astack.hs

{- ORACLE_TEST pass -}
module AStack( Stack, push, pop, top, size ) 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}}) -- (pop Empty) is an error
;top :: Stack a -> a
;top (MkStack x s) = x -- (top Empty) is an error
}