Unified Algebras and Action Semantics

The recently-developed framework of Unified Algebras is intended for axiomatic specification of abstract data types. In contrast, the somewhat older framework of Action Semantics (earlier known as ''Abstract Semantic Algebras'') is for denotational specification of programming languages. This paper gives an introduction to the main features of Unified Algebras and Action Semantics, and discusses the relation between them. The two frameworks both exploit nondeterministic choice in unconventional ways.