Nominal Automata with Name Binding

TL;DR

This paper introduces a formalism for data languages with local freshness semantics using nominal automata with name binding, establishing equivalences, a Kleene theorem, and decidability results for inclusion.

Contribution

It extends session automata to a full Kleene theorem for RNNAs with unscoped name binding and explores local freshness semantics, linking to orbit-finite automata.

Findings

Full Kleene theorem for RNNAs with extended expression language

Decidability of inclusion under local freshness semantics

Equivalence of RNNAs to a subclass of orbit-finite automata

Abstract

Automata models for data languages (i.e. languages over infinite alphabets) often feature either global or local freshness operators. We show that Bollig et al.'s session automata, which focus on global freshness, are equivalent to regular nondeterministic nominal automata (RNNA), a natural nominal automaton model with explicit name binding that has appeared implicitly in the semantics of nominal Kleene algebra (NKA), an extension of Kleene algebra with name binding. The expected Kleene theorem for NKA is known to fail in one direction, i.e. there are nominal languages that can be accepted by an RNNA but are not definable in NKA; via session automata, we obtain a full Kleene theorem for RNNAs for an expression language that extends NKA with unscoped name binding. Based on the equivalence with RNNAs, we then slightly rephrase the known equivalence checking algorithm for session automata. Reinterpreting the data language semantics of name binding by unrestricted instead of clean alpha-equivalence, we obtain a local freshness semantics as a quotient of the global freshness semantics. Under local freshness semantics, RNNAs turn out to be equivalent to a natural subclass of Bojanczyk et al.'s nondeterministic orbit-finite automata. We establish decidability of inclusion under local freshness by modifying the RNNA-based algorithm; in summary, we obtain a formalism for local freshness in data languages that is reasonably expressive and has a decidable inclusion problem.