open import Cat.Functor.Naturality
open import Cat.Monoidal.Diagonals
open import Cat.Instances.Product
open import Cat.Monoidal.Braided
open import Cat.Monoidal.Base
open import Cat.Prelude

import Cat.Functor.Reasoning
import Cat.Reasoning

module Cat.Monoidal.Functor {oc ℓc od ℓd}
  {C : Precategory oc ℓc} (Cᵐ : Monoidal-category C)
  {D : Precategory od ℓd} (Dᵐ : Monoidal-category D)
  where

Monoidal functors🔗

open Cat.Reasoning D

private
  module C = Monoidal-category Cᵐ
  module D = Monoidal-category Dᵐ

Categorifying the fact that a morphism between monoids is expected to preserve the unit and multiplication, a functor between monoidal categories should come equipped with natural isomorphisms

witnessing that it preserves the tensor product and unit.

However, just like for lax functors between bicategories, we have the option of requiring only a natural transformation in one direction or the other. If we choose the forward direction $FA\otimes FB\to F(A\otimes B)\text{,}$ we obtain the notion of a lax monoidal functor; choosing the opposite direction instead would yield an oplax monoidal functor.

We begin by defining the structure of a lax monoidal functor on a given functor $F\text{:}$ this consists of the aforementioned morphisms, as well as some coherence conditions similar to the ones for a lax functor.

record Lax-monoidal-functor-on (F : Functor C D) : Type (oc ⊔ ℓc ⊔ od ⊔ ℓd) where
  private module F = Cat.Functor.Reasoning F

  field
    ε : Hom D.Unit (F.₀ C.Unit)
    F-mult : D.-⊗- F∘ (F F× F) => F F∘ C.-⊗-

  module φ = _=>_ F-mult

  φ : ∀ {A B} → Hom (F.₀ A D.⊗ F.₀ B) (F.₀ (A C.⊗ B))
  φ = φ.η _

  field
    F-α→ : ∀ {A B C}
      → F.₁ (C.α→ A B C) ∘ φ ∘ (φ D.⊗₁ id) ≡ φ ∘ (id D.⊗₁ φ) ∘ D.α→ _ _ _
    F-λ← : ∀ {A} → F.₁ (C.λ← {A}) ∘ φ ∘ (ε D.⊗₁ id) ≡ D.λ←
    F-ρ← : ∀ {A} → F.₁ (C.ρ← {A}) ∘ φ ∘ (id D.⊗₁ ε) ≡ D.ρ←

  F-α← : ∀ {A B C}
    → F.₁ (C.α← A B C) ∘ φ ∘ (id D.⊗₁ φ) ≡ φ ∘ (φ D.⊗₁ id) ∘ D.α← _ _ _
  F-α← = swizzle (sym (F-α→ ∙ assoc _ _ _)) (D.α≅ .invl) (F.F-map-iso C.α≅ .invr)
    ∙ sym (assoc _ _ _)

private unquoteDecl eqv = declare-record-iso eqv (quote Lax-monoidal-functor-on)
Lax-monoidal-functor-on-path
  : ∀ {F} {l l' : Lax-monoidal-functor-on F}
  → l .Lax-monoidal-functor-on.ε ≡ l' .Lax-monoidal-functor-on.ε
  → l .Lax-monoidal-functor-on.F-mult ≡ l' .Lax-monoidal-functor-on.F-mult
  → l ≡ l'
Lax-monoidal-functor-on-path p q = Iso.injective eqv
  (Σ-pathp p (Σ-prop-pathp (λ _ _ → hlevel ) q))

Lax-monoidal-functor : Type (oc ⊔ ℓc ⊔ od ⊔ ℓd)
Lax-monoidal-functor = Σ (Functor C D) Lax-monoidal-functor-on

A monoidal functor, or strong monoidal functor¹, is then simply a lax monoidal functor whose structure morphisms are invertible.

record Monoidal-functor-on (F : Functor C D) : Type (oc ⊔ ℓc ⊔ od ⊔ ℓd) where
  field
    lax : Lax-monoidal-functor-on F

  open Lax-monoidal-functor-on lax public

  field
    ε-inv : is-invertible ε
    F-mult-inv : is-invertibleⁿ F-mult

Monoidal-functor : Type (oc ⊔ ℓc ⊔ od ⊔ ℓd)
Monoidal-functor = Σ (Functor C D) Monoidal-functor-on

Braided and symmetric monoidal functors🔗

A monoidal functor between braided monoidal categories can additionally preserve the braiding in the sense that the following diagram commutes, yielding the notion of a braided monoidal functor.

module _
  (Cᵇ : Braided-monoidal Cᵐ)
  (Dᵇ : Braided-monoidal Dᵐ)
  where
  module Cᵇ = Braided-monoidal Cᵇ
  module Dᵇ = Braided-monoidal Dᵇ

  is-braided-functor : Lax-monoidal-functor → Type (oc ⊔ ℓd)
  is-braided-functor (F , lax) = ∀ {A B} → φ ∘ Dᵇ.β→ ≡ F.₁ Cᵇ.β→ ∘ φ {A} {B}
    where
      module F = Functor F
      open Lax-monoidal-functor-on lax

A symmetric monoidal functor between symmetric monoidal categories is just a braided monoidal functor, since there is no extra structure to preserve.

module _
  (Cˢ : Symmetric-monoidal Cᵐ)
  (Dˢ : Symmetric-monoidal Dᵐ)
  where
  open Symmetric-monoidal Cˢ using (Cᵇ)
  open Symmetric-monoidal Dˢ using () renaming (Cᵇ to Dᵇ)

  is-symmetric-functor : Lax-monoidal-functor → Type (oc ⊔ ℓd)
  is-symmetric-functor = is-braided-functor Cᵇ Dᵇ

Diagonal monoidal functors🔗

If the source and target categories are equipped with diagonal morphisms, then a diagonal monoidal functor, or idempotent monoidal functor is a monoidal functor that makes the following diagram commute:

module _
  (Cᵈ : Diagonals Cᵐ)
  (Dᵈ : Diagonals Dᵐ)
  where
  module Cᵈ = Diagonals Cᵈ
  module Dᵈ = Diagonals Dᵈ

  is-diagonal-functor : Lax-monoidal-functor → Type (oc ⊔ ℓd)
  is-diagonal-functor (F , lax) = ∀ {A} → φ ∘ Dᵈ.δ ≡ F.₁ (Cᵈ.δ {A})
    where
      module F = Functor F
      open Lax-monoidal-functor-on lax

The “idempotent” terminology comes from the semantics of programming languages, where lax monoidal functors are used to model certain kinds of effectful computations, as a “static” alternative to monads. In that setting, an idempotent monoidal functor (or “idempotent applicative functor”) represents an effect that can be executed multiple times with the same effect as executing it once: for example, reading from an immutable data source or throwing an exception.

Monoidal natural transformations🔗

The notion of natural transformation between functors can also be refined in the case of monoidal functors: a monoidal natural transformation $\alpha :F\Rightarrow G$ is one such that the following diagrams commute.

module _ ((F , F-monoidal) (G , G-monoidal) : Lax-monoidal-functor) where
  module FM = Lax-monoidal-functor-on F-monoidal
  module GM = Lax-monoidal-functor-on G-monoidal
  open _=>_

  record is-monoidal-transformation (α : F => G) : Type (oc ⊔ ℓc ⊔ ℓd) where
    field
      nat-ε : α .η C.Unit ∘ FM.ε ≡ GM.ε
      nat-φ : ∀ {A B} → α .η _ ∘ FM.φ {A} {B} ≡ GM.φ ∘ (α .η _ D.⊗₁ α .η _)

Note that, since monoidal categories can be thought of as one-object bicategories, we may expect to also have modifications between monoidal natural transformations, but this is not the case: the categorical ladder ends here. This is analogous to the fact that monoids only form a category and not a bicategory, even when viewed as one-object categories: there simply aren’t enough objects to have interesting 3-cells (resp. 2-cells)!