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

[PATRICK BROWNE]

David,
Thanks for the help.
Much appreciated,
Pat

On 11 May 2017 at 21:00, David McBride <toad3k at gmail.com> wrote:

> 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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Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to 
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