12 - Liftings and Extensions from Set

Summary

This chapter studies results whereby a set functor is lifted to other categories, paying attention to whether the initial algebra and terminal coalgebra structures also lift. For example, given a set functor F having a terminal coalgebra and a lifting on either complete partial orders and complete metric spaces, the terminal coalgebra can be equipped with a canonical order or metric, respectively, so that this yields the terminal coalgebra for the lifting. Initial algebras, however, need not lift from Set to the other categories. We are also interested in specific liftings of F to pseudometric spaces, such as the Kantorovich and Wasserstein liftings. We study extensions to Kleisli categories and liftings to Eilenberg–Moore categories. We present results on coalgebraic trace semantics, and discuss examples such as the classical trace semantics of (probabilistic) labelled transition systems and languages accepted by nominal automata. We also study generalized determinization of coalgebras of functors arising from liftings to Eilenberg–Moore categories, leading to the coalgebraic language semantics. We see many instances of this semantics: the language semantics of non-deterministic weighted, probabilistic, and nominal automata; and also context-free languages.