Henry Laxen
s :: Map String (Map Int Char) s = M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])]
`ix` will give you a `Traversal` that targets a given key. If it's not present in the map, you get a `Traversal` that hits no targets. t :: Map String (Map Int Char) t = s & ix "bob" . ix 6 %~ succ *Main> t fromList [("alice",fromList [(5,'a')]),("bob",fromList [(6,'c')])] If you want more control (to set or delete values, for instance), the `at` function gives you a lens that views a `Maybe`. Writing `Just` into the lens creates the value at the key, and writing `Nothing` into the lens removes the value at the key. The `(?~)` operator, and the `_Just` prism may be useful to you if you use `at`. HTH, -- Jack