cover does not work when actual percentage 0

[nh2]

If you write with cover: "I want that x% (e.g. 100%) of my generated characters are a", and no as are generated at all, cover doesn't warn you when it should.

Example 1:

quickCheck $ forAll (return 'b') $ \c -> cover (c == 'a') 100 "a" True
+++ OK, passed 100 tests.

The problem is that quickcheck only considers stamps that actually occur.

This also happens if the percentage is very small so that with high probability no suitable sample is found; example 2:

> quickCheck $ forAll arbitrary $ \c -> cover (c == 'a') 100 "a" True
+++ OK, passed 100 tests (only 1% a; not 100%).
> quickCheck $ forAll arbitrary $ \c -> cover (c == 'a') 100 "a" True
+++ OK, passed 100 tests.

The location where cover is handled is covers in success :: State -> IO () in QuickCheck.Test.

[nh2]

Our temporary workaround for this: http://git.ganeti.org/?p=ganeti.git;a=commitdiff;h=8a1b6f9bc3d63c101404bfa320887309a890a9a7

-- | A stable version of QuickCheck's `cover`. In its current implementation,
-- cover will not detect insufficient coverage if the actual coverage in the
-- sample is 0. Work around this by lifting the probability to at least
-- 10 percent.
-- The underlying issue is tracked at
-- https://github.com/nick8325/quickcheck/issues/26
stableCover :: Testable prop => Bool -> Int -> String -> prop -> Property
stableCover pred percent label prop =
  let newlabel = "(stabilized to at least 10%) " ++ label
  in forAll (frequency [(1, return True), (9, return False)]) $ \ basechance ->
     cover (basechance || pred) (10 + (percent * 9 `div` 10)) newlabel prop

[nick8325]

Thanks for the bug report, this should be fixed now!