[Haskell-cafe] quickCheck problem

[Rogan Creswick]

On Wed, Oct 7, 2015 at 11:33 AM, Ozgur Akgun <ozgurakgun at gmail.com> wrote:
> This is not true, ==> still does generate and test.

I could have phrased that better.  Sure, it generates values, but it
will never provide them to your test, so your logic is simpler.

> Prelude> import Test.QuickCheck
> Prelude Test.QuickCheck> let f x = x == 13 ==> mod x 2 == 1
> Prelude Test.QuickCheck> quickCheck f
> *** Gave up! Passed only 6 tests.

That's unlikely to be a problem given the test inputs in the original
question, but yes, it is certainly an issue to know about.

Roelof -

The issue Ozgur is demonstrating is that the ==> operator will just
discard test inputs that were generated but which did not satisfy the
predicate (here that x == 13).  QuickCheck will eventually stop trying
to generate inputs (you can tune this), and produce something like the
above result.

--Rogan

>
> Also try verboseCheck to see what values are generated.
>
> Ozgur
>