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

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

empty :: Stack a
empty =  Stack []

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

top  :: Stack a-> a
top  (Stack [])  = error "Stack: top on empty stack"
top  (Stack s)   = head s

pop  :: Stack a-> Stack a
pop  (Stack [])  = error "Stack: pop on empty stack"
pop  (Stack s)   = Stack (tail s)

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