**Authors:**Daniel Gratzer (Aarhus University), G. A. Kavvos (Aarhus University), Andreas Nuyts (KU Leuven), and Lars Birkedal (Aarhus University)

**Reference:**Daniel Gratzer, G. A. Kavvos, Andreas Nuyts, and Lars Birkedal. 2021. Multimodal Dependent Type Theory. In Logical Methods in Computer Science, Volume 17, Issue 3 - https://doi.org/10.46298/lmcs-17(3:11)2021

**Cited by A Categorical Normalization Proof for the Modal Lambda-Calculus, Fitch-Style Applicative Functors, and Commuting Cohesions**

Why this reference?

Why this reference?

I’ve discussed the links between types and modal logic a few times on this blog, most notably two weeks ago, when I discussed Davies and Pfenning’s addition of a certain modality into a simply typed functional language. This paper, based on the shorter conference paper of the previous year, extends the state of play by introducing a general recipe for integrating modalities, including multiple modalities at once, into dependent type theory. As long as the recipe is followed (as we will see, this is not always exactly possible) certain desirable properties such as canonicity can be guaranteed, rather than working from scratch to derive such properties every time a formally new modal type theory is proposed.

The two main technologies making this possible are Fitch-style type theory, which I discussed two weeks ago, and the

*mode theories*introduced recently by Licata and Shulman. Given how recently I’ve discussed Fitch-style types I’ll restrain myself to briefly say that these provide a setting for modal type theory that is appealingly general and close to informal modal reasoning, and that their usual formulation is modified here to push some of the complexity of the system from the modal elimination rules to the variable rules, which turns out to be beneficial due to avoidance of certain ‘trimming’ operations on contexts.

Mode theories are three layer structures organised as 2-categories (for some applications, some of these layers may be trivial). These layers are best illustrated through the example of guarded recursion, a setting in which types are regarded as unfolding over time, which is helpful for ensuring that certain recursive constructions avoid the impossible circularity of parts that are constructed in terms of other parts that are not constructed yet. The first layer of a mode theory are the

*modes*, which correspond to different dependent type theories. Often one mode suffices, but with guarded recursion it is convenient to have two, allowing the programmer to switch back and forth between regarding types as varying over time, or as lacking this property; in the latter mode, ‘vanilla’ dependent type theory suffices. The second layer are the

*modalities*, which map from mode to mode. With guarded recursion, the ‘later’ modality starts and finishes in the time-varying mode, but is meaningless in constant mode; two further modalities allow passage between the varying and constant modes. The third layer, of 2-cells, define maps between modalities, which give fine control over details such as whether an S4-like modality is idempotent.

The guarded recursion example is also helpful for observing some of the limitations of this setup. The ‘Löb induction’ property of the later modality, a technical requirement that gives guarded recursion its power and significance, cannot be expressed in a mode theory and must be imposed on top of the iteration of the general recipe. By leaving the recipe, the powerful general theorems proved about recipe-obeying type theories are no longer guaranteed to apply. More generally, the notion of mode theory as 2-category is quite abstract and it will be interesting to see development of how they are realised as syntax, and how their limitations can be understood.

Another limitation, not so significant in the case of guarded recursion but of interest elsewhere, is that every modality that fits into the framework is a ‘necessity’ or ‘box’ style of modality, commuting with products. Some modalities of interest elsewhere are the ‘possibility’ or ‘diamond’ modalities, which (usually) commute with coproducts instead. Admittedly such modalities currently have considerably less application as types that the modalities of this paper, but I am not completely clear whether that will be an enduring truth, or it is something that is true

*because*we do not have good general understanding of how to fit them into type theory.

Next week my paper-picking algorithm will see me consider a direct sequel paper to this one, so it will be interesting to see how these important and relevant ideas continue to develop.

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