[QuickCheck] Obtaining value of QC counter example

[Koen Claessen]

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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