My understanding is that SmallCheck and lazy SmallCheck[1] are much better suited to handle existential properties, and indeed have combinators especially for that task. --Sterl. [1] http://www-users.cs.york.ac.uk/~mfn/lazysmallcheck/ smallcheck.pdf, http://hackage.haskell.org/cgi-bin/hackage-scripts/ package/smallcheck, http://hackage.haskell.org/cgi-bin/hackage- scripts/package/lazysmallcheck On Oct 12, 2008, at 1:38 PM, Norman Ramsey wrote:
I recently used QuickCheck to check on some calculations for image compression. (I love exact rational arithmetic!) But I thought only to check for inverse properties, and I realized afterward I had failed to check for ranges. For example I should have checked that
boundedB block = -1 <= b && b <= 1 where Cos a b c d = cos_of_block block
which turns out to be correct. But actually my calculations were wrong, and the real bounds on b are not +/- 1 but rather +/- 0.5, so I might equally well have passed a more stringent test. It's quite embarrassing as I have only 5 bits of precision to represent b, and throwing away a bit on values that don't exist is not the best idea I have had recently.
What I would really like to do is write some combinators for interval checking that say
* It is a *universal* property that for *every* input, the output lies within the stated interval.
* It is an *existential* property of *some* input that the output lies close to the ends of the stated interval.
But how do I use QuickCheck to check an existential? I realize I could run
boundsNotAchieved block = -0.9 <= b && b <= 0.9 where Cos a b c d = cos_of_block block
and then if the test of boundsNotAcheived fails, my test passes (by exists x : P(x) iff not forall(x) : not P(x)).
But if I set up a test suite in which some tests are supposed to fail and others are supposed to succeed, I will never keep track of which is which. I would like to build an existential test. Is it sufficient for me to write
boundsAchieved block = expectFailure (-0.9 <= b && b <= 0.9) where Cos a b c d = cos_of_block block
and then run 'quickCheck boundsAchieved'?
In the long run I'd love something like
fillsIntervalWithin :: (Num a, Ord a, Arbitrary a) => (a, a) -> a -> a -> Property
with the idea that I want to test the partial application
fillsIntervalWithin (lo, hi) epsilon
and have the test succeed if every input x satisfies lo <= x <= hi and if some input x satisfies not (lo + epsilon <= x <= hi - epsilon).
I'm betting someone on this list has already thought about similar problems and can advise me. (Because I have yet to write the specialized instance declaration for Arbitrary that guarantees the numerical invariants of the inputs, I have yet to test any of the examples in this email.)
Norman _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell