Re: [Haskell-cafe] class Arbitrary in quickcheck

There is a typo, in my previous post, It should have been: prop_2 v1 v2 = v1 == v2 not: prop_2 v1 v2 = v2 == v2 Moreover, I realised that the (nice) function verboseCheck doesn't exist in QuickCheck 2. However you can always do the following in ghci, to see whether my suggestion works or not: sample (arbitrary :: Gen (Maybe Double, Maybe Double) ) On 5 May 2010 09:01, Ozgur Akgun wrote:

your quick check property (in a different way of writing) is the following:

prop_1 :: Maybe Double -> Bool prop_1 v = v == v

but what you want is actually the following:

prop_2 :: Maybe Double -> Maybe Double -> Bool prop_2 v1 v2 = v2 == v2

if I understood the problem correctly, using prop_2 should solve it. just run verboseCheckhttp://hackage.haskell.org/packages/archive/QuickCheck/1.1.0.0/doc/html/Test...on these two and observe the results.

Best,

On 5 May 2010 08:47, Tim Docker wrote:

...

I've notice a behaviour of quickcheck that is unexpected to me. With this code:

import Test.QuickCheck

main = check myconfig ((\v -> v == v) :: (Maybe Double,Maybe Double) -> Bool)

myconfig = defaultConfig{configMaxTest=100000, configEvery = \n args -> show n ++ ":\n" ++ unlines args}

I am relying on the default Arbitrary instances to generate pairs of maybe doubles. But to my surprise, all of the pairs have these patterns:

(Just _ Just _) (Nothing, Nothing)

I never see patterns:

(Just _, Nothing) (Nothing, Just _)

Why is this the case?

Thanks,

Tim

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-- Ozgur Akgun

-- Ozgur Akgun