupM :: (Ord val, Ord key) => val -> Map key val -> [key]
MissingH Data.Map.Utils
Returns a list of all keys in the Map whose value matches the parameter. If the value does not occur in the Map, the empty list is returned.