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

David, Thanks for the help. Much appreciated, Pat On 11 May 2017 at 21:00, David McBride 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

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

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Hi, I am trying to use Quickcheck to check that pushing an element increases

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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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On Thu, May 11, 2017 at 3:38 PM, PATRICK BROWNE wrote: the please

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