open import Cat.Monoidal.Instances.Cartesian
open import Cat.Functor.Coherence
open import Cat.Instances.Product
open import Cat.Monoidal.Strength
open import Cat.Monoidal.Braided
open import Cat.Monoidal.Functor
open import Cat.Monoidal.Reverse
open import Cat.Diagram.Monad
open import Cat.Monoidal.Base
open import Cat.Functor.Base
open import Cat.Prelude

import Cat.Functor.Reasoning
import Cat.Reasoning

module Cat.Monoidal.Strength.Monad where

Strong monads🔗

Recall that a (left) strength for an endofunctor $M:\mathcal{C}\to \mathcal{C}$ on a monoidal category consists of a natural transformation $A\otimes MB\to M(A\otimes B)$ allowing us to “slide” things from the ambient context into the functor. If $M$ is equipped with the structure of a monad, then it is natural to refine this notion to be compatible with the monad, yielding the notion of a (left/right-) strong monad.

module _ {o ℓ} {C : Precategory o ℓ} (Cᵐ : Monoidal-category C) (monad : Monad C) where
  open Cat.Reasoning C
  open Monoidal-category Cᵐ
  open Monad monad

  record Left-monad-strength : Type (o ⊔ ℓ) where
    field
      functor-strength : Left-strength Cᵐ M

    open Left-strength functor-strength public

    field
      left-strength-η : ∀ {A B} → σ ∘ (id ⊗₁ η B) ≡ η (A ⊗ B)
      left-strength-μ : ∀ {A B} → σ ∘ (id ⊗₁ μ B) ≡ μ (A ⊗ B) ∘ M₁ σ ∘ σ

  record Right-monad-strength : Type (o ⊔ ℓ) where
    field
      functor-strength : Right-strength Cᵐ M

    open Right-strength functor-strength public

    field
      right-strength-η : ∀ {A B} → τ ∘ (η A ⊗₁ id) ≡ η (A ⊗ B)
      right-strength-μ : ∀ {A B} → τ ∘ (μ A ⊗₁ id) ≡ μ (A ⊗ B) ∘ M₁ τ ∘ τ

  record Monad-strength : Type (o ⊔ ℓ) where
    field
      strength-left : Left-monad-strength
      strength-right : Right-monad-strength

    open Left-monad-strength strength-left hiding (functor-strength) public
    open Right-monad-strength strength-right hiding (functor-strength) public

    field
      strength-α→ : ∀ {A B C}
        → M₁ (α→ A B C) ∘ τ ∘ (σ ⊗₁ id) ≡ σ ∘ (id ⊗₁ τ) ∘ α→ A (M₀ B) C

Strong monads are of particular importance in the semantics of effectful programming languages: while monads are used to model effects, they do not capture the fact that monadic computations can make use of information from the context; for example, consider the following pseudo-Haskell program (in do notation, then in two possible desugared forms):

do
  a ← ma ∷ M A
  b ← mb ∷ M B
  pure (a , b)

ma >>= λ a →
  mb >>= λ b →
    pure (a , b)

join (fmap (λ a →
  fmap (λ b →
    (a , b)) mb) ma)

Notice that mb, and then a, are available under $\lambda \text{-}$abstractions: this is no problem in a functional programming language like Haskell, because monads are automatically enriched in the sense that the functorial action fmap is an internal morphism; in other words, $\mathbf{Sets}$-monads are strong. But the mathematical denotation of the above program in a general monoidal category makes crucial use of the strengths, as we will see below.

With this perspective in mind, the additional coherences imposed on a monad strength are quite natural: using the strength to slide into a “pure” computation (that is, one in the image of the unit) should yield a pure computation, and using the strength twice before multiplying should be the same as using it once after multiplying: they express a sort of “internal naturality” condition for the unit and multiplication with respect to the enrichment induced by the strength.

    functor-strength : Strength Cᵐ M
    functor-strength .Strength.strength-left = strength-left .Left-monad-strength.functor-strength
    functor-strength .Strength.strength-right = strength-right .Right-monad-strength.functor-strength
    functor-strength .Strength.strength-α→ = strength-α→

  private unquoteDecl left-eqv = declare-record-iso left-eqv (quote Left-monad-strength)
  Left-monad-strength-path
    : ∀ {a b}
    → a .Left-monad-strength.functor-strength ≡ b .Left-monad-strength.functor-strength
    → a ≡ b
  Left-monad-strength-path p = Iso.injective left-eqv (Σ-prop-path (λ _ → hlevel ) p)

  private unquoteDecl right-eqv = declare-record-iso right-eqv (quote Right-monad-strength)
  Right-monad-strength-path
    : ∀ {a b}
    → a .Right-monad-strength.functor-strength ≡ b .Right-monad-strength.functor-strength
    → a ≡ b
  Right-monad-strength-path p = Iso.injective right-eqv (Σ-prop-path (λ _ → hlevel ) p)

  private unquoteDecl strength-eqv = declare-record-iso strength-eqv (quote Monad-strength)
  Monad-strength-path
    : ∀ {a b}
    → a .Monad-strength.strength-left ≡ b .Monad-strength.strength-left
    → a .Monad-strength.strength-right ≡ b .Monad-strength.strength-right
    → a ≡ b
  Monad-strength-path p q = Iso.injective strength-eqv (Σ-pathp p (Σ-prop-pathp (λ _ _ → hlevel ) q))

Monoidal functors from strong monads🔗

module _ {o ℓ}
  {C : Precategory o ℓ} {Cᵐ : Monoidal-category C}
  {monad : Monad C}
  where
  open Cat.Reasoning C
  open Monoidal-category Cᵐ
  open Monad monad
  private
    module M = Cat.Functor.Reasoning M
  open is-iso

  module _ (s : Monad-strength Cᵐ monad) where
    open Monad-strength s
    open Lax-monoidal-functor-on

The above program wasn’t picked at random – it witnesses the common functional programming wisdom that “every monad is an applicative functor¹”, whose theoretical underpinning is that, given a strong monad $M\text{,}$ we can equip $M$ with the structure of a lax monoidal functor.

In fact, we can do so in two different ways, corresponding to sequencing the effects from left to right or from right to left:

    left-φ right-φ : -⊗- F∘ (M F× M) => M F∘ -⊗-

    left-φ = (mult ◂ -⊗-) ∘nt nat-assoc-to (M ▸ left-strength) ∘nt right-strength'
      where
        unquoteDecl right-strength' =
          cohere-into right-strength' (-⊗- F∘ (M F× M) => M F∘ -⊗- F∘ (Id F× M))
            (right-strength ◂ (Id F× M))

    right-φ = (mult ◂ -⊗-) ∘nt nat-assoc-to (M ▸ right-strength) ∘nt left-strength'
      where
        unquoteDecl left-strength' =
          cohere-into left-strength' (-⊗- F∘ (M F× M) => M F∘ -⊗- F∘ (M F× Id))
            (left-strength ◂ (M F× Id))

    is-commutative-strength : Type (o ⊔ ℓ)
    is-commutative-strength = right-φ ≡ left-φ

We now complete the definition of the left-to-right monoidal structure, which requires a bit of work. For the unit, we pick ${\eta }_1\text{,}$ the unit of the monad.

    strength→monoidal : Lax-monoidal-functor-on Cᵐ Cᵐ M
    strength→monoidal .ε = η Unit
    strength→monoidal .F-mult = left-φ

The associator coherence is witnessed by the following monstrosity commutative diagram.

    strength→monoidal .F-α→ =
      M₁ (α→ _ _ _) ∘ (μ _ ∘ M₁ σ ∘ τ) ∘ ((μ _ ∘ M₁ σ ∘ τ) ⊗₁ id)                                    ≡⟨ pulll (extendl (sym (mult.is-natural _ _ _))) ⟩≡
      (μ _ ∘ M₁ (M₁ (α→ _ _ _)) ∘ M₁ σ ∘ τ) ∘ ((μ _ ∘ M₁ σ ∘ τ) ⊗₁ id)                               ≡⟨ pullr (pullr (pullr refl)) ⟩≡
      μ _ ∘ M₁ (M₁ (α→ _ _ _)) ∘ M₁ σ ∘ τ ∘ ((μ _ ∘ M₁ σ ∘ τ) ⊗₁ id)                                 ≡⟨ refl⟩∘⟨ M.pulll left-strength-α→ ⟩≡
      μ _ ∘ M₁ (σ ∘ (id ⊗₁ σ) ∘ α→ _ _ _) ∘ τ ∘ ((μ _ ∘ M₁ σ ∘ τ) ⊗₁ id)                             ≡⟨ refl⟩∘⟨ refl⟩∘⟨ ◀.popl right-strength-μ ⟩≡
      μ _ ∘ M₁ (σ ∘ (id ⊗₁ σ) ∘ α→ _ _ _) ∘ (μ _ ∘ M₁ τ ∘ τ) ∘ ((M₁ σ ∘ τ) ⊗₁ id)                    ≡⟨ refl⟩∘⟨ refl⟩∘⟨ pullr (pullr (◀.popl (τ.is-natural _ _ _))) ⟩≡
      μ _ ∘ M₁ (σ ∘ (id ⊗₁ σ) ∘ α→ _ _ _) ∘ μ _ ∘ M₁ τ ∘ (M₁ (σ ⊗₁ id) ∘ τ) ∘ (τ ⊗₁ id)              ≡⟨ refl⟩∘⟨ M.popr (M.popr (pulll (sym (mult.is-natural _ _ _)))) ⟩≡
      μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ σ) ∘ (μ _ ∘ M₁ (M₁ (α→ _ _ _))) ∘ M₁ τ ∘ (M₁ (σ ⊗₁ id) ∘ τ) ∘ (τ ⊗₁ id) ≡⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ pullr (refl⟩∘⟨ refl⟩∘⟨ pullr refl) ⟩≡
      μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ σ) ∘ μ _ ∘ M₁ (M₁ (α→ _ _ _)) ∘ M₁ τ ∘ M₁ (σ ⊗₁ id) ∘ τ ∘ (τ ⊗₁ id)     ≡⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ M.pulll3 strength-α→ ⟩≡
      μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ σ) ∘ μ _ ∘ M₁ (σ ∘ (id ⊗₁ τ) ∘ α→ _ _ _) ∘ τ ∘ (τ ⊗₁ id)                ≡⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ M.popr (M.popr (sym right-strength-α→)) ⟩≡
      μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ σ) ∘ μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ τ) ∘ τ ∘ α→ _ _ _                           ≡˘⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ extendl (τ.is-natural _ _ _) ⟩≡˘
      μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ σ) ∘ μ _ ∘ M₁ σ ∘ τ ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _                           ≡˘⟨ refl⟩∘⟨ refl⟩∘⟨ extendl (mult.is-natural _ _ _) ⟩≡˘
      μ _ ∘ M₁ σ ∘ μ _ ∘ M₁ (M₁ (id ⊗₁ σ)) ∘ M₁ σ ∘ τ ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _                      ≡˘⟨ refl⟩∘⟨ extendl (mult.is-natural _ _ _) ⟩≡˘
      μ _ ∘ μ _ ∘ M₁ (M₁ σ) ∘ M₁ (M₁ (id ⊗₁ σ)) ∘ M₁ σ ∘ τ ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _                 ≡˘⟨ extendl mult-assoc ⟩≡˘
      μ _ ∘ M₁ (μ _) ∘ M₁ (M₁ σ) ∘ M₁ (M₁ (id ⊗₁ σ)) ∘ M₁ σ ∘ τ ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _            ≡˘⟨ refl⟩∘⟨ refl⟩∘⟨ refl⟩∘⟨ M.extendl (σ.is-natural _ _ _) ⟩≡˘
      μ _ ∘ M₁ (μ _) ∘ M₁ (M₁ σ) ∘ M₁ σ ∘ M₁ (id ⊗₁ M₁ σ) ∘ τ ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _              ≡⟨ refl⟩∘⟨ M.pulll3 (sym left-strength-μ) ⟩≡
      μ _ ∘ M₁ (σ ∘ (id ⊗₁ μ _)) ∘ M₁ (id ⊗₁ M₁ σ) ∘ τ ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _                     ≡˘⟨ refl⟩∘⟨ refl⟩∘⟨ extendl (τ.is-natural _ _ _) ⟩≡˘
      μ _ ∘ M₁ (σ ∘ (id ⊗₁ μ _)) ∘ τ ∘ (M₁ id ⊗₁ M₁ σ) ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _                     ≡⟨ refl⟩∘⟨ M.popr (extendl (sym (τ.is-natural _ _ _))) ⟩≡
      μ _ ∘ M₁ σ ∘ τ ∘ (M₁ id ⊗₁ μ _) ∘ (M₁ id ⊗₁ M₁ σ) ∘ (M₁ id ⊗₁ τ) ∘ α→ _ _ _                    ≡⟨ pushr (pushr (refl⟩∘⟨ ⊗.pulll3 ((refl⟩∘⟨ M.annihilate (idl _)) ∙ M.eliml refl ,ₚ refl))) ⟩≡
      (μ _ ∘ M₁ σ ∘ τ) ∘ (id ⊗₁ (μ _ ∘ M₁ σ ∘ τ)) ∘ α→ _ _ _                                         ∎

The unitor coherences are relatively easy to prove.

    strength→monoidal .F-λ← =
      M₁ λ← ∘ (μ _ ∘ M₁ σ ∘ τ) ∘ (η _ ⊗₁ id) ≡⟨ refl⟩∘⟨ pullr (pullr right-strength-η) ⟩≡
      M₁ λ← ∘ μ _ ∘ M₁ σ ∘ η _               ≡˘⟨ refl⟩∘⟨ refl⟩∘⟨ unit.is-natural _ _ _ ⟩≡˘
      M₁ λ← ∘ μ _ ∘ η _ ∘ σ                  ≡⟨ refl⟩∘⟨ cancell right-ident ⟩≡
      M₁ λ← ∘ σ                              ≡⟨ left-strength-λ← ⟩≡
      λ←                                     ∎
    strength→monoidal .F-ρ← =
      M₁ ρ← ∘ (μ _ ∘ M₁ σ ∘ τ) ∘ (⌜ id ⌝ ⊗₁ η _) ≡˘⟨ ap¡ M-id ⟩≡˘
      M₁ ρ← ∘ (μ _ ∘ M₁ σ ∘ τ) ∘ (M₁ id ⊗₁ η _)  ≡⟨ refl⟩∘⟨ pullr (pullr (τ.is-natural _ _ _)) ⟩≡
      M₁ ρ← ∘ μ _ ∘ M₁ σ ∘ M₁ (id ⊗₁ η _) ∘ τ    ≡⟨ refl⟩∘⟨ refl⟩∘⟨ M.pulll left-strength-η ⟩≡
      M₁ ρ← ∘ μ _ ∘ M₁ (η _) ∘ τ                 ≡⟨ refl⟩∘⟨ cancell left-ident ⟩≡
      M₁ ρ← ∘ τ                                  ≡⟨ right-strength-ρ← ⟩≡
      ρ←                                         ∎

Symmetry🔗

In a braided monoidal category, we unsurprisingly say that a monad strength is symmetric if the underlying functor strength is: a strength with this property is equivalent to the data of a left (or right) strength, with the other one obtained by the braiding.

  module _ (Cᵇ : Braided-monoidal Cᵐ) where
    is-symmetric-monad-strength : Monad-strength Cᵐ monad → Type (o ⊔ ℓ)
    is-symmetric-monad-strength s =
      is-symmetric-strength Cᵐ M Cᵇ functor-strength
      where open Monad-strength s

Duality🔗

Just as with functor strengths, the definitions of left and right monad strengths are completely dual up to reversing the tensor product.

  monad-strength^rev
    : Left-monad-strength (Cᵐ ^rev) monad ≃ Right-monad-strength Cᵐ monad
  monad-strength^rev = Iso→Equiv is where
    is : Iso _ _
    is .fst l = record
      { functor-strength = strength^rev Cᵐ M .fst functor-strength
      ; right-strength-η = left-strength-η
      ; right-strength-μ = left-strength-μ
      } where open Left-monad-strength l
    is .snd .inv r = record
      { functor-strength = Equiv.from (strength^rev Cᵐ M) functor-strength
      ; left-strength-η = right-strength-η
      ; left-strength-μ = right-strength-μ
      } where open Right-monad-strength r
    is .snd .rinv _ = Right-monad-strength-path Cᵐ monad
      (Equiv.ε (strength^rev Cᵐ M) _)
    is .snd .linv _ = Left-monad-strength-path (Cᵐ ^rev) monad
      (Equiv.η (strength^rev Cᵐ M) _)

Sets-monads are strong🔗

module _ {ℓ} (monad : Monad (Sets ℓ)) where
  open Monad monad
  open Left-monad-strength

The fact that $\mathbf{Sets}$-endofunctors are strong straightforwardly extends to the fact that $\mathbf{Sets}\text{-}$monads are strong, by naturality of the unit and multiplication.

  Sets-monad-strength : Left-monad-strength Setsₓ monad
  Sets-monad-strength .functor-strength = Sets-strength M
  Sets-monad-strength .left-strength-η = ext λ a b →
    sym (unit.is-natural _ _ (a ,_) $ₚ _)
  Sets-monad-strength .left-strength-μ = ext λ a mmb →
    sym (mult.is-natural _ _ (a ,_) $ₚ _) ∙ ap (μ _) (M-∘ _ _ $ₚ _)