We present MLSOS, a functional metalanguage for encoding definitions of structural operational semantics. The key features of this language are inbuilt support for representing object-language binding structures and performing proof-search. MLSOS uses the nominal approach to dealing with binders and a FreshML-style generative treatment of atoms. This allows us to prototype systems in a natural way, starting from a semi-formal specification. We outline the main design choices behind the language and illustrate its use. 1

Abstract. This paper studies inductive definitions involving binders, in which aliasing between free and bound names is permitted. Such aliasing occurs in informal specifications of operational semantics, but is excluded by the common representation of binding as meta-level λ-abstraction. Drawing upon ideas from functional logic programming, we represent such definitions with aliasing as recursively defined functions in a higher-order typed functional programming language that extends core ML with types for name-binding, a type of “semi-decidable propositions” and existential quantification for types with decidable equality. We show that the representation is sound and complete with respect to the language’s operational semantics, which combines the use of evaluation contexts with constraint programming. We also give a new and simple proof that the associated constraint problem is NP-complete. 1