Haskell implementation

Haskell/maybe.hs

@@ -0,0 +1,19 @@
+import Prelude hiding (Maybe(..), Functor, fmap, (<$>))
+
+data Maybe a = Just a | Nothing
+  deriving (Show)
+
+class Functor f where
+  fmap  :: (a -> b) -> f a -> f b
+
+(<$>) :: Functor f => (a -> b) -> f a -> f b
+(<$>) = fmap
+infixl 4 <$>
+
+instance Functor Maybe where
+  fmap f (Just x) = Just $ f x
+  fmap _ Nothing  = Nothing
+
+main = do
+ print $ (* 2) <$> Just 5
+ print $ (* 2) <$> Nothing

Haskell/maybe2.hs

@@ -0,0 +1,3 @@
+main = do
+  print $ (* 2) . (+ 5) . (^ 2) <$> Just 5
+  print $ (* 2) . (+ 5) . (^ 2) <$> Nothing

Haskell/maybe3.hs

@@ -0,0 +1,24 @@
+import Prelude hiding (Applicative(..))
+
+class Functor f => Applicative f where
+  pure  :: a -> f a
+  (<*>) :: f (a -> b) -> f a -> f b
+  (*>)  :: f a -> f b -> f b
+  (<*)  :: f a -> f b -> f a
+
+  a *> b = fmap const b <*> a
+  (<*)   = flip (*>)
+  {-# MINIMAL pure, (<*>) #-}
+
+instance Applicative Maybe where
+  pure           = Just
+  (Just f) <*> v = fmap f v
+  Nothing  <*> _ = Nothing
+
+main = do
+  let a = Just 5
+      b = Just $ (* 2) . (+ 5) . (^ 2)
+      c = b <*> a
+  print c
+
+