Rename functor composition - implement monads...

In their monoidal form.

src/Cat/Category/Functor.agda

@@ -124,8 +124,9 @@ module _ {ℓ ℓ' : Level} {A B C : Category ℓ ℓ'} (F : Functor B C) (G : F
         ; isDistributive = dist
         }
 
-  _∘f_ : Functor A C
-  raw _∘f_ = _∘fr_
+  F[_∘_] _∘f_ : Functor A C
+  raw F[_∘_] = _∘fr_
+  _∘f_ = F[_∘_]
 
 -- The identity functor
 identity : ∀ {ℓ ℓ'} → {C : Category ℓ ℓ'} → Functor C C

src/Cat/Category/Monad.agda

@@ -1,8 +1,71 @@
 {-# OPTIONS --cubical #-}
 module Cat.Category.Monad where
 
+open import Agda.Primitive
+
+open import Data.Product
+
 open import Cubical
 
 open import Cat.Category
-open import Cat.Category.Functor
+open import Cat.Category.Functor as F
+open import Cat.Category.NaturalTransformation
 open import Cat.Categories.Fun
+
+-- "A monad in the monoidal form" [vlad]
+module Monoidal {ℓa ℓb : Level} (ℂ : Category ℓa ℓb) where
+  private
+    ℓ = ℓa ⊔ ℓb
+
+  open Category ℂ hiding (IsAssociative)
+  open NaturalTransformation ℂ ℂ
+  record RawMonad : Set ℓ where
+    field
+      R : Functor ℂ ℂ
+      -- pure
+      ηNat : NaturalTransformation F.identity R
+      -- (>=>)
+      μNat : NaturalTransformation F[ R ∘ R ] R
+
+    module R  = Functor R
+    module RR = Functor F[ R ∘ R ]
+    private
+      module _ {X : Object} where
+        -- module IdRX = Functor (F.identity {C = RX})
+
+        η : Transformation F.identity R
+        η = proj₁ ηNat
+        ηX  : ℂ [ X                    , R.func* X ]
+        ηX = η X
+        RηX : ℂ [ R.func* X            , R.func* (R.func* X) ] -- ℂ [ R.func* X , {!R.func* (R.func* X))!} ]
+        RηX = R.func→ ηX
+        ηRX = η (R.func* X)
+        IdRX : Arrow (R.func* X) (R.func* X)
+        IdRX = 𝟙 {R.func* X}
+
+        μ : Transformation F[ R ∘ R ] R
+        μ = proj₁ μNat
+        μX  : ℂ [ RR.func* X           , R.func* X ]
+        μX = μ X
+        RμX : ℂ [ R.func* (RR.func* X) , RR.func* X ]
+        RμX = R.func→ μX
+        μRX : ℂ [ RR.func* (R.func* X) , R.func* (R.func* X) ]
+        μRX = μ (R.func* X)
+
+        IsAssociative' : Set _
+        IsAssociative' = ℂ [ μX ∘ RμX ] ≡ ℂ [ μX ∘ μRX ]
+        IsInverse' : Set _
+        IsInverse'
+          = ℂ [ μX ∘ ηRX ] ≡ IdRX
+          × ℂ [ μX ∘ RηX ] ≡ IdRX
+
+    -- We don't want the objects to be indexes of the type, but rather just
+    -- universally quantify over *all* objects of the category.
+    IsAssociative = {X : Object} → IsAssociative' {X}
+    IsInverse = {X : Object} → IsInverse' {X}
+
+  record IsMonad (raw : RawMonad) : Set ℓ where
+    open RawMonad raw public
+    field
+      isAssociative : IsAssociative
+      isInverse : IsInverse