Haskell Language QuickCheck Randomly generating data for custom types
Example
The Arbitrary class is for types that can be randomly generated by QuickCheck.
The minimal implementation of Arbitrary is the arbitrary method, which runs in the Gen monad to produce a random value.
Here is an instance of Arbitrary for the following datatype of non-empty lists.
import Test.QuickCheck.Arbitrary (Arbitrary(..))
import Test.QuickCheck.Gen (oneof)
import Control.Applicative ((<$>), (<*>))
data NonEmpty a = End a | Cons a (NonEmpty a)
instance Arbitrary a => Arbitrary (NonEmpty a) where
arbitrary = oneof [ -- randomly select one of the cases from the list
End <$> arbitrary, -- call a's instance of Arbitrary
Cons <$>
arbitrary <*> -- call a's instance of Arbitrary
arbitrary -- recursively call NonEmpty's instance of Arbitrary
]
