Observational Equivalence of 3rd-Order Idealized Algol is Decidable

  • Authors:
  • C.-H. Luke Ong

  • Affiliations:
  • -

  • Venue:
  • LICS '02 Proceedings of the 17th Annual IEEE Symposium on Logic in Computer Science
  • Year:
  • 2002

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Abstract

We prove that observational equivalence of 3rd-order finitary Idealized Algol (IA) is decidable using Game Semantics. By modelling state explicitly in our games, we show that the denotation of a term M of this fragment of IA (built up from finite base types) is a compactly innocent strategy-with-state i.e. the strategy is generated by a finite view function fM . Given any such fM , we construct a real-time deterministic pushdown automata (DPDA) that recognizes the complete plays of the knowing-strategy denotation of M. Since such plays characterize observational equivalence, and there is an algorithm for deciding whether any two DPDAs recognize the same language, we obtain a procedure for deciding observational equivalence of 3rd-order finitary IA. This algorithmic representation of program meanings, which is compositional, provides a foundation for model-checking a wide range of behavioural properties of IA and other cognate programming languages. Another result concerns 2nd-order IA with recursion: we show that observational equivalence for this fragment is undecidable.