data List a = Nil | Cons a (List a) deriving(Eq)
instance Show a => Show (List a) where
show Nil = ""
show (Cons a Nil) = show a
show (Cons a as) = show a ++ "," ++ show as
instance Functor List where
fmap f Nil = Nil
fmap f (Cons a as) = Cons (f a) (fmap f as)
instance Applicative List where
(<*>) Nil _ = Nil
(<*>) _ Nil = Nil
(<*>) (Cons f fs) xs = fmap f xs `append` (<*>) fs xs
pure x = Cons x Nil
instance Monad List where
(>>=) Nil f = Nil
(>>=) (Cons a as) f = f a `append` (>>=) as f
return = pure
-- instance traversable
-- instance foldable
append :: List a -> List a -> List a
append Nil y = y
append x Nil = x
append (Cons x xs) y = Cons x $ append xs y
reverseL :: List a -> List a
reverseL Nil = Nil
reverseL (Cons a Nil) = Cons a Nil
reverseL (Cons a (Cons b bs)) = append (reverseL bs) (Cons b $ Cons a Nil)
-- push
-- pop
-- insertNth
-- init
-- last
-- head
-- tail
-- fromList
-- (!) , (!?)
showL :: Show a => List a -> String
showL l = "[" ++ show l ++ "]"
main = do
putStrLn $ showL $ reverseL $ append (Cons 1 $ Cons 2 $ Cons 3 Nil) (Cons 4 $ Cons 5 Nil)
putStrLn $ showL $ (+) <$> (Cons 1 $ Cons 2 Nil) <*> (Cons 3 $ Cons 4 Nil)
putStrLn $ showL $ (Cons 3 $ Cons 4 Nil) >>= (\x -> Cons x $ Cons x Nil)
putStrLn $ showL $ (Cons 3 $ Cons 4 Nil) >>= (\x -> pure $ x*2)
