-- Implementation of the Stack ADT in Haskell
module Stack
  (Stack, empty, push, pop)
  where

data Stack a = MkStack [a]

empty :: Stack a
empty = MkStack []

push :: a -> Stack a -> Stack a
push a (MkStack s) = MkStack (a:s)

pop :: Stack a -> Maybe (a, Stack a)
pop (MkStack [])    = Nothing
pop (MkStack (a:s)) = Just (a, MkStack s)