The Interpretation of Intuitionistic Type Theory in Locally Cartesian Closed Categories -- an Intuitionistic Perspective

  • Authors:
  • Alexandre Buisse;Peter Dybjer

  • Affiliations:
  • Programming, Logic and Semantics Group, IT University of Copenhagen, Rued Langgaards Vej 7, 2300 København S;Department of Computer Science and Engineering, Chalmers University of Technology, Rännvägen 6, S-41296 Göteborg

  • Venue:
  • Electronic Notes in Theoretical Computer Science (ENTCS)
  • Year:
  • 2008

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Abstract

We give an intuitionistic view of Seely's interpretation of Martin-Lof's intuitionistic type theory in locally cartesian closed categories. The idea is to use Martin-Lof type theory itself as metalanguage, and E-categories, the appropriate notion of categories when working in this metalanguage. As an E-categorical substitute for the formal system of Martin-Lof type theory we use E-categories with families (E-cwfs). These come in two flavours: groupoid-style E-cwfs and proof-irrelevant E-cwfs. We then analyze Seely's interpretation as consisting of three parts. The first part is purely categorical: the interpretation of groupoid-style E-cwfs in E-locally cartesian closed categories. (The key part of this interpretation has been type-checked in the Coq system.) The second is a coherence problem which relates groupoid-style E-cwfs with proof-irrelevant ones. The third is a purely syntactic problem: that proof-irrelevant E-cwfs are equivalent to traditional lambda calculus based formulations of Martin-Lof type theory.