[Haskell-cafe] How to generate dependend values with QuickCheck

[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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