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 :

Hello Han

On 11-06-21 17:16, Han Joosten wrote:

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[..] *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 -- pgp: 4EE9 28F7 932E F4AD

I think you're looking for something like this: ``` prop_parser :: Foo -> Property prop_parser foo  =    case parserFoo pretty of       Checked foo' [] | foo == foo' -> property True       a  -> counterexample (show a ++ "\n" ++ show pretty) False   where     pretty = prettyFoo foo ``` Sent with ProtonMail Secure Email. ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Saturday, June 12th, 2021 at 12:49, 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. 

Op vr 11 jun. 2021 om 23:31 schreef Ruben Astudillo :

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Hello Han

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On 11-06-21 17:16, Han Joosten wrote:

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...

[..]

...

...

*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

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    propOnAllFooText :: Property

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    propOnAllFooText = forAll (arbitrary :: Gen Foo) $ \exFoo ->

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        parseFoo (prettyFoo exFoo) == Checked exFoo []

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What you are missing is a way to generate the test cases (the `arbitrary'

...

generator above). You need a way to declare this instance

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    instance Arbitrary Foo where

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        -- arbitrary :: Gen Foo

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        arbitrary = <something>

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

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

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--

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-- Rubén

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-- pgp: 4EE9 28F7 932E F4AD

Thanks all for the responses. I managed to put them all together and I [got it all to work]( https://github.com/AmpersandTarski/Ampersand/blob/cefbc08ad8fb3152ce1a55dce5... ). Cheers, Han. Op za 12 jun. 2021 om 13:28 schreef Henning Thielemann < lemming@henning-thielemann.de>:

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.h...