Generic Weakest Precondition Semantics from Monads Enriched with Order

Abstract. We devise a generic framework where a weakest precondi-tion semantics, in the form of indexed posets, is derived from a monad whose Kleisli category is enriched by posets. It is inspired by Jacobs’ recent identification of a categorical structure that is common in various predicate transformers, but adds generality in the following aspects: 1) different notions of modality (such as “may ” vs. “must”) are captured by Eilenberg-Moore algebras; 2) nested branching—like in games and in probabilistic systems with nondeterministic environments—is modularly modeled by a monad on the Eilenberg-Moore category of another.