Quick, Dirty & Naive GroupBy test hs/elm

GroupBy.elm

module GroupBy exposing (..)

groupBy : List v -> (v -> k) -> List (k, List v)
groupBy items f =
  case items of
    []      -> []
    (x::xs) ->
      let key = f x
          rest = groupBy xs f
      in insertTo rest key x

insertTo : List (k, List v) -> k -> v -> List (k, List v)
insertTo m key value =
  case m of
    []      -> [(key, [value])]
    (x::xs) -> if fst x == key
        then [(key, value :: snd x)] ++ xs
        else x :: (insertTo xs key value)

GroupBy.hs

module GroupBy where

groupBy :: Eq k => [v] -> (v -> k) -> [(k, [v])]
groupBy [] _ = []
groupBy (x:xs) f =
  let key = f x
      rest = groupBy xs f
  in insertTo rest key x

insertTo :: Eq k => [(k, [v])] -> k -> v -> [(k, [v])]
insertTo [] k v = [(k, [v])]
insertTo (x:xs) key value =
  if fst x == key
    then [(key, value : snd x)] ++ xs
    else x : insertTo xs key value