The Biequivalence of Locally Cartesian Closed Categories and Martin-Löf Type Theories

Abstract : Seely's paper Locally cartesian closed categories and type theory contains a well-known result in categorical type theory: that the category of locally cartesian closed categories is equivalent to the category of Martin-Löf type theories with Π, Σ, and extensional identity types. However, Seely's proof relies on the problematic assumption that substitution in types can be interpreted by pullbacks. Here we prove a corrected version of Seely's theorem: that the Bénabou-Hofmann interpretation of Martin-Löf type theory in locally cartesian closed categories yields a biequivalence of 2-categories. To facilitate the technical development we employ categories with families as a substitute for syntactic Martin-Löf type theories. As a second result we prove that if we remove Π-types the resulting categories with families with only Σ and extensional identity types are biequivalent to left exact categories.
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Pierre Clairambault, Peter Dybjer. The Biequivalence of Locally Cartesian Closed Categories and Martin-Löf Type Theories. Mathematical Structures in Computer Science, Cambridge University Press (CUP), 2014, 24 (05), pp.e240501. ⟨10.1017/S0960129513000881⟩. ⟨hal-00990027⟩

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