-- Implements a stack data structure
-- by Scot Drysdale on 8/25/07

module Stack (Stack(Stack), makeStack, push, pop, isEmpty) where

data Stack a = Stack [a]
     deriving Show

-- Creates an empty stack
makeStack         :: Stack a
makeStack         =  Stack []

-- Adds a new value to the top of a stack, returning the updated stack
push               :: a -> Stack a -> Stack a
push x (Stack s)   =  Stack (x:s)

-- Removes the top value in a stack, returning both the value and 
-- the updated stack as a pair.
pop                :: Stack a -> (a, Stack a)
pop (Stack (x:s))  =  (x, Stack s)
pop _              =  error "Cannot pop an empty stack"

-- returns true if stack is empty
isEmpty            :: Stack a -> Bool
isEmpty (Stack []) = True
isEmpty _          = False