Improve the NullObject example

src/NullObject.hs

@@ -1,6 +1,7 @@
 module NullObject where
 import           Data.Map (Map, fromList) 
 import qualified Data.Map as Map (lookup) -- avoid clash with Prelude.lookup
+import Control.Monad ((>=>)) -- importing the Kleisli 'fish' operator for composing monadic functions
 
 type Song   = String
 type Album  = String
@@ -62,6 +63,10 @@ findUrlFromSong'' :: Song -> Maybe URL
 findUrlFromSong'' song =
     findAlbum song >>= findArtist >>= findWebSite      
 
+findUrlFromSong''' :: Song -> Maybe URL
+findUrlFromSong''' =
+    findAlbum >=> findArtist >=> findWebSite   
+
 nullObjectDemo = do
     putStrLn "NullObject -> Maybe"
 
@@ -74,4 +79,31 @@ nullObjectDemo = do
 
     print $ findUrlFromSong' "An Ending"
     print $ findUrlFromSong'' "Baby Satellite"
+    print $ findUrlFromSong''' "A Never Ending Story"
 
+    print $ safeRootReciprocal 0
+    print $ safeRootReciprocal (-10)
+    print $ safeRootReciprocal 0.01
+
+{-- --This is how >=> could be implemented for Maybe:
+(>=>) :: (a -> Maybe b) -> (b -> Maybe c) -> (a -> Maybe c)   
+m1 >=> m2 = \x ->
+    case m1 x of
+        Nothing -> Nothing
+        Just y  -> case m2 y of
+            Nothing          -> Nothing
+            result@(Just z)  -> result
+--}
+
+safeRoot :: Double -> Maybe Double
+safeRoot x
+    | x >= 0    = Just (sqrt x)
+    | otherwise = Nothing
+
+safeReciprocal :: Double -> Maybe Double
+safeReciprocal x
+    | x /= 0    = Just (1/x)
+    | otherwise = Nothing
+
+safeRootReciprocal :: Double -> Maybe Double
+safeRootReciprocal = safeReciprocal >=> safeRoot