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.

Op vr 11 jun. 2021 om 23:31 schreef Ruben Astudillo <ruben.astud@gmail.com>:

Hello Han

On 11-06-21 17:16, Han Joosten wrote:
> [..]
> *parseFoo :: Text -> Guarded Foo*
> *prettyFoo :: Foo -> Text*
>
> The idea is that for all foo :: Foo, the following holds:
> *parseFoo . prettyFoo $ foo* should be equal to *Checked foo []*
>
> Now, if there is a counterexample, I would like to see prettyFoo foo as
> output, folowed by the show of the Errors part.

It seems you know what property should hold for `parseFoo` and `prettyFoo`.
Per the QuickCheck manual [1] this could be written as

import Test.QuickCheck

propOnAllFooText :: Property
propOnAllFooText = forAll (arbitrary :: Gen Foo) $ \exFoo ->
parseFoo (prettyFoo exFoo) == Checked exFoo []

What you are missing is a way to generate the test cases (the `arbitrary'
generator above). You need a way to declare this instance

instance Arbitrary Foo where
-- arbitrary :: Gen Foo
arbitrary = <something>

You can see example on how to define it on the manual [1]. Good luck.

[1]: http://www.cse.chalmers.se/~rjmh/QuickCheck/manual.html

--
-- Rubén
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