[QuickCheck] Obtaining value of QC counter example

[J. Stutterheim]

Hi Koen,


Thank you for the example! But suppose we have some monomorphic property prop_Foo :: [Int] -> Bool. Do we then not know the type of the counter example? The type of prop_Foo indicates that it would be some list of Int (e.g. [1,0]). Could we use this knowledge to somehow cast the value into a concrete counter-example, without relying on IORefs?

Likewise, if we have some property prop_Bar :: [Int] -> Int -> Bool and we get some counter example (e.g. [1,2,3] and 4), would we then not have sufficient type information to produce a tuple of type ([Int], Int)?


Jurriën

On 15 Oct 2012, at 10:51, Koen Claessen <koen at chalmers.se> wrote:

> Hi Jurien,
> 
> Since it is impossible to know on the outside of the property what the
> type is of the values you are interested in, this is not very easy.
> 
> I usually use the following trick (a bit ugly, but it works).
> 
> Suppose you have a property:
> 
>  prop_Monkey =
>    forAll ....
>      ...
>        x == y
> 
> And you want to get the value of x and y. x and y can be quantified
> variables, or other values that live inside the property.
> 
> If you want to print them, you can say:
> 
>  prop_Monkey =
>    forAll ....
>      ...
>        whenFail (print (x,y)) $
>          x == y
> 
> If you want to get them and use them as Haskell values, you can say:
> 
>  prop_Monkey ref =
>    forAll ....
>      ...
>        whenFail (writeIORef ref (Just (x,y))) $
>          x == y
> 
> Now, before you QuickCheck the property, you create an IORef like so:
> 
>  ref <- newIORef Nothing
>  quickCheck (prop_Monkey ref)
> 
> Afterwards you can look in the ref to see if there is a counter example.
> 
> A bit ugly, but since the type of the internal values is not known,
> the only way I know how to do it.
> 
> Let me know if this works for you.
> 
> /Koen
> 
> On Sat, Oct 13, 2012 at 11:36 AM, Jurriën Stutterheim
> <j.stutterheim at me.com> wrote:
>> Dear QuickCheck developers,
>> 
>> 
>> Is it possible to obtain the value of the shrunk counter example that QuickCheck produces, so that I may use it in the rest of my program? If so, how? :)
>> I could imagine that it's possible (at least for non-function values) to return a single counter example value for functions with a single argument, and a tuple of values for functions with more than one argument.
>> 
>> Cheers,
>> 
>> 
>> Jurriën
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