A Normalizing Computation Rule for Propositional Extensionality in Higher-Order Minimal Logic
Paper in proceedings, 2018
The univalence axiom expresses the principle of extensionality for dependent type theory. How-
ever, if we simply add the univalence axiom to type theory, then we lose the property of canonicity
— that every closed term computes to a canonical form. A computation becomes ‘stuck’ when it
reaches the point that it needs to evaluate a proof term that is an application of the univalence
axiom. So we wish to find a way to compute with the univalence axiom. While this problem has
been solved with the formulation of cubical type theory, where the computations are expressed us-
ing a nominal extension of lambda-calculus, it may be interesting to explore alternative solutions,
which do not require such an extension.
As a first step, we present here a system of propositional higher-order minimal logic (PHOML).
There are three kinds of typing judgement in PHOML. There are terms which inhabit types, which
are the simple types over Ω. There are proofs which inhabit propositions, which are the terms
of type Ω. The canonical propositions are those constructed from ⊥ by implication ⊃. Thirdly,
there are paths which inhabit equations M = A N , where M and N are terms of type A. There are
two ways to prove an equality: reflexivity, and propositional extensionality — logically equivalent
propositions are equal. This system allows for some definitional equalities that are not present
in cubical type theory, namely that transport along the trivial path is identity.
We present a call-by-name reduction relation for this system, and prove that the system
satisfies canonicity: every closed typable term head-reduces to a canonical form. This work has
been formalised in Agda.