groupBy in Erlang and Haskell

groupBy.erl

groupBy(F, L) -> lists:foldr(fun({K,V}, D) -> dict:append(K, V, D) end , dict:new(), [ {F(X), X} || X <- L ]).

GroupBy.hs

module GroupBy where

import Data.Map (Map)
import qualified Data.Map as Map

groupBy :: Ord b => (a -> b) -> [a] -> Map b [a] 
groupBy f = foldr (\ v -> Map.insertWith (++) (f v) [v]) Map.empty


groupBy' :: Ord b => (a -> b) -> [a] -> Map b [a] 
groupBy' f lst = Map.fromListWith (++) [(f x, [x]) | x <- lst]

groupBy'' :: Ord b => (a -> b) -> [a] -> Map b [a] 
groupBy'' f = Map.fromListWith (++) . map (\x -> (f x, [x]))

GroupByTest.hs

module Main where

import GroupBy

import Test.HUnit
import qualified Data.Map as Map

main = runTestTT $ "Should group by" ~: 
    TestList [
        groupBy length ["foo", "bar", "baz"] ~?= Map.fromList [(3, ["foo", "bar", "baz"])],


        groupBy head ["foo", "bar", "baz"] ~?= Map.fromList [('b', ["bar", "baz"]),  ('f', ["foo"])]


    ]