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New Results on Morris's Observational Theory: The Benefits of Separating the Inseparable

Authors Flavien Breuvart, Giulio Manzonetto, Andrew Polonsky, Domenico Ruoppolo

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Flavien Breuvart
Giulio Manzonetto
Andrew Polonsky
Domenico Ruoppolo

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Flavien Breuvart, Giulio Manzonetto, Andrew Polonsky, and Domenico Ruoppolo. New Results on Morris's Observational Theory: The Benefits of Separating the Inseparable. In 1st International Conference on Formal Structures for Computation and Deduction (FSCD 2016). Leibniz International Proceedings in Informatics (LIPIcs), Volume 52, pp. 15:1-15:18, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2016)


Working in the untyped lambda calculus, we study Morris's lambda-theory H+. Introduced in 1968, this is the original extensional theory of contextual equivalence. On the syntactic side, we show that this lambda-theory validates the omega-rule, thus settling a long-standing open problem.On the semantic side, we provide sufficient and necessary conditions for relational graph models to be fully abstract for H+. We show that a relational graph model captures Morris's observational preorder exactly when it is extensional and lambda-Konig. Intuitively, a model is lambda-Konig when every lambda-definable tree has an infinite path which is witnessed by some element of the model. Both results follow from a weak separability property enjoyed by terms differing only because of some infinite eta-expansion, which is proved through a refined version of the Böhm-out technique.
  • Lambda calculus
  • relational models
  • fully abstract
  • Böhm-out
  • omega-rule


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