Product Rules and Distributive Laws

Abstract : We give a categorical perspective on various product rules, including Brzozowski’s product rule $(st)_a = s_a t + o(s) t_a$ and the familiar rule of calculus $(st)_a = s_a t + s t_a$ It is already known that these product rules can be represented using distributive laws, e.g. via a suitable quotient of a GSOS law. In this paper, we cast these product rules into a general setting where we have two monads S andT, a (possibly copointed) behavioural functor F, a distributive law of T over S, a distributive law of S over F, and a suitably defined distributive law $TF \Rightarrow FST$ We introduce a coherence axiom giving a sufficient and necessary condition for such triples of distributive laws to yield a new distributive law of the composite monad ST over F, allowing us to determinize FST-coalgebras into lifted F coalgebras via a two step process whenever this axiom holds.