On Sat, 12 Jun 2021, Han Joosten wrote:

> Thanks for this respons, Rubén. I might not have explained what I am looking for very well. I already have 
> instance Arbitrary Foo 
> That is not the problem. The problem is, that when a counterexample is found, it is dumped to the output with the `show` function.
> I would like to be able to use another function for that, say show'. In order to do so, I need to obtain the counterexample itself
> in some way, so I can use it to generate a useful message after the tests are done. 

https://hackage.haskell.org/package/QuickCheck-2.14.2/docs/Test-QuickCheck.html#v:forAllShow