[Haskell-cafe] QuickCheck's co-arbitrary

[Joel Reymont]

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

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