anagram: Make the test suite as flexible as possible

exercises/anagram/HINTS.md

@@ -0,0 +1,22 @@
+## Hints
+
+To complete this exercise you need to implement the function `anagramsFor`,
+that takes a *word* and a group of *words*, returning the ones that are
+anagrams of the given *word*.
+
+If it is your first time solving this exercise, it is recommended that you
+stick to the provided signature:
+
+```haskell
+anagramsFor :: String -> [String] -> [String]
+```
+
+Later, it may be a good idea to revisit this problem and play with other data
+types and libraries:
+
+- `Text`, from package *text*.
+- `Sequence` and `Set`, from package *containers*.
+- `MultiSet`, from package *multiset*
+
+The test suite was intentionally designed to accept almost any type signature
+that makes sense, so you are encouraged to find the one you think is the best.

exercises/anagram/package.yaml

@@ -9,6 +9,9 @@ library:
   dependencies:
   # - foo       # List here the packages you
   # - bar       # want to use in your solution.
+    - containers
+    - multiset
+    - text
 
 tests:
   test:

exercises/anagram/src/Example.hs

@@ -1,10 +1,13 @@
 module Anagram (anagramsFor) where
-import Data.List (sort)
-import Data.Char (toLower)
 
-anagramsFor :: String -> [String] -> [String]
-anagramsFor word = filter (isAnagram . normalize)
+import Data.Function  (on)
+import Data.MultiSet  (fromList)
+import Data.Set       (Set, filter)
+import Data.Text      (Text, toLower, unpack)
+import Prelude hiding (filter)
+
+anagramsFor :: Text -> Set Text -> Set Text
+anagramsFor x = filter (\w -> x `isDistinctOf` w && x `isAnagramOf` w)
   where
-    normalize xs = let nxs = map toLower xs in (nxs, sort nxs)
-    (nw, sw) = normalize word
-    isAnagram (w, s) = nw /= w && sw == s
+    isDistinctOf = (/=) `on` toLower
+    isAnagramOf  = (==) `on` fromList . unpack . toLower

exercises/anagram/test/Tests.hs

@@ -1,7 +1,8 @@
 {-# LANGUAGE RecordWildCards #-}
 
 import Data.Foldable     (for_)
-import Test.Hspec        (Spec, describe, it, shouldBe)
+import GHC.Exts          (fromList, toList)
+import Test.Hspec        (Spec, describe, it, shouldMatchList)
 import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
 
 import Anagram (anagramsFor)
@@ -14,9 +15,14 @@ specs = describe "anagram" $
           describe "anagramsFor" $ for_ cases test
   where
 
-    test Case{..} = it description $ expression `shouldBe` expected
+    test Case{..} = it description $ expression `shouldMatchList` expected
       where
-        expression = anagramsFor subject candidates
+        expression = map toList
+                   . toList
+                   . anagramsFor (fromList subject)
+                   . fromList
+                   . map fromList
+                   $ candidates
 
 -- Test cases adapted from `exercism/x-common/anagrams.json` on 2016-07-25.