Action Semantics

Action Semantics (1985–2005) is a hybrid of denotational and operational semantics, developed in collaboration with David Watt (Glasgow).

An action semantics of a programming language maps syntax compositionally to action notation, and the semantics of action notation is defined using SOS (structural operational semantics).

See the Wikipedia page and the Action Semantics book.

The non-modularity of the original SOS of action notation prompted the development of MSOS (a modular variant of SOS using labeled transitions) and I-MSOS (a version of MSOS using conventional SOS notation).