Hi Jonas,
As far as i can tell, derive only works for regular and linear recursive types and Regular uses frequencies to regulate size. (Also Regular doesn't seem to work for QuickCheck-2).
Derive will generate instances for all types, but uses a fairly standard formulation (pick between each constructor equally), meaning that in some cases the generators won't be that good, and will tend to generate infinite branches.
Another feature of Agata generators is improved scalability compared to other QuickCheck generators, especially for nested collection data types (analog to [[[[a]]]] and such). The details of how this works in Agata will one day be explained in the documentation, but the principle is explained in my masters thesis[1].
Very neat :-) I'd welcome an instance generator based on your ideas as a patch to Derive. Derive provides 3 things: * Infrastructure - given a definition, it provides tests, ability to run as template haskell, ability to write to files, auto-generated documentation etc. I hope to add more infrastructure, such as the ability to run derivations via a website, so the user need not even install derive. * Derivation inference - a method infer derivations given a single example (explained in this paper: http://community.haskell.org/~ndm/downloads/paper-deriving_a_relationship_fr...) * Lots of instances, about 34 different ones. I/you/we could replace the Arbitrary instance with one based on your implementation. You don't have to use the derivation inference (and it probably wouldn't be suitable), but you do get to benefit from the rest of the infrastructure. People can then run your program via the command line, integrate easily with preprocessors etc - and still do the Template Haskell bit too.
[1] http://gupea.ub.gu.se/bitstream/2077/22087/1/gupea_2077_22087_1.pdf
Nice, I'll have a read (if it's in English) Thanks, Neil