Thank you all, (you are responding so quick that I consider to stop thinking myself in the future and just ask). Jürgen Am Donnerstag, den 26.08.2010, 00:02 -0400 schrieb Edward Z. Yang:
Excerpts from Jürgen Nicklisch-Franken's message of Wed Aug 25 23:32:51 -0400 2010:
instance Arbitrary QCExample where arbitrary = let i1 = arbitrary i2 = fmap (* 2) i1 in liftM2 QCExample i1 i2
What's happening here is that you are "evaluating" the random value generator too late in the game: when you set i1 = arbitrary, no random value has been generated yet, and i2 is thus another generator which has no relation to what i1 might produce. You could instead do this:
arbitrary = do i1 <- arbitrary let i2 = i1 * 2 return (QCExample i1 i2)
Cheers, Edward