PNL to HOL: From the logic of nominal sets to the logic of higher-order functions

  • Authors:
  • Gilles Dowek;Murdoch J. Gabbay

  • Affiliations:
  • INRIA, 23 avenue dItalie, CS 81321, 75214 Paris Cedex 13, France;School of Mathematical and Computer Sciences, HeriotWatt University, Riccarton Edinburgh, EH14 4AS, Great Britain, United Kingdom

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2012

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Abstract

Permissive-Nominal Logic (PNL) extends first-order predicate logic with term-formers that can bind names in their arguments. It takes a semantics in (permissive-)nominal sets. In PNL, the @?-quantifier or @l-binder are just term-formers satisfying axioms, and their denotation is functions on nominal atoms-abstraction. Then we have higher-order logic (HOL) and its models in ordinary (i.e. Zermelo-Fraenkel) sets; the denotation of @? or @l is functions on full or partial function spaces. This raises the following question: how are these two models of binding connected? What translation is possible between PNL and HOL, and between nominal sets and functions? We exhibit a translation of PNL into HOL, and from models of PNL to certain models of HOL. It is natural, but also partial: we translate a restricted subsystem of full PNL to HOL. The extra part which does not translate is the symmetry properties of nominal sets with respect to permutations. To use a little nominal jargon: we can translate names and binding, but not their nominal equivariance properties. This seems reasonable since HOL-and ordinary sets-are not equivariant. Thus viewed through this translation, PNL and HOL and their models do different things, but they enjoy non-trivial and rich subsystems which are isomorphic.