If I am writing a rener/parser for e.g. HTTP Requests and Responses, I would like to be able to guarantee that the parser really is the inverse of the generator. I assume that if I define a pair of computations as reversible then I can put those computations in some monad such that I can do: parse = inverse generate and parse2 = inverse do {generate;generate2} Actually, I assume this is an arrow rather than a monad because a render is a computation that takes a value and produces a value rather than simply a computation that produces a value. Whether its an arrow or not, I also assume this problem is sufficiently common that there is a well worked out idiom for this such that I don't have to hand-code and prove inverse functions for all my data structures. I just don't know what it is. -Alex- _________________________________________________________________ S. Alexander Jacobson mailto:me@alexjacobson.com tel:917-770-6565 http://alexjacobson.com