Re: [Haskell-cafe] Need advise with using Quickcheck

12 Jun 2021

      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 :

...
...
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
...
...
-- pgp: 4EE9 28F7 932E F4AD

Re: [Haskell-cafe] Need advise with using Quickcheck

coot＠coot.me