exam2.hs

data MyList a = Empty | List (a, (MyList a)) deriving (Show, Eq)

instance Functor MyList where
    fmap _ Empty = Empty
    fmap f (List (x, xs)) = List (f x, fmap f xs)

flatten :: (MyList (MyList (a))) -> (MyList a)
flatten Empty = Empty
flatten (List (x, xs)) = myConcat x (flatten xs) 


myConcat :: (MyList a) -> (MyList a) -> (MyList a)
myConcat xs Empty = xs
myConcat Empty ys = ys
myConcat (List (x, xs)) ys = List (x ,(myConcat xs ys))

addToEnd :: a -> (MyList a) -> (MyList a)
addToEnd a Empty = List (a, Empty)
addToEnd a xs = myConcat xs (List (a, Empty))

instance Monad MyList where
    return x = List (x, Empty)
    Empty >>= _ = Empty
    xs >>= f = flatten $ fmap f xs

instance Applicative MyList where
    pure = return
    (<*>) Empty xs = Empty
    (<*>) xs Empty = Empty
    (<*>) (List (f, fs)) xs = myConcat (fmap f xs ) ((<*>) fs xs)