counts.elm

counts : List Int -> Dict Int Int
counts items =
  List.map (\x -> Dict.singleton x 1) items
  |> List.foldr (merge combine) Dict.empty
  
  
combine : Maybe Int -> Maybe Int -> Int
combine a b =
  case (a, b) of
    (Just a', Just b') -> a' + b'
    (Just a', Nothing) -> a'
    (Nothing, Just b') -> b'
    (Nothing, Nothing) -> 0
  
  
merge : (Maybe Int -> Maybe Int -> Int) -> Dict Int Int -> Dict Int Int -> Dict Int Int
merge f d1 d2 =
  Dict.union d1 d2
  |> Dict.keys
  |> List.map (\k -> (k, Dict.get k d1, Dict.get k d2))
  |> List.map (\(k, v1, v2) -> (k, f v1 v2))
  |> Dict.fromList