September 2017 Update

Task/Permutations-Derangements/Haskell/permutations-derangements-1.hs

@@ -1,24 +1,33 @@
-import Control.Monad
-import Data.List
+import Control.Monad (forM_)
+
+import Data.List (permutations)
 
 -- Compute all derangements of a list
-derangements xs = filter (and . zipWith (/=) xs) $ permutations xs
+derangements
+  :: Eq a
+  => [a] -> [[a]]
+derangements = (\x -> filter (and . zipWith (/=) x)) <*> permutations
 
 -- Compute the number of derangements of n elements
+subfactorial
+  :: (Eq a, Num a)
+  => a -> a
 subfactorial 0 = 1
 subfactorial 1 = 0
-subfactorial n = (n-1) * (subfactorial (n-1) + subfactorial (n-2))
+subfactorial n = (n - 1) * (subfactorial (n - 1) + subfactorial (n - 2))
 
-main = do
-  -- Generate and show all the derangements of four integers
-  print $ derangements [1..4]
+main :: IO ()
+main
+-- Generate and show all the derangements of four integers
+ = do
+  print $ derangements [1 .. 4]
   putStrLn ""
-
   -- Print the count of derangements vs subfactorial
-  forM_ [1..9] $ \i ->
-      putStrLn $ show (length (derangements [1..i])) ++ " " ++
-                 show (subfactorial i)
+  forM_ [1 .. 9] $
+    \i ->
+       putStrLn $
+       mconcat
+         [show (length (derangements [1 .. i])), " ", show (subfactorial i)]
   putStrLn ""
-
   -- Print the number of derangements in a list of 20 items
   print $ subfactorial 20