Multimodal Dependent Type Theory Daniel Gratzer 0 Alex Kavvos 0 - - PowerPoint PPT Presentation

Multimodal Dependent Type Theory

Daniel Gratzer0 Alex Kavvos0 Andreas Nuyts1 Lars Birkedal0 ICMS 2020

0Aarhus University 1imec-DistriNet, KU Leuven

The problem

We’d like extend Martin-L¨

• f Type Theory and apply it to new situations.
• Staged programming [PD01].
• Proof-irrelevance [Pfe01].
• Guarded recursion [Clo+15; BGM17; Gua18].
• Parametric quantification [ND18].
• Exotic models of computation [Bir00].
• (Abstract) topology [Shu18].
• Differential geometry [Wel18].

Let’s extend MLTT to these situations...

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The problem

We’d like extend Martin-L¨

• f Type Theory and apply it to new situations.
• Staged programming [PD01].
• Proof-irrelevance [Pfe01].
• Guarded recursion [Clo+15; BGM17; Gua18].
• Parametric quantification [ND18].
• Exotic models of computation [Bir00].
• (Abstract) topology [Shu18].
• Differential geometry [Wel18].

Let’s extend MLTT to these situations... and use modalities to tame these extensions.

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A tangent: what exactly is a modality?

In general, people use modality to mean many different things:

• 1. Any unary type constructor.
• 2. A unary type constructor which is an internal functor.
• 3. A unary type constructor equipped with a monad structure.

For our work, a modality is essentially a right adjoint.1 This restriction yields a practical syntax and still includes many examples.

1More specifically, a modality is essentially a dependent right adjoint [Bir+20]

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A story of several type theories

Let us consider a representative example of how modal type theories are developed.

• 1. Work on guarded recursion converges towards the Fitch-style [BGM17; Clo18].
• 2. Birkedal et al. [Bir+20] isolate this into paradigmatic type theory.
• 3. Gratzer, Sterling, and Birkedal [GSB19] prove normalization for a similar system.

Each of these type theories build upon each other... but no reuse is possible.

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Our Contribution: MTT

We introduce MTT: a type theory parameterized by a collection of modalities.

• MTT features usual connectives of Martin-L¨
• f Type Theory, including a universe.
• The user can instantiate MTT with different collections of modalities.
• Important results such as canonicity are proven irrespective of the modalities.

We have applied MTT to several different situations:

• Axiomatic cohesion [Shu18]
• Parametricity [ND18]
• Guarded recursion [Biz+16]
• And many more...

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Modes and mode theories

MTT is a multimode type theory, not just multimodal.

• Each mode is its own separate type theory, with modalities bridging between them.
• As an example, spatial type theory has two modes: sets and spaces.

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Modes and mode theories

MTT is a multimode type theory, not just multimodal.

• Each mode is its own separate type theory, with modalities bridging between them.
• As an example, spatial type theory has two modes: sets and spaces.

We follow [LS16] and specify our modalities as a mode theory, a (strict) 2-category:

• bject ∼ mode

morphism ∼ modality 2-cell ∼ coercion (a natural map between modalities)

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An example: an idempotent comonad

The mode theory for an idempotent comonad is generated from the following data:

• bjects: {m}

morphisms: {µ : m → m} 2-cells: {ǫ : µ ⇒ 1} Furthermore, µ ◦ µ = µ and that α = β for any pair of 2-cells.

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An example: an idempotent comonad

The mode theory for an idempotent comonad is generated from the following data:

• bjects: {m}

morphisms: {µ : m → m} 2-cells: {ǫ : µ ⇒ 1} Furthermore, µ ◦ µ = µ and that α = β for any pair of 2-cells. This induces a single modality µ | − with the following coercions: Γ ⊢ M : µ | A @ m Γ ⊢ extract(M) : A @ m Γ ⊢ M : µ | A @ m Γ ⊢ duplicate(M) : µ | µ | A @ m These satisfy the comonad laws.

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An example: an idempotent comonad

The mode theory for an idempotent comonad is generated from the following data:

• bjects: {m}

morphisms: {µ : m → m} 2-cells: {ǫ : µ ⇒ 1} Furthermore, µ ◦ µ = µ and that α = β for any pair of 2-cells. This induces a single modality µ | − with the following coercions: Γ ⊢ M : µ | A @ m Γ ⊢ extract(M) : A @ m Γ ⊢ M : µ | A @ m Γ ⊢ duplicate(M) : µ | µ | A @ m A mode annotation. These satisfy the comonad laws.

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MTT, more formally

We will now introduce MTT a bit more carefully. Let us fix a mode theory M. MTT is stratified into the following judgments: Γ ctx @ m Γ ⊢ A type @ m Γ ⊢ M : A @ m Each judgment is localized to a mode and each mode contains a copy of MLTT.

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Modal types

Slogan: modalities act like functors between modes. Given a closed type A @ n and µ : n → m, there is a closed type µ | A @ m.

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Modal types

Slogan: modalities act like functors between modes. Given a closed type A @ n and µ : n → m, there is a closed type µ | A @ m. This doesn’t easily scale to open types: Γ ⊢ A type @ n µ : n → m Γ ⊢ µ | A type @ m One of these must live in the wrong mode.

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Modal types

Slogan: modalities act like functors between modes. Given a closed type A @ n and µ : n → m, there is a closed type µ | A @ m. This doesn’t easily scale to open types: Γ ⊢ A type @ n µ : n → m Γ ⊢ µ | A type @ m One of these must live in the wrong mode. We require additional judgmental structure to make sense of modal types.

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Fitch-style contexts

MTT uses a Fitch-style context so modalities to have an action on contexts: µ : n → m Γ ctx @ m Γ, µ ctx @ n Intuition: −, µ ⊣ µ | −

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Fitch-style contexts

MTT uses a Fitch-style context so modalities to have an action on contexts: µ : n → m Γ ctx @ m Γ, µ ctx @ n Intuition: −, µ ⊣ µ | − Accordingly, the introduction and formation rules are transposition: µ : n → m Γ, µ ⊢ M : A @ n Γ ⊢ modµ(M) : µ | A @ m Follows the Fitch-style [BGM17; Clo18; Bir+20; GSB19] (whence the notation!)

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Fitch-style contexts with multiple modalities

Prior work had one modality, hence one lock. How do we scale to many modalities? Γ ctx @ m Γ = Γ, 1 ctx @ m ν : o → n µ : n → m Γ ctx @ m Γ, µ, ν = Γ, µ◦ν ctx @ o

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Fitch-style contexts with multiple modalities

Prior work had one modality, hence one lock. How do we scale to many modalities? Γ ctx @ m Γ = Γ, 1 ctx @ m ν : o → n µ : n → m Γ ctx @ m Γ, µ, ν = Γ, µ◦ν ctx @ o In fact, is part of a 2-functor from Mcoop to contexts giving an admissible rule µ, ν : n → m α : µ ⇒ ν Γ, ν ⊢ M : A @ n Γ, µ ⊢ Mα : Aα @ n (NB: −α must be applied to both the term and the type.)

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What about variables?

Locks allow us to state the formation rule for modalities, but what about variables? With the standard variable rule, we again have a mode error! µ : n → m x : A, µ ⊢ x : A @ n

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What about variables?

Locks allow us to state the formation rule for modalities, but what about variables? With the standard variable rule, we again have a mode error! µ : n → m x : A, µ ⊢ x : A @ n A must live in mode m A must live in mode n

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What about variables?

Locks allow us to state the formation rule for modalities, but what about variables? With the standard variable rule, we again have a mode error! µ : n → m x : A, µ ⊢ x : A @ n

• Previous Fitch-style type theories handled this through an elimination rule.
• In MTT, we will introduce a final piece of judgmental structure.

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Variable annotations

In addition to locks, each variable in the context will be annotated with a modality. µ : n → m Γ ctx @ m Γ, µ ⊢ A type @ n Γ, x : (µ | A) ctx @ m Intuition: Γ, x : (µ | A) is roughly Γ, x : µ | A.

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Variable annotations

In addition to locks, each variable in the context will be annotated with a modality. µ : n → m Γ ctx @ m Γ, µ ⊢ A type @ n Γ, x : (µ | A) ctx @ m Intuition: Γ, x : (µ | A) is roughly Γ, x : µ | A. µ : n → m Γ, x : (µ | A), µ ⊢ x : A @ n Γ ⊢ M0 : µ | A @ m Γ, y : (µ | A) ⊢ M1 : B[modµ(y)/x] @ m Γ ⊢ let modµ(y) ← M0 in M1 : B[M0/x] @ m

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Variable annotations

In addition to locks, each variable in the context will be annotated with a modality. µ : n → m Γ ctx @ m Γ, µ ⊢ A type @ n Γ, x : (µ | A) ctx @ m Intuition: Γ, x : (µ | A) is roughly Γ, x : µ | A. µ : n → m Γ, x : (µ | A), µ ⊢ x : A @ n Γ ⊢ M0 : µ | A @ m Γ, y : (µ | A) ⊢ M1 : B[modµ(y)/x] @ m Γ ⊢ let modµ(y) ← M0 in M1 : B[M0/x] @ m Counit of −, µ ⊣ µ | −

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Variable annotations

In addition to locks, each variable in the context will be annotated with a modality. µ : n → m Γ ctx @ m Γ, µ ⊢ A type @ n Γ, x : (µ | A) ctx @ m Intuition: Γ, x : (µ | A) is roughly Γ, x : µ | A. µ : n → m Γ, x : (µ | A), µ ⊢ x : A @ n Γ ⊢ M0 : µ | A @ m Γ, y : (µ | A) ⊢ M1 : B[modµ(y)/x] @ m Γ ⊢ let modµ(y) ← M0 in M1 : B[M0/x] @ m Shift from Γ, x : µ | A to Γ, x : (µ | A)

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Variable annotations

In addition to locks, each variable in the context will be annotated with a modality. µ : n → m Γ ctx @ m Γ, µ ⊢ A type @ n Γ, x : (µ | A) ctx @ m Intuition: Γ, x : (µ | A) is roughly Γ, x : µ | A. µ, ν : n → m α : µ → ν Γ, x : (µ | A), ν ⊢ xα : Aα @ n Γ ⊢ M0 : µ | A @ m Γ, y : (µ | A) ⊢ M1 : B[modµ(y)/x] @ m Γ ⊢ let modµ(y) ← M0 in M1 : B[M0/x] @ m

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Taking stock of MTT

It’s easy to feel this is just “one damn rule after another”, but at a high-level: a modality for each morphism in M a coercion for each 2-cell in M modal introduction needs richer contexts we equip contexts with locks and make modalities ‘right adjoints’ we must factor locks into the variable rule we equip variables with modalities and occurences with 2-cells there is now a mismatch between modal annotations and types modal elimination moves from a modal type to an annotation

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Modal combinators

The mode theory is reflected into MTT as a series of modal combinators: 1 | A ≃ A µ | ν | A ≃ µ ◦ ν | A µ | A → ν | A (For each α : µ ⇒ ν) µ | A → B → (µ | A → µ | B) All of these follow from the 2-functoriality of .

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Example definitions of modal combinators

To get a feel for MTT, let us define some of these combinators. Programs coe[α : µ ⇒ ν](−) : µ | A → ν | Aα coe[α](x) Holes x : (1 | µ | A) ⊢ : ν | A

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Example definitions of modal combinators

To get a feel for MTT, let us define some of these combinators. Programs coe[α : µ ⇒ ν](−) : µ | A → ν | Aα coe[α](x) let modµ(y) ← x in Holes x : (1 | µ | A), y : (µ | A) ⊢ : ν | A

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Example definitions of modal combinators

To get a feel for MTT, let us define some of these combinators. Programs coe[α : µ ⇒ ν](−) : µ | A → ν | Aα coe[α](x) letν modµ(y) ← x in modν() Holes x : (1 | µ | A), y : (µ | A), ν ⊢ : A

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Example definitions of modal combinators

To get a feel for MTT, let us define some of these combinators. Programs coe[α : µ ⇒ ν](−) : µ | A → ν | Aα coe[α](x) letν modµ(y) ← x in modν(yα) Holes

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Results about MTT

A major strength of MTT is that we can prove theorems irrespective of M. Theorem (Consistency) There is no term · ⊢ M : IdB(tt, ff) @ m. Theorem (Canonicity) Subject to a technical restriction, if · ⊢ M : A @ m is a closed term, then the following conditions hold:

• If A = B, then · ⊢ M = tt : B @ m or · ⊢ M = ff : B @ m.
• If A = IdA0(N0, N1) then · ⊢ M = refl(N0) : IdA0(N0, N1) @ m.
• If A = µ | A0 then · ⊢ M = modµ(N) : µ | A0 @ m for some N.

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Example: axiomatic cohesion

The other major strength of MTT is that we can use it to model interesting examples!

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Axiomatic cohesion: a brief introduction

Axiomatic cohesion uses a handful of modalities to relate spaces and sets

• 1. Semantically, we demand two categories and a collection of functors between

them: E B ⊣ ⊣ ⊣ Γ ∆ Π0 ∇ (Where Π0 preserves finite limits and ∆ and ∇ are full and faithful)

• 2. Type-theoretic approaches have considered only E and the monad induced by

♯ = ∇ ◦ Γ and its left adjoint, the comonad ♭ = ∆ ◦ Γ [Shu18].

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s Spaces. Sets.

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s κ γ δ

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s κ γ δ ∆A δ | A ΓA γ | A ∇A κ | A

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s κ γ δ ∆A δ | A ΓA γ | A ∇A κ | A ǫ0 : δ ◦ γ ⇒ 1 η0 : 1 ⇒ γ ◦ δ ǫ1 : γ ◦ κ ⇒ 1 η1 : 1 ⇒ κ ◦ γ ... triangle equalities

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s κ γ δ ∆A δ | A ΓA γ | A ∇A κ | A ǫ0 : δ ◦ γ ⇒ 1 η0 : 1 ⇒ γ ◦ δ ǫ1 : γ ◦ κ ⇒ 1 η1 : 1 ⇒ κ ◦ γ ... triangle equalities ... force η0, ǫ1 to be isos

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Splitting axiomatic cohesion into 2 modes

It’s more natural with MTT to work with two modes directly. t s κ γ δ ∆A δ | A ΓA γ | A ∇A κ | A ǫ0 : δ ◦ γ ⇒ 1 η0 : 1 ⇒ γ ◦ δ ǫ1 : γ ◦ κ ⇒ 1 η1 : 1 ⇒ κ ◦ γ ... triangle equalities ... force η0, ǫ1 to be isos We cannot capture the leftmost adjoint: it’s not a dependent right adjoint.

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Modal operations

Instantiating MTT gives us a workable cohesive type theory [Shu18]:

• We can derive transposition-type formulas.

(Γ(∆A → B)) → (Aη0 → ∆B)

• Various crisp induction principles are provable.

Γ(IdA(M, N)) ≃ IdΓ(A)(modγ(M), modγ(N))

• And (with funext) we can prove something akin to full and faithfulness.

(A → B) ≃ Γ(∆A → ∆B) Detailed proofs of these results (and more!) are given in the technical report.

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Conclusions

We introduce MTT: a type theory parameterized by a collection of modalities.

• MTT features usual connectives of Martin-L¨
• f Type Theory, including a universe.
• The user can instantiate MTT with different collections of modalities.
• Important results such as canonicity are proven irrespective of the modalities.

We have applied MTT to several different situations:

• Axiomatic cohesion
• Degrees of relatedness
• Guarded recursion and warps
• Various classic modal type theories

https://jozefg.github.io/papers/multimodal-dependent-type-theory.pdf https://jozefg.github.io/papers/type-theory-a-la-mode.pdf

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