You're generating two random values, where you probably want to just
generate one and then calculate the second from it. Try this:
instance Arbitrary QCExample where
arbitrary = do
i1 <- arbitrary
return (QCExample i1 (i1 * 2))
2010/8/25 Jürgen Nicklisch-Franken
I want to generate values, so that i have some arbitrary object, which has a certain set of signals, and each set of signals has a certain set of sensor value pairs, were the values are arbitrary. However to demonstrate my problem I have an easy example, were I want to generate an instance of
data QCExample = QCExample Int Int deriving Show
and the second value should always be the double of the first. So I tried this:
instance Arbitrary QCExample where arbitrary = let i1 = arbitrary i2 = fmap (* 2) i1 in liftM2 QCExample i1 i2
but showing some of the generated test cases in ghci does not give me what I expected:
let gen :: Gen (QCExample) = arbitrary Test.QuickCheck.Gen.sample gen
QCExample (-2) 0 QCExample (-4) (-6) QCExample 3 30 ... I know that I can filter, but this would be to inefficient.
Jürgen
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