modifying Map of Maps in Haskell

Question

What would be a proper way of working with a map of maps in Haskell? Assuming that I want to have something like

import qualified Data.Map as M

type Key1   = String
type MyMap1 = M.Map Key1 Int
type Key    = String
type MyMap  = M.Map Key MyMap1

how am I supposed to implement the function which adds elements into the map? The best I can think of is

addE :: Key -> Key1 -> Int -> MyMap -> MyMap
addE k k1 v = M.insertWith M.union k (M.singleton k1 v)

but M.union gives me O(n) complexity instead of expected O(log n)

Is there a better way to do this or another data structure that should be used instead?

Have you tried the update function? Something like addE k k1 v = M.update (Just . M.insert k1 v) should work. — bheklilr
– bheklilr, Commented Mar 10, 2015 at 18:33
A map of maps is fine, but depending on what you actually do with it, you might want a Map (Key,Key1) Int instead. — chi
– chi, Commented Mar 10, 2015 at 19:22

Louis Wasserman · Accepted Answer · 2015-03-10 18:51:05Z

3

addE :: Key -> Key1 -> Int -> MyMap -> MyMap
addE k k1 v = M.alter (Just . M.insert k1 v . fromMaybe M.empty) k

answered Mar 10, 2015 at 18:51

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

1 Answer 1