[Haskell-beginners] Quickcheck: pushing element onto stack increases stack size

[PATRICK BROWNE]

Hi,
I am trying to use Quickcheck to check that pushing an element increases
the size of a stack.
Something like:
prop_size2 x s = (size (Push x s)) > (size s)

Is this possible? Below is my effort using an empty stack.
Thanks in advance,
Pat


module Stack (empty, push, pop, top, isEmpty) where
import Test.QuickCheck

data Stack a = Empty | Push a (Stack a) deriving  Show

empty :: Stack a
empty = Empty

push :: a -> Stack a -> Stack a
push x ss = Push x ss

pop :: Stack a -> Stack a
pop Empty = error "pop emptyStack"
pop (Push x ss) = ss

top :: Stack a -> a
top Empty = error "top emptyStack"
top (Push x ss) = x

isEmpty :: Stack a -> Bool
isEmpty Empty = True
isEmpty (Push x ss) = False


size :: (Stack a) -> Int
size Empty = 0
size (Push x ss) = succ (size ss)


prop_size x  = (size (Push x Empty)) > (size Empty)

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