An Intersection Type System for Deterministic Pushdown Automata

Takeshi Tsukada, Naoki Kobayashi

Abstract: We propose a generic method for deciding the language inclusion problem between context-free languages and deterministic contextfree languages. Our method extends a given decision procedure for a subclass to another decision procedure for a more general subclass called a refinement of the former. To decide \(\mathcal{L}_0 \subseteq \mathcal{L}_1\), we take two additional arguments: a language \(\mathcal{L}_2\) of which \(\mathcal{L}_1\) is a refinement, and a proof of \(\mathcal{L}_0 \subseteq \mathcal{L}_2\). Our technique then refines the proof of \(\mathcal{L}_0 \subseteq \mathcal{L}_2\) to a proof or a refutation of \(\mathcal{L}_0 \subseteq \mathcal{L}_1\). Although the refinement procedure may not terminate in general, we give a sufficient condition for the termination. We employ a type-based approach to formalize the idea, inspired from Kobayashi’s intersection type system for model-checking recursion schemes. To demonstrate the usefulness, we apply this method to obtain simpler proofs of the previous results of Minamide and Tozawa on the inclusion between context-free languages and regular hedge languages, and of Greibach and Friedman on the inclusion between context-free languages and superdeterministic languages.