simple haskell stack implementation, work in progress

astack.hs

module AStack(Stack, push, pop, top, size, stackTo) where
data Stack a = Empty
             | MkStack a (Stack a)

push :: a -> Stack a -> Stack a
push x s = MkStack x s

size :: Stack a -> Int
size s = length (stkToLst s) where
           stkToLst  Empty        = []
           stkToLst (MkStack x s) = x:xs where
             xs = stkToLst s

pop :: Stack a -> (a, Stack a)
pop (MkStack x s)
  = (x, case s of
      r -> i r where i x = x)

top :: Stack a -> a
top (MkStack x s) = x

stackTo :: Int -> Stack Int
stackTo x = foldl (\s n -> push n s) Empty [1..x]

showStack :: (Show a) => Stack a -> String
showStack (MkStack x s) = show x ++ " - " ++ showStack s
showStack Empty = "}"

instance Show a => Show (Stack a) where
  show x = showStack x

instance Functor Stack where
  fmap f (MkStack x s) = push (f x) $ fmap f s
  fmap f Empty = Empty

http://www.ki.informatik.uni-frankfurt.de/doc/html/Haskell1.3/intro.html