Hi
> Would someone kindly explain why we need co-arbitrary in QuickCheck
> and how to define it?
To write your own 'coarbitrary', you are aiming to convert values of
your datatype into a disjoint partition of the set of all Ints. In
the end it is rather mechanical: e.g.
data Foo a = Foo String a
| Bar Bool
instance Arbitrary (Foo a) where
coarbitrary (Foo s a) = variant 0 . coarbitrary s . coarbitrary a
coarbitrary (Bar b) = variant 1 . coarbitrary b
The Derive tool now implements coarbitrary for the instances it
generates - following the scheme outlined in this email.
http://www-users.cs.york.ac.uk/~ndm/derive/
Thanks
Neil
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