Is there documentation on the multi-parameter type classes?
Sections 7.4.2. Class declarations, 7.4.3 Functional dependencies and 7.4.4. Instance declarations of the GHC user guide give the short description of these features. These section refer to a couple of papers. The best explanation can be found in papers by Mark P. Jones: http://web.cecs.pdx.edu/~mpj/pubs.html (see especially `qualified types').
i think i see what you've done, but i'd like to read up on it to make sure that i understand.
The approach in the previous message was quite close to that used for representing collections. The type of a particular collection implies the type of collection's elements. In your case, the type of the agent implies the type of the processor for that particular agent (and vice versa). Incidentally, instance declarations can be recursive and mutually recursive.