\[ \require{mathtools} \let\DeclarePairedDelimiter\DeclarePairedDelimiters % MathJax typo % sup \newcommand{\op}{^\mathrm{op}} \newcommand{\inv}{^{-1}} % rm \newcommand{\Kl}{\mathrm{Kl}} % symbol \newcommand{\xto}{\xrightarrow} \newcommand{\toot}{\rightleftarrows} \newcommand{\comma}{\downarrow} \newcommand{\incl}{\hookrightarrow} \newcommand{\mono}{\rightarrowtail} \newcommand{\epi}{\twoheadrightarrow} \newcommand{\iso}{\cong} \newcommand{\nat}{\Rightarrow} \newcommand{\adj}{\dashv} % delimiter \DeclarePairedDelimiter{\parens}{(}{)} % parentheses ( ) \DeclarePairedDelimiter{\bracks}{[}{]} % brackets [ ] \DeclarePairedDelimiter{\braces}{\{}{\}} % braces { } \DeclarePairedDelimiter{\angles}{\langle}{\rangle} % angles \newcommand{\set}{\braces} % operator \DeclareMathOperator{\Obj}{Obj} \DeclareMathOperator{\Hom}{Hom} \DeclareMathOperator{\Eq}{Eq} \DeclareMathOperator{\Coeq}{Coeq} \DeclareMathOperator{\Pull}{Pull} \DeclareMathOperator{\Push}{Push} \DeclareMathOperator{\Lan}{Lan} \DeclareMathOperator{\Ran}{Ran} \DeclareMathOperator{\Lift}{Lift} \DeclareMathOperator{\Rift}{Rift} % \DeclareMathOperator{\id}{id} \DeclareMathOperator{\act}{act} \DeclareMathOperator{\colim}{colim} % exponential \DeclareMathOperator{\partf}{partial} \DeclareMathOperator{\curry}{curry} % natural number \DeclareMathOperator{\zero}{zero} \DeclareMathOperator{\successor}{succ} % list \DeclareMathOperator{\nil}{nil} \DeclareMathOperator{\cons}{cons} \newcommand{\concat}{\mathbin{{+}\mspace{-8mu}{+}}} % fold \DeclareMathOperator{\fold}{fold} \DeclareMathOperator{\map}{map} \DeclareMathOperator{\filter}{filter} % bool \DeclareMathOperator{\cond}{cond} % cd \DeclareMathOperator{\copy}{copy} \DeclareMathOperator{\del}{del} % sets \newcommand{\N}{\mathbb{N}} % natural numbers \newcommand{\Z}{\mathbb{Z}} % integers \newcommand{\Q}{\mathbb{Q}} % rational numbers \newcommand{\R}{\mathbb{R}} % real numbers % category \newcommand{\catf}[1]{{\mathbf{#1}}} \newcommand{\cA}{\catf{A}} \newcommand{\cB}{\catf{B}} \newcommand{\cC}{\catf{C}} \newcommand{\cD}{\catf{D}} \newcommand{\cE}{\catf{E}} \newcommand{\cI}{\catf{I}} \newcommand{\cJ}{\catf{J}} \newcommand{\cS}{\catf{S}} \newcommand{\cT}{\catf{T}} \newcommand{\cV}{\catf{V}} \newcommand{\cZero}{\catf{0}} \newcommand{\cOne}{\catf{1}} \newcommand{\cTwo}{\catf{2}} % \newcommand{\cArr}{\catf{Arr}} % arrow \newcommand{\cPSh}{\catf{PSh}} % presheaf \newcommand{\cCone}{\catf{Cone}} % cone \newcommand{\cCocone}{\catf{Cocone}} % cocone % \newcommand{\cSet}{\catf{Set}} % sets \newcommand{\cFinSet}{\catf{FinSet}} % finite sets \newcommand{\cRel}{\catf{Rel}} % relations \newcommand{\cPos}{\catf{Pos}} % posets \newcommand{\cMon}{\catf{Mon}} % monoids \newcommand{\cGrp}{\catf{Grp}} % groups \newcommand{\cAb}{\catf{Ab}} % Abelian groups \newcommand{\cVect}{\catf{Vect}} % vector spaces \newcommand{\cCat}{\catf{Cat}} % categories % logic \newcommand{\lfal}{\bot} \newcommand{\ltru}{\top} \]

A monoidal category is a category where morphisms can be composed in parallel.

1 Monoidal category

A monoidal category is a category \(\cC\) equipped with

monoidal multiplication: a bifunctor \(\otimes: \cC \times \cC \to \cC\)
monoidal unit: a \(\cC\)-object \(I\)
associator: a natural isomorphism \(\alpha_{A, B, C}: (A \otimes B) \otimes C \to A \otimes (B \otimes C)\)
left unitor: a natural isomorphism \(\lambda_A: I \otimes A \to A\)
right unitor: a natural isomorphism \(\rho_A: A \otimes I \to A\)

such that the triangle and pentagon diagrams commute.

A monoidal category is strict if \(\alpha\), \(\lambda\), and \(\rho\) are all identity morphisms.

1.1 Braided monoidal category

A braided monoidal category is a monoidal category \((\cC, \otimes, I, \alpha, \lambda, \rho)\) equipped with

braiding: a natural isomorphism \(\beta_{A, B}: A \otimes B \to B \otimes A\)

such that the hexagon diagram commutes.

1.2 Symmetric monoidal category

A symmetric monoidal category is a braided monoidal category \((\cC, \otimes, I, \alpha, \lambda, \rho, \beta)\) whose braiding \(\beta\) satisfies \(\beta_{B, A} \circ \beta_{A, B} = \id_{A \otimes B}\).

2 Monoidal functor

For monoidal categories \(\cC\) and \(\cD\), a lax monoidal functor is a functor \(F: \cC \to \cD\) equipped with

a natural transformation \(\mu_{A, B}: FA \otimes_\cD FB \to F(A \otimes_\cC B)\)
a \(\cD\)-morphism \(\eta: I_\cD \to FI_\cC\)

subject to

associativity: \(\mu (\id \otimes \mu) \alpha_\cD = (F\alpha_\cC) \mu (\mu \otimes \id)\)
identity: \((F \lambda_\cC) \mu (\eta \otimes \id) = \lambda_\cD\) and \((F \rho_\cC) \mu (\id \otimes \eta) = \rho_\cD\)

Type:

lax: morphisms
strong: isomorphisms
strict: identity morphisms

-- applicative function application
(<*>) :: Applicative f => f (a -> b) -> f a -> f b
(<*)  :: Applicative f => f a -> f b -> f a
 (*>) :: Applicative f => f a -> f b -> f b

2.1 Braided monoidal functor

A braided monoidal functor is a monoidal functor that is compatible with

braiding: \(\mu \beta_\cD = (F\beta_\cC) \mu\)

2.2 Symmetric monoidal functor

A symmetric monoidal functor is a braided monoidal functor between symmetric monoidal categories.

3 Monoidal natural transformation

A monoidal natural transformation \(\alpha: F \to G\) between two monoidal categories is a natural transformation that is compatible with

monoidal multiplication: \(\mu_G (\alpha \otimes \alpha) = \alpha \mu_F\)
monoidal unit: \(\alpha \eta_F = \eta_G\)

4 Monoid object

A monoid object in a monoidal category \((\cC, \otimes, I, \alpha, \lambda, \rho)\) is a \(\cC\)-object \(M\) equipped with two \(\cC\)-morphisms:

multiplication \(\mu: M \otimes M \to M\)
unit \(\eta: I \to M\)

subject to

associativity: \(\mu (\id \otimes \mu) \alpha = \mu (\mu \otimes \id)\)
identity: \(\mu (\eta \otimes \id) = \lambda\) and \(\mu (\id \otimes \eta) = \rho\)

-- mappend
(<>) :: Monoid m => m -> m -> m

A monoid homomorphism between two monoid objects \(M\) and \(N\) in \(\cC\) is a \(\cC\)-morphism \(f: M \to N\) that preserves

monoidal multiplication: \(\mu_N \circ (f \otimes f) = f \circ \mu_M\)
monoidal unit: \(f \circ \eta_M = \eta_N\)

a monoid object = a lax monoidal functor \(M: (\cOne, \otimes, *) \to (\cC, \otimes, I)\)
a monoid homomorphism = a monoidal natural transformation \(f: M \nat N\)

Examples:

monoid: \(\cSet\)
ring: \(\cAb\)
monad: \([\cC, \cC]\)

A comonoid object in \(\cC\) is a monoid object in \(\cC\op\).

comultiplication \(\Delta: M \to M \otimes M\)
counit \(\epsilon: M \to I\)

5 M-module

For a monoid object \((M, \mu, \eta)\) in a monoidal category \((\cC, \otimes, I, \alpha, \lambda, \rho)\), a (left) module over a monoid object (M-module) is an F-algebra \((A, \act_A: M \otimes A \to A)\) for the endofunctor

\[\begin{aligned} (M \otimes -): \cC &\to \cC \\ A &\mapsto M \otimes A \\ f &\mapsto \id_M \otimes f \end{aligned}\]

that is compatible with

monoid multiplication: \(\act (\id \otimes \act) \alpha = \act (\mu \otimes \id)\)
monoid unit: \(\act (\eta \otimes \id) = \lambda\)

An M-module homomorphism is an F-algebra homomorphism.

Monoidal Category