Folks, I understand that arbitrary defines the possible values. How do I generally come up with co-arbitrary, though? Would someone kindly explain the choice of co-arbitrary in the following cases, specially the very last bit with "variant 1 . coarbitrary a"? instance Arbitrary Char where arbitrary = elements ([' ', '\n', '\0'] ++ ['a'..'h']) coarbitrary c = variant (fromEnum c `rem` 4) instance Arbitrary Ordering where arbitrary = elements [LT, EQ, GT] coarbitrary LT = variant 0 coarbitrary EQ = variant 1 coarbitrary GT = variant 2 instance Arbitrary a => Arbitrary (Maybe a) where arbitrary = frequency [ (1, return Nothing) , (3, liftM Just arbitrary) ] coarbitrary Nothing = variant 0 coarbitrary (Just a) = variant 1 . coarbitrary a Thanks, Joel -- http://wagerlabs.com/