Functors.

category-theory/functor-example1.hs

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+main = do line <- fmap reverse getLine  
+          putStrLn $ "You said " ++ line ++ " backwards!"  
+          putStrLn $ "Yes, you really said" ++ line ++ " backwards!"

category-theory/functor-example2.hs

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+import Data.Char  
+import Data.List  
+
+main = do line <- fmap (intersperse '-' . reverse . map toUpper) getLine  
+          putStrLn line

category-theory/functors-module.hs

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+-- few examples on List, Mabe and Either
+a1 = fmap (replicate 3) [1,2,3,4]  
+a2 = fmap (replicate 3) (Just 4)  
+a3 = fmap (replicate 3) (Right "blah")  
+a4 = fmap (replicate 3) Nothing  
+a5 = fmap (replicate 3) (Left "foo")
+
+----------- functor laws in action 
+
+-- identity function (\x -> x). Formerly: fmap id = id
+b1 = fmap id (Just 3)
+b2 = id (Just 3)  
+b3 = fmap id [1..5]  
+b4 = id [1..5]  
+b5 = fmap id []  
+b6 = fmap id Nothing
+
+-- composition: fmap (f . g) F = fmap f (fmap g F)

category-theory/functors.hs

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+-- Defined in the standard library
+instance Functor IO where  
+    fmap f action = do  
+        result <- action  
+        return (f result)
+
+-- defined in Control.Monad.Instances
+instance Functor ((->) r) where  
+    fmap f g = (\x -> f (g x))
+
+-- same as aboved using composition
+instance Functor ((->) r) where  
+    fmap = (.)