Hi Henry, Yes, the "ix" function traverses into particular keys of map-like structures https://www.stackage.org/haddock/lts-13.25/lens-4.17.1/Control-Lens-At.html#... So, (s & ix "bob" . ix 6 .~ 'c') will yield a modified Map with 'b' changed to 'c'. -Michael On Mon, Jun 17, 2019 at 9:45 PM Henry Laxen wrote:

Hi Cafe,

Suppose I have:

s :: Map String (Map Int Char) s = M.fromList [("alice", M.fromList [(5,'a')]), ("bob", M.fromList [(6,'b')])]

Is there any way to modify the 'b' entry easily using lenses?

Thanks in advance. Best wishes, Henry Laxen

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Henry Laxen writes:

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