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arXiv · 2103.07397

An extensible equality checking algorithm for dependent type theories

Andrej Bauer·Anja Petković Komel
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Abstract

We present a general and user-extensible equality checking algorithm that is applicable to a large class of type theories. The algorithm has a type-directed phase for applying extensionality rules and a normalization phase based on computation rules, where both kinds of rules are defined using the type-theoretic concept of object-invertible rules. We also give sufficient syntactic criteria for recognizing such rules, as well as a simple pattern-matching algorithm for applying them. A third component of the algorithm is a suitable notion of principal arguments, which determines a notion of normal form. By varying these, we obtain known notions, such as weak head-normal and strong normal forms. We prove that our algorithm is sound. We implemented it in the Andromeda 2 proof assistant, which supports user-definable type theories. The user need only provide the equality rules they wish to use, which the algorithm automatically classifies as computation or extensionality rules, and select appropriate principal arguments.

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Andrej Bauer, Anja Petković Komel. 2021-03-12. An extensible equality checking algorithm for dependent type theories. https://doi.org/10.46298/lmcs-18(1%3A17)2022

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