module StackADT (Stack, createStack, push, empty, top, pop) where 

import Prelude

data Stack a = Stack [a] 
               deriving (Eq, Show) 

createStack :: Stack a 
createStack = Stack [] 

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

empty :: Stack a -> Bool 
empty (Stack s) = null s 

top :: Stack a -> a 
top (Stack s) 
    | null s = error "stack is empty" 
    | otherwise = head s 


poptop :: Stack a -> (a, Stack a)
poptop s = (top s, pop s)


pop :: Stack a -> Stack a 
pop (Stack s) 
    | null s = error "stack is empty" 
    | otherwise = Stack (tail s) 

---

-- Using stack

s = createStack

s2 = push 1 s

s3 = ((push 1) . (push 2)) s

t = top s3