[Haskell-cafe] class Arbitrary in quickcheck

[Ozgur Akgun]

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 <ozgurakgun at gmail.com> 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 verboseCheck<http://hackage.haskell.org/packages/archive/QuickCheck/1.1.0.0/doc/html/Test-QuickCheck.html#v:verboseCheck>on these two and observe the results.
>
> Best,
>
>
>
>
> On 5 May 2010 08:47, Tim Docker <twd2 at dockerz.net> 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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>> Haskell-Cafe at haskell.org
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>>
>
>
>
> --
> Ozgur Akgun
>



-- 
Ozgur Akgun
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