Actually use parallelism in Haskell exercise

Kinda forgot that in the original commit. Whoops.

assignments/haskell/parallel-letter-frequency/example.hs

@@ -1,6 +1,9 @@
 module Frequency (frequency) where
 
+import           Control.Monad.Par (IVar, Par)
+import qualified Control.Monad.Par as Par
 import           Data.Char (isLetter, toLower)
+import           Data.List.Split (chunksOf)
 import           Data.Map (Map)
 import qualified Data.Map as Map
 import           Data.Text (Text)
@@ -9,7 +12,15 @@ import qualified Data.Text as T
 -- | Compute the frequency of letters in the text using the given number of
 --   parallel workers.
 frequency :: Int -> [Text] -> Map Char Int
-frequency _workers = Map.unionsWith (+) . map countLetters
+frequency workers texts = Par.runPar $ do
+    let chunks = chunksOf workers texts
+    ivars <- mapM (\_ -> Par.new) chunks
+    mapM_ (Par.fork . work) $ zip chunks ivars
+    freqs <- mapM Par.get ivars
+    return $ Map.unionsWith (+) freqs
+
+work :: ([Text], IVar (Map Char Int)) -> Par ()
+work (texts, ivar) = Par.put ivar .  Map.unionsWith (+) . map countLetters $ texts
 
 countLetters :: Text -> Map Char Int
 countLetters = T.foldl' process Map.empty