Anagrams kata in Haskell, based on http://www.cyber-dojo.org/ exercise

Anagrams.hs

module Anagrams where

anagrams :: String -> [String]
anagrams [] = []
anagrams [x] = [[x]]
anagrams word = anagrams' [] word

anagrams' :: String -> String -> [String]
anagrams' _ [] = []
anagrams' prev (x:xs) =
  [x:anagram | anagram <- anagrams(prev ++ xs)]
    ++ anagrams' (x:prev) xs

test_Anagrams.hs

module Test_Anagrams where

import Test.HUnit
import Anagrams
import Data.List (sort)

tests = TestList [
 TestCase (assertEqual "empty string"
  []
  (anagrams "")),

 TestCase (assertEqual "single character string"
  ["a"]
  (anagrams "a")),

 TestCase (assertEqual "two character string"
  (sort ["ab", "ba"])
  (sort $ anagrams "ab")),

 TestCase (assertEqual "three character string"
  (sort ["abc", "acb", "bac", "bca", "cab", "cba"])
  (sort $ anagrams "abc")),

 TestCase (assertEqual "biro"
  (sort [
   "biro", "bior", "brio", "broi", "boir", "bori",
   "ibro", "ibor", "irbo", "irob", "iobr", "iorb",
   "rbio", "rboi", "ribo", "riob", "roib", "robi",
   "obir", "obri", "oibr", "oirb", "orbi", "orib"])
  (sort $ anagrams "biro"))
 ]

main = do runTestTT tests