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arXiv · 2607.23671

Setoids in Intensional Type Theory

Andrew M. Pitts
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Abstract

We show that a certain notion of displayed setoid (family of setoids) in intensional type theory can be used to give a semantics for extensional type theory with universes (ETU). Safe Agda serves as a machine-checkable formalization of intentional type theory augmented with a universe closed under inductive-recursive definitions (IRU). The syntax of ETU is defined in IRU in a traditional extrinsic form, using a well-scoped locally nameless representation of its terms. Giving the semantics of ETU in terms of displayed setoids is complicated by the very limited means of expression afforded by IRU. As a corollary we obtain a proof within IRU of the consistency of ETU.

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Andrew M. Pitts. 2026-07-26. Setoids in Intensional Type Theory. https://arxiv.org/abs/2607.23671

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