Modular structural operational semantics

Modular SOS (MSOS) is a variant of conventional Structural Operational Semantics (SOS). Using MSOS, the transition rules for each construct of a programming language can be given definitively, once and for all, and do not need reformulation when further constructs are added to the language. MSOS thus provides an exceptionally high degree of modularity in language descriptions, thereby removing a shortcoming of the original SOS framework. After sketching the background and reviewing the main features of SOS, this paper explains the crucial differences between SOS and MSOS, and indicates how MSOS descriptions are written. It discusses notions of semantic equivalence based on MSOS, and the possibility of modularity in proofs of properties. It gives illustrative examples of MSOS rules, directly comparable to examples of SOS given by Plotkin in his original Aarhus notes.