Data.Map

Test.hs

import qualified Data.Map as M
import qualified Data.Set as S
import Data.Monoid

type Classification = M.Map String String

type Value = Int

src :: M.Map Classification Value
src =
  let key1 = M.fromList [("cls1", "v1"), ("cls2", "v2")]
      key2 = M.fromList [("cls1", "v1"), ("cls2", "v3")]
      key3 = M.fromList [("cls1", "v4"), ("cls2", "v2")]
  in  M.fromList [(key1, 5), (key2, 10), (key3, 7)]

main :: IO ()
main = do
  let keys = S.fromList ["cls1"]
  let result = M.mapKeysWith (+) (\cls -> M.restrictKeys cls keys) src
  print result