[Haskell-cafe] How to generate dependend values with QuickCheck
Ivan Lazar Miljenovic
ivan.miljenovic at gmail.com
Thu Aug 26 00:03:01 EDT 2010
2010/8/26 Jürgen Nicklisch-Franken <jnf at arcor.de>:
> 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
You're saying that i1 is equal to the arbitrary action, not the result
of such an action. As such, i2 is based upon a different arbitrary
action.
Consider this:
instance Arbitrary QCExample where
arbitrary = do i1 <- arbitrary
let i2 = 2*i1
return $ QCExample i1 i2
--
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com
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