Double-functorial representation of regular structures

Abstract: It is well-known that bicategories of spans capture the Beck-Chevalley relationship underlying the adjunctions suitable for the semantics of existential quantification. By viewing hyperdoctrines from a double-categorical lens, this paper shows that we can also characterise the Frobenius property: (generalised) regular hyperdoctrines correspond to those lax symmetric monoidal double pseudofunctors from spans to quintets whose monoidal laxators provide companion commuter cells (in the sense of Par\'e). This suggests a new notion of regular double hyperdoctrine. As an application, we discuss how we can recover a form of graphical regular logic suitable for modelling specifications of systems (e.g., port-plugging systems) that compose operadically.