September 2017 Update

Task/Sort-using-a-custom-comparator/Haskell/sort-using-a-custom-comparator-1.hs

@@ -1,9 +1,26 @@
-import List
-import Char
+import Data.List (sortBy)
+import Data.Function (on)
+import Data.Char (toLower)
 
-mycmp s1 s2 = case compare (length s2) (length s1) of
-                 EQ -> compare (map toLower s1) (map toLower s2)
-                 x  -> x
+lengthThenAZ :: String -> String -> Ordering
+lengthThenAZ a b
+  | d == EQ = on compare (toLower <$>) a b
+  | otherwise = d
+  where
+    d = on compare length a b
 
-strings = ["Here", "are", "some", "sample", "strings", "to", "be", "sorted"]
-sorted = sortBy mycmp strings
+descLengthThenAZ :: String -> String -> Ordering
+descLengthThenAZ a b
+  | d == EQ = on compare (toLower <$>) a b
+  | otherwise = d
+  where
+    d = on (flip compare) length a b
+
+xs :: [String]
+xs = ["Here", "are", "some", "sample", "strings", "to", "be", "sorted"]
+
+main :: IO ()
+main =
+  mapM_
+    putStrLn
+    [unlines $ sortBy lengthThenAZ xs, unlines $ sortBy descLengthThenAZ xs]

Task/Sort-using-a-custom-comparator/Haskell/sort-using-a-custom-comparator-2.hs

@@ -1,3 +1,26 @@
+import Data.List (sortBy)
+import Data.Char (toLower)
+import Data.Ord (comparing)
 import Data.Monoid
-mycmp s1 s2 = mappend (compare (length s2) (length s1))
-                       (compare (map toLower s1) (map toLower s2))
+
+-- the Ordering instance of mappend is defined to yield
+-- lexicographical ordering.
+-- instance Monoid Ordering where
+--        mempty         = EQ
+--        LT `mappend` _ = LT
+--        EQ `mappend` y = y
+--        GT `mappend` _ = GT
+
+xs :: [String]
+xs = ["Here", "are", "some", "sample", "strings", "to", "be", "sorted"]
+
+lowerCase :: String -> String
+lowerCase = (toLower <$>)
+
+main :: IO ()
+main =
+  mapM_ putStrLn $
+  (unlines . flip sortBy xs) <$>
+  [       comparing length  <> comparing lowerCase -- Ascending  length
+  , flip (comparing length) <> comparing lowerCase -- Descending length
+  ]