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

[David McBride]

First you need a way to generate an arbitrary stack.  Here's a simple
method that just gives you a 50% chance at each level of generating an
extra layer.

instance Arbitrary a => Arbitrary (Stack a) where
  arbitrary = oneof [return Empty, Push <$> arbitrary <*> arbitrary ]

prop_size_succeeds x s = size (Push x s) > size s
prop_size_fails x s = size s > size Empty

>quickCheck prop_size_fails
*** Failed! Falsifiable (after 6 tests):
()
Empty


On Thu, May 11, 2017 at 3:38 PM, PATRICK BROWNE <patrick.browne at dit.ie> wrote:
> 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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