# CatsCheatSheet

This project is to contain a listing of common theorems in elementary category theory.

The repo contains other articles I've written on Category Theory; which may be read in a blog-format at: https://alhassy.github.io/blog/categories/#categorytheory

The listing sheet, as PDF, can be found here, while below is an unruly html rendition.

This reference sheet is built around the system https://awesomeopensource.com/project/alhassy/CheatSheet

# Categories

A category 𝒞 consists of a collection of “objects” (\Obj, 𝒞), a collection of “(homo)morphisms” (\Hom_𝒞(a,b)) for any (a,b : \Obj,𝒞) —also denoted “(a ,\to_𝒞, b)”—, an operation (\Id) associating a morphism (\Idₐ : \Hom(a,a)) to each object (a), and a dependently-typed “composition” operation (_∘_ : ∀{A , B , C : \Obj} → \Hom(B,C) → \Hom(A,B) → \Hom(A,C)) that is required to be associative with (\Id) as identity.

It is convenient to define a pair of operations (\src, \tgt) from morphisms to objects as follows:

\begin{flalign*} f : X \to_𝒞 Y \quad\equiv\quad \mathsf{src}; f = X ;\land; \mathsf{tgt}; f = Y && \tag*{$\src,\tgt$-Definition} \label{src-tgt-Definition} \end{flalign*}

Instead of (\Hom_𝒞) we can instead assume primitive a ternary relation (_:_→_𝒞_) and regain (\Hom_𝒞) precisely when the relation is functional in its last two arguments: \eqn{type-Unique}{f : A \to_𝒞 B ;;\land;; f : A' \to_𝒞 B' ;\implies; A=A' ;\land; B=B'} When this condition is dropped, we obtain a pre-category; e.g., the familiar Sets is a pre-category that is usually treated as a category by making morphisms contain the information about their source and target: ((A, f, B) : A → B) rather than just (f). \newline This is sometimes easier to give than Hom! C.f. Alg(F). \room

Here's an equivalence-preserving property that is useful in algebraic calculations,

Examples:

• Linear Algebra: Matrices with real number values determine a category whose objects are the natural numbers, morphisms (n → m) are (n × m) matrices, (\Id) is the identity matrix, and composition is matrix multiplication.

• Each preorder determines a category: The objects are the elements and there is a morphism (a → b) named, say, “((a, b))”, precisely when (a \leq b); composition boils down to transitivity of (\leq).

• Each monoid ((M, ⊕, e)) gives rise to a category: The objects and the arrows are both the elements of(M), and (k : m → n ;≡; k ⊕ m = n). E.g., ((ℕ, ×, 1)) gives rise to a category whose products are gcd's and so properties of products are thus gcd theorems!

• Each digraph determines a category: The objects are the nodes and the paths are the morphisms typed with their starting and ending node. Composition is catenation of paths and identity is the empty path.

• Suppose we have an interface', in the programming sense, of constant, function, and relation symbols —this is also called a signature.

Let 𝒯 be any collection of sentences in the first-order language of signature (\Sigma). Then we can define a category (\mathsf{Mod},𝒯) whose objects are implementations of interface (\Sigma) satisfying constraints 𝒯, and whose morphisms are functions that preserve the (\Sigma) structure. Ignoring constraints 𝒯 gives us functor algebras'.

Particular examples include monoids and structure-preserving maps between them; likewise digraphs, posets, rings, etc and their homomorphisms.

\room

Even when morphisms are functions, the objects need not be sets: Sometimes the objects are operations —with an appropriate definition of typing for the functions. The categories of F-algebras are an example of this.

# “Gluing” Morphisms Together

Traditional function application is replaced by the more generic concept of functional composition suggested by morphism-arrow chaining:

Whenever we have two morphisms such that the target type of one of them, say (g : B ← A) is the same as the source type of the other, say (f : C ← B) then “(f) after (g)”, their composite morphism, (f ∘ g : C ← A) can be defined. It “glues” (f) and (g) together, “sequentially”:

Composition is the basis for gluing morphisms together to build more complex morphisms. However, not every two morphisms can be glued together by composition.

\room

Types provide the interface for putting morphisms together to obtain more complex functions.

\room

A split arises wherever two morphisms do not compose but share the same source.

• Since they share the same source, their outputs can be paired: (c ↦ (f, c, g, c)).
• This duplicates the input so that the functions can be executed in “parallel” on it.

\room

A product appears when there is no explicit relationship between the types of the morphisms.

• We regard their sources as projections of a product, whence they can be seen as splits.
• This ((c, d) ↦ (f, c, g, d)) corresponds to the “parallel” application of (f) and (g), each with its own input.

\room

An either arises wherever two morphisms do not compose but share the same target.

• Apply (f) if the input is from the “(A) side” or apply (g) if it is from the “(B) side”.
• This is a “case analysis” of the input with branches being either (f) or (g).

\room

A sum appears when there is no explicit relationship between the types of the morphisms.

• We regard their targets as injections into a sum, whence they can be seen as eithers.

\room

A transpose arises when we need to combine a binary morphism with a unary morphism.

• I.e., it arises when a composition chain is interrupted by an extra product argument.
• Express (f) as a C-indexed family, (f_c : A → B), of morphisms such that applying a function at any index behaves like (f); i.e., (f_c , a = f(c, a)). Each (f_c) can now be composed with (g). Let (\transpose{(;)}) denote the operation (f ↦ f_c).

\vspace{-0.5em}

\vspace{1em}

# Functors

A functor F : 𝒜 → ℬ is a pair of mappings, denoted by one name, from the objects, and morphisms, of 𝒜 to those of ℬ such that it respects the categorical structure:

\vspace{1em}

The two axioms are equivalent to the single statement that functors distribute over finite compositions, with (\Id) being the empty composition: [ F(f_0 ∘ \cdots ∘ f_{n-1}) ;=; F, f_0 ∘ \cdots ∘ F, f_{n-1} ]

Use of Functors.

• In the definition of a category, “objects” are “just things” for which no internal structure is observable by categorical means —composition, identities, morphisms, typing. Functors form the tool to deal with “structured” objects.

Indeed in 𝒮ℯ𝓉 the aspect of a structure is that it has “constituents”, and that it is possible to apply a function to all the individual constituents; this is done by F f : F A → F B.

• For example, let (\bin A = A × A) and (\bin f = (x, y) ↦ (f, x, f, y)). So (\bin) is or represents the structure of pairs; (\bin, A) is the set of pairs of A, and (\bin, f) is the function that applies f to each constituent of a pair.

• A binary operation on A is then just a function (\bin A → A); in the same sense we obtain F-ary operations.
• Also, Seq is or represents the structure of sequences; Seq A is the structure of sequences over A, and Seq f is the function that applies f to each constituent of a sequence.

• Even though F A is still just an object, a thing with no observable internal structure, the functor properties enable to exploit the “structure” of F A by allowing us to “apply” an f to each “constituent” by using F f.

\vspace{1em}

Category (𝒜lℊ(F))

• For a functor F : 𝒜 → 𝒟, this category has F-algebras, F-ary operations in 𝒟 as, objects — i.e., objects are 𝒟-arrows (F, A → A) — and F-homomorphisms as morphisms, and it inherits composition and identities from 𝒟.

Note that category axiom \eqref{unique-Type} is not fulfilled since a function can be a homomorphism between several distinct operations. However, we pretend it is a category in the way discussed earlier, and so the carrier of an algebra is fully determined by the operation itself, so that the operation itself can be considered the algebra.

*\ref{comp-Homomorhism} renders a semantic property as a syntactic condition!*

\vspace{1em}

• A contravariant functor 𝒞 → 𝒟 is just a functor 𝒞ᵒᵖ → 𝒟.
• A bifunctor from 𝒞 to 𝒟 is just a functor 𝒞² → 𝒟.

# Naturality

A natural transformation is nothing but a structure preserving map between functors. “Structure preservation” makes sense, here, since we've seen already that a functor is, or represents, a structure that objects might have.

\room

As discussed before for the case F : 𝒞 → 𝒮ℯ𝓉, each F A denotes a structured set and F denotes the structure itself.

\hspace{-1em}: (\bin) is the structure of pairs, Seq is the structure of sequences, Seq Seq the structure of sequences of sequences, (\bin , Seq) the structure of pairs of sequences —which is naturally isomorphic to (Seq , \bin) the structure of sequences of pairs!—, and so on.

\room

A “transformation” from structure F to structure G is a family of functions \newline (η : ∀{A} → F, A → G, A); and it is “natural” if each (η_A) doesn't affect the constituents of the structured elements in F A but only reshapes the structure of the elements, from an F-structure to a G-structure.

\vspace{0em}

*Reshaping the structure by η commutes with subjecting the constituents to an arbitrary morphism.*

\vspace{-2em}

This is naturally' remembered: Morphism (η_{\tgt, f}) has type (F (\tgt, f) → G(\tgt, f)) and therefore appears at the target side of an occurrence of f; similarly (η_{\src, f}) occurs at the source side of an f. Moreover since η is a transformation from F to G, functor F occurs at the source side of an η and functor G at the target side.

\room

• One also says ηₐ is natural in its parameter a.

• If we take (G = \Id), then natural transformations are F-homomorphisms. Thus, naturality is a kind of homomorphism condition.

• Indeed, a natural transformation is a sort-of homomorphism in that the image of a morphism after reshaping is the same as the reshaping of the image.

\room

Example natural transformations

• rev : Seq →̣ Seq : [a₁, …, aₙ] ↦ [aₙ, …, a₁] reverses its argument thereby reshaping a sequence structure into a sequence structure without affecting the constituents.

• inits : Seq →̣ Seq Seq : [a₁, …, aₙ] ↦ [[], [a₁], ⋯, [a₁, …, aₙ]] yields all initial parts of its argument thereby reshaping a sequence structure into a sequence of sequences structure, not affecting the constituents of its argument.

\room

\room

Category ℱ𝓊𝓃𝒸(𝒞, 𝒟) consists of functors 𝒞 → 𝒟 as objects and natural transformations between them as arrows. The identity transformation is indeed an identity for transformation composition, which is associative.

\room

Heuristic To prove (φ = φ₁ ∘ ⋯ ∘ φₙ : F →̣ G) is a natural transformation, it suffices to show that each (φᵢ) is a natural transformation. E.g., without even knowing the definitions, naturality of tails = Seq rev ∘ inits ∘ rev can be proven —just type checking!

\iffalse

• Theorem \eqref{ntrf-Compose} renders proofs of semantic properties to be trivial type checking!
• E.g., It's trivial to prove tails = rev ﹔ inits ﹔ Seq rev is a natural transformation by type checking, but to prove the naturality equation by using the naturality equations of rev and inits —no definitions required— necessitates more writing, and worse: Actual thought!

\fi

An adjunction is a particular one-one correspondence between different kinds of morphisms in different categories.

\room

An adjunction consists of two functors (L : 𝒜 → ℬ) and (R : ℬ → 𝒜), as well as two (not necessarily natural!) transformations (η : \Id → RL) and (ε : LR → \Id) such that

\vspace{-1em}

Reading right-to-left: In the equation (ε_B ∘ L f = g) there is a unique solution to the unknown (f). Dually for the other direction.

\room

That is, each L-algebra g is uniquely determined —as an L-map followed by an ε-reduce--- by its restriction to the adjunction's unit η.

\room

A famous example is “Free ⊣ Forgetful”, e.g. to define the list datatype, for which the above becomes: Homomorphisms on lists are uniquely determined, as a map followed by a reduce, by their restriction to the singleton sequences.

\room

We may call (f) the restriction, or lowering, of (g) to the “unital case” and write (f = ⌊g⌋ = R g ∘ η_A). Also, we may call (g) the extension, or raising, of (f) to an L-homomorphism and write (g = ⌈f⌉ = ε_B ∘ L f). The above equivalence now reads:

\room \vspace{1ex} Note that ⌈ is like r' and the argument to ⌈⌉ must involve the R-ight adjoint in its type;

\room

This equivalence expresses that lad' (⌊⌋), from \emph{l}eft \emph{ad}jungate, and rad' (⌈⌉), from \emph{r}ight \emph{ad}jungate, are each other's inverses and constitute a correspondence between certain morphisms. Being a bijective pair, lad and rad are injective, surjective, and undo one another.

\room

We may think of ℬ as having all complicated problems so we abstract away some difficulties by \emph{r}aising up to a cleaner, simpler, domain via rad ⌈⌉; we then solve our problem there, then go back \emph{down} to the more complicated concrete issue via ⌊⌋, lad. \newline ( E.g., ℬ is the category of monoids, and 𝒜 is the category of sets; (L) is the list functor. )

Also,

• Left adjoints preserve colimits such as initial objects and sums.
• Right adjoints preserve limits such as terminal objects and products.

# Constant Combinators

Opposite to the identity functions which do not lose any information, we find functions which lose all, or almost all, information. Regardless of their input, the output of these functions is always the same value.

(\K : 𝒞 → ℱ𝓊𝓃𝒸(𝒟,𝒞))

• For objects (x), the constant functor'': (\K{x}, y = x) and (\K{x}, f = \Id_x) for objects (y) and morphisms (f).
• For morphisms (f), the constant natural transformation'': (\K{f} : \K{(\src f)} →̣ \K{(\tgt f)}) sending objects (y) to morphism (\K{f}, y = f).

\room Sometimes it is convenient to notate (\const{c} = \K , c) and refer to this as the everywhere c operation.

The following property defines constant functors at the pointfree level':

Constant functors force any difference in behaviour for any two functors to disappear:

Interestingly, functor composition and application are bridged explicitly by the constant functor:

# Monics and Epics

Identity functions and constant functions are limit points of the functional spectrum with respect to information preservation. All the other functions are in-between: They “lose” some information, which is regarded as uninteresting for some reason.

\room

How do functions lose information? Basically in two ways: They may be “blind” enough to confuse different inputs, by mapping them to the same output, or they may ignore values of their target. For instance, (\const{c}) confuses all inputs by mapping them all onto (c). Moreover, it ignores all values of its target apart from (c).

\room

Functions which do not confuse their inputs are called monics: They are “post-cancellable”:

Functions which do not ignore values of their target are called epics: They are “pre-cancellable”:

Intuitively, (h = k) on all points of their source precisely when they are equal on all image points of (f), since (f) being epic means it outputs all values of their source.

\room

It is easy to check that “the” identity function is monic and epic, while any constant function (\const{c}) is not monic and is only epic when its target consists only of (c).

# Isos

An arrow is an iso iff it is invertible; i.e., there is an “inverse” morphism (f\inverse) with \eqn{inverse-Char}{ f ∘ f\inverse = \Id \landS f\inverse ∘ f = \Id}

To construct (f\inverse), we begin by identifying its type which may give insight into its necessary shape' —e.g., as a sum or a product— then we pick one of these equations and try to reduce it as much as possible until we arrive at a definition of (f˘), or its components'.

• E.g., (coassocr = [\Id + \inl, \inr ∘ \inr]) of type ((A + B) + C ≅ A + (B + C)), its inverse coassocl must be of the shape ([x, [y, z]]) for unknowns (x,y,z) which can be calculated by solving the equation ([x, [y, z]] ∘ coassocr = \Id) —Do it!

\room

The following rules can be of help if (f\inverse) is found handier than isomorphism (f) in reasoning,

\begineqns

\eqn{inverse-Shunting1}{ f ∘ x = y \equivS x = f\inverse ∘ y}

\eqn{inverse-Shunting2}{ x ∘ f = y \equivS x = y ∘ f\inverse}

\endeqns

\room \room

Isos are necessarily monic and epic, but in general the other way around is not true.

\room

Isomorphisms are very important because they convert data from one “format”, say (A), to another format, say (B), without losing information. So (f) and (f˘) are faithful protocols between the two formats (A) and (B). Of course, these formats contain the same “amount” of information although the same data adopts a “different” shape in each of them. ─c.f. \nameref{SeqPair-is-Pair-Seq}.

\room

Isomorphic data domains are regarded as “abstractly” the same; then one write (A ≅ B).

Finally, note that all classes of functions referred to so far —identities, constants, epics, monics, and isos— are closed under composition.

Monics to the initial object are necessarily isos!

# Skolemisation

If a property (P) holds for precisely one class of isomorphic objects, and for any two objects in the same class there is precisely one isomorphism from one to the other, then we say that the P-object is unique up to unique isomorphism. For example, in 𝒮ℯ𝓉 the one-point set is unique up to a unique isomorphism, but the two-point set is not.

\room

For example, an object A is initial'' iff (∀ B ;•; ∃₁ f ;•; f : A → B), and such objects are unique up to unique isomorphism —prove it! The formulation of the definition is clear but it's not very well suited for algebraic manipulation.

\room

A convenient formulation is obtained by skolemisation': An assertion of the form [ ∀ x ;•; ∃₁ y ;•; R , x , y ] is equivalent to: There's a function ℱ such that [ , \hspace{13em} ∀ x, y ;•; R , x , y ;≡; y = ℱ, x \hspace{8em}\text{\sc(ℱ-Char)} ] In the former formulation it is the existential quantification “(∃ y)” inside the scope of a universal one that hinders effective calculation. In the latter formulation the existence claim is brought to a more global level: A reasoning need no longer be interrupted by the declaration and naming of the existence of a unique (y) that depends on (x); it can be denoted just (ℱ, x). As usual, the final universal quantification can be omitted, thus simplifying the formulation once more.

\room

In view of the important role of the various (y)'s, these (y)'s deserve a particular notation that triggers the reader of their particular properties. We employ bracket notation such as (⦇ x ⦈) for such (ℱ, x): An advantage of the bracket notation is that no extra parentheses are needed for composite arguments (x), which we expect to occur often.

\room

The formula characterising (ℱ) may be called ℱ-Char' and it immediately give us some results by truthifying each side, namely Self' and Id'. A bit more on the naming:

 Type Possibly non-syntactic constraint on notation being well-formed Self It, itself, is a solution Id How $$\Id$$ can be expressed using it Uniq Its problem has a unique solution Fusion How it behaves with respect to composition Composition How two instances, in full subcategories, compose

Note that the last 3 indicate how the concept interacts with the categorical structure: (=, ﹔, \Id). Also note that Self says there's at least one solution and Uniq says there is at most one solution, so together they are equivalent to ℱ-Char —however those two proofs are usually not easier nor more elegant than a proof of ℱ-Char directly.

\room

Proving ℱ-Char is straightforwardly accomplished by providing a definition for ℱ and establishing ℱ-Char —these two steps can be done in parallel! Almost every such proof has the following format, or a circular implication thereof: For arbitrary (x) and (y),

R \kern0.5ex x \kern0.5ex y \step[≡]{} ⋮ \step[≡]{} y = \text{“an expression not involving $y$”} \step[≡]{ 𝒹ℯ𝒻𝒾𝓃ℯ $ℱ \, x$ to be the right side of the previous equation } y = ℱ \kern0.5ex x

# Initiality

An object 0 is initial if there's a mapping (⦇-⦈), from objects to morphisms, such that \ref{initial-Char} holds; from which we obtain a host of useful corollaries. Alternative notations for (⦇ B ⦈) are (\text{!}_B), or (⦇0 → B⦈) to make the dependency on 0 explicit.

\vspace{1em}

These laws become much more interesting when the category is built upon another one and the typing is expressed as one or more equations in the underlying category. In particular the importance of fusion laws cannot be over-emphasised; it is proven by a strengthening step of the form (f ∘ ⦇B⦈ : 0 → C \providedS ⦇B⦈ : 0 → B \lands f : B → C).

\room

For example, it can be seen that the datatype of sequences is the' initial object in a suitable category, and the mediator (⦇-⦈) captures “definitions by induction on the structure”! Hence induction arguments can be replaced by initiality arguments! Woah!

# Colimits

Each colimit is a certain initial object, and each initial object is a certain colimit.

• A diagram in 𝒞 is a functor (D : 𝒟 → 𝒞).

• Recall \nameref{constant-combinator} yielding a functor on objects ---(\const{C}, x = C) for objects (x) and (\const{C}, f = \Id_C) for morphisms f— and a natural transformation on arrows ---(\const{g} = x \mapsto g : \const{A} →̣ \const{B}) for morphism (g : A → B).

• The category (⋁D), built upon 𝒞, has objects (γ : D →̣ \const{C}) called “co-cones”, for some object (C =: \tgt, γ), and a morphism from (γ) to (δ) is a 𝒞-morphism (x) such that (\const{x} ∘ γ = δ).

Cones' sit upright on their base, (D), on a table; CoCones' sit upright on a co-table!

• A colimit for D is an initial object in (⋁ D); which may or may not exist.

\room

Writing (-╱γ) for (⦇-⦈) and working out the definition of co-cone in terms of equations in 𝒞, we obtain: (γ : Obj(⋁D)) is a colimit for (D) if there is a mapping (-╱γ) such that ╱-Type and ╱-Char hold.

\room \room

Let (()_x : \varnothing →̣ \K{x}) be the natural transformation from the empty functor (\varnothing : \mathbf{0} \to 𝒞) to the constant functor. \vspace{-0.8em}

Cocones under (D) correspond one-to-one with arrows from its colimit: \vspace{-0.8em}

# Limits

Dually, the category (⋀D) has objects being “cones” (γ : \const{C} →̣ D) where (C =: \src, γ) is a 𝒞-object, and a morphism to (γ) from (δ) is a 𝒞-morphism (x) such that (γ ∘ \const{x} = δ). In terms of 𝒞, (γ : Obj(⋀ D)) is a limit for (D) if there is a mapping (γ╲-) such that the following ╲-Type and ╲-Char hold, from which we obtain a host of corollaries. As usual, there is the implicit well-formedness condition.

\vspace{-1em}

\newpage

# Sums

Take (D) and (𝒟) as suggested by (D𝒟 = \left( \overset{A}{•} ;;; \overset{B}{•} \right)). Then a cocone δ for (D) is a two-member family (δ = (f, g)) with (f : A → C) and (g : B → C), where (C = \tgt, δ).

\room

Let (γ=(\inl, \inr)) be a colimit for (D), let (A + B = \tgt,γ), and write ([f, g]) in-place of (γ╲(f, g)), then the ╲-laws yield: ((\inl, \inr, A+B)) form a sum of (A) and (B) if there is a mapping ([-,-]) such that \ref{[]-Type} and \ref{[]-Char} hold.

\room \room

In the pointwise setting, notice that the cancellation law serves to define the casing construct [-,-]. Then that casing is a form of conditional can be read from the characterisation. With this view, fusion, post-distributivity of composition over casing is just the usual law that function application distributes over conditionals and the casing extensionality law is the body-idempotency of conditionals.

\room

For categories in which sums exist, we define for (f : A → B) and (g : C → D),

\begineqns

\eqn{+-Definition}{ f + g = [ \inl ∘ f, \inr ∘ g] : A + C → B + D}

\eqn{Injections-Naturality}{ (f + g) ∘ \inl = f ∘ \inl \landS (f + g) ∘ \inr = \inr ∘ g }

\eqn{Extensionality}{ [h ∘ \inl , h ∘ \inr] = h}

\eqn{Absorption}{ [h, j] ∘ (f + g) = [h ∘ f, j ∘ g] }

\eqn{+-BiFunctoriality}{ \Id + \Id = \Id \landS (f + g) ∘ (h + j) = (f ∘ h) + (g ∘ j)}

\eqn{Structural Equality}{ [f,g] = [h, j] \equivS f = h \lands g = j }

\endeqns

\newpage

# Products

Take (D) and (𝒟) as suggested by (D𝒟 = \left( \overset{A}{•} ;;; \overset{B}{•} \right)). Then a cone δ for (D) is a two-member family (δ = (f, g)) with (f : C → A) and (g : C → B), where (C = \tgt, δ).

\room

Let (γ=(\fst, \snd)) be a limit for (D), let (A + B = \tgt,γ), and write (⟨f, g⟩) in-place of ((f, g)╱γ), then the ╱-laws yield: ((\fst, \snd, A × B)) form a product of A and B if there is an operation (⟨-,-⟩) satisfying the Char and Type laws below; from which we obtain a host of corollaries.

\room \room

The characterisation says that the essential properties of ordered pairs is that their components are retrievable and they are completely determined by their components.

\room

Notice that the cancellation rule is essentially the definitions of projections in the pointwise setting; likewise absorption is akin to the pointwise definition of the product bi-map.

\room

The fusion laws give us a pointfree rendition of their usual pointwise definitions: All applications have been lifted to compositions!

\room

For categories in which products exist, we define for (f : A → B) and (g : C → D),

\begineqns

\eqn{x-Definition}{ f × g = ⟨ f ∘ \fst, g ∘ \snd ⟩ : A × C → B × D}

\eqn{Projections-Naturality}{ \fst ∘ (f × g) = f ∘ \fst \landS \snd ∘ (f × g) = g ∘ \snd }

\eqn{Extensionality}{ ⟨\fst ∘ h, \snd ∘ h⟩ = h}

\eqn{Absorption}{ (f × g) ∘ ⟨h, j⟩ = ⟨f ∘ h, g ∘ j⟩ }

\eqn{x-BiFunctoriality}{ \Id × \Id = \Id \landS (f × g) ∘ (h × j) = (f ∘ h) × (g ∘ j)}

\eqn{Structural Equality}{ ⟨f,g⟩ = ⟨h, j⟩ \equivS f = h \lands g = j }

\endeqns

# Finitary Sums and Products

All properties studied for binary splits and binary eithers extend to the finitary case. For the particular situation (n = 1), we will have (⟨f⟩=[f]=f) and (\inl = \fst = \Id), of course.

\room

For the particular situation (n = 0), finitary products “degenerate” to terminal object 1 and finitary sums “degenerate” to initial object 0. The standard notation for the empty split (⟨⟩) is (!_C), where (C) is the source. Dually, the standard notation for the empty either ([]) is (?_C).

\eqn{Empty Exchange Rule}{ ⟨⟩_0 = []_1 }

# Mixing products and coproducts

Any (f : A + B → C × D) can be expressed alternatively as an either or as a split. It turns out that both formats are identical:

\vspace{-1.5em} \eqn{Exchange Rule}{ ⟨[f,g], [h,j]⟩ = [⟨f,h⟩,⟨g,j⟩] }

E.g., (\mathsf{undistr} = ⟨[\fst, \fst], \snd + \snd⟩ = [\Id × \inl, \Id × \inr] : (A × B) + (A × C) → A × (B + C)).

\begineqns

\eqn{Cool-Property}{ [f × g, h × k] ;=; ⟨ [f, h] ∘ (\fst + \fst), [g, k] ∘ (\snd + \snd)⟩ }

\eqn{Co-cool-Property}{ ⟨f + g, h + k⟩ ;=; [ (\inl × \inl) ∘ ⟨f, h⟩, (\inr × \inr) ∘ ⟨g, k⟩] }

\endeqns

\room

Also, since constants ignore their inputs,

# References

\vspace{1em}

An excellent introduction to category theory with examples motivated from programming, in-particular working with sequences. All steps are shown in a calculational style —which Fokkinga has made available for use with LaTeX— thereby making it suitable for self-study.

\vspace{1em}

Clear, concise, and an illuminating read.

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I've deviated from his exposition by using backwards composition ∘' rather than diagrammatic composition ;', as such my limit notation is his colimit notation! Be careful.

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I've also consulted the delightful read Program Design by Calculation of José Oliveira.

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Very accessible for anyone who wants an introduction to functional programming! The category theory is mostly implicit, but presented elegantly!

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Monoidal:

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# Monoidal and Closed Categories

It is rather common that we have a notion of pairing for types for which there is a unit type. Examples include products with the initial object, sums with the terminal object, or for the category of endofunctors: Functor composition with the identity functor.

\room

A monoidal category (𝒞, ⊗, I, α, λ, ρ) consists of a category 𝒞 with bifunctor \newline (_{}⊗_{} : 𝒞^2 → 𝒞) and object I : Obj 𝒞 —referred to as the ‘tensor product’ and ‘tensor unit— and three natural isomorphisms: \newline The ‘(right-to-left) associator’ αA, B, C : A ⊗ (B ⊗ C) ≅ (A ⊗ B) ⊗ C and \newline the ‘unitors’ λA : I ⊗ A ≅ A and ρA : A ⊗ I ≅ A such that:

1. The order of re-parensization, outer-most or inner-most first, does not matter; i.e., the two obvious maps witnessing (A ⊗ (B ⊗ (C ⊗ D)) → ((A ⊗ B) ⊗ C) ⊗ D) are identical: (α_{A ⊗ B, C, D} ;∘; α_{A, B, C ⊗ D} \eqs α_{A, B, C} ⊗ \Id_D ;∘; α_{A, B ⊗ C, D};∘; \Id_A ⊗ α_{B, C, D}).

2. Unit elimination paths are the same even if unnecessary associtivity is performed; i.e., the two obvious maps witnessing (A ⊗ (I ⊗ B) → A ⊗ B) are identical: \newline (\Id_A ⊗ λ_B \eqs (ρ_A ⊗ \Id_B) ;∘; α_{A, I, B}).

Mnemonic: λ, ‘L’ambda, is for ‘L’eft unitor; ρ, ‘R’ho, is for ‘R’ight unitor.

\room

Unfolding some of that up yields:

• (\Id ⊕ \Id = \Id) and ((f ∘ g) ⊗ (h ∘ k) = (f ⊗ h) ∘ (g ⊗ k))
• (α ∘ (f ⊗ (g ⊗ h)) = ((f ⊗ g) ⊗ h) ∘ α)
• (λ ∘ (\Id ⊗ f) = f ∘ λ)
• (ρ ∘ (f ⊗ \Id) = f ∘ ρ)

Mac Lane's coherence theorem: Any well-typed diagram built from ⊗, α, λ, ρ commutes.

\room

\eqn{Unit-Equivalence-Left}{ \Id ⊗ f = \Id ⊗ g \equivS f = g } \eqn{Unit-Equivalence-Right}{ f ⊗ \Id = g ⊗ \Id \equivS f = g }

\room

The apparent complexity of the definition of monoidal categories vanishes when a geometrical notation is used —the coherence laws simply become expected geometric operations on the plane. The geometric interpretation is sound and complete —i.e., equal morphisms yield ‘equal’ pictures, and conversely.

\room

Examples

• Common examples include preordered monoids thought of as monoidal categories.
• Functor categories (𝒞^𝒞) with tensor being functor composition.
• Any category with finite co/products is monoidal using sums or products; e.g., 𝒮ℯ𝓉.
• ℛℯ𝓁 with Cartesian product is monoidal, even though this is not a categorical product.
• The ‘free strict monoidal category’ on 𝒞 has objects being finite lists of 𝒞-objects where an arrow exists only between equal length lists and it is a list of 𝒞-morphisms between the corresponding components; tensor is catenation with the empty list as the unit object.
• Symmetric monoidal categories are closed under products, and this has a right adjoint yielding functor categories of sym. mon. cats.
• (Mat(K), ⊗, 1): The category whose objects are natural numbers and whose arrows M : m → n are n × m matrices taking values in field K. Composition is matrix multiplication, the monoidal

product is multiplication of natural numbers (on objects) and the Kronecker product of matrices (on arrows). This category is essentially the category of finite-dimensional vector spaces over K, with a chosen basis for all of its objects.

\room

Interestingly, tensor distributes over sums: (A + B) ⊗ C ≅ (A ⊗ C) + (B ⊗ C).

\room

A lax monoidal functor (F : 𝒱 ⟶ 𝒱′) is a functor that sub-factors over product: (F , x_0 ⊗ ⋯ ⊗ F, x_n ⟶̇ F(x₀ ⊗ ⋯ ⊗ x_n)).

\room

\room A Cartesian-closed category is a monoidal category where the tensor is categorical product and all exponentials exist. These categories are in correspondence with the models of simply typed lambda-calculus. If it has all finite sums as well, then it's known as bicartesian closed, in which case products necessarily distribute over sums.

𝒱 is semicartesian if any of the following equivalent statements is true.

1. Unit object (I) is terminal; in which case one says 𝒱 is semicartesian.
2. It has a natural ‘deletion’ (‼_X : X ⟶ I) with (‼_I = \Id_I).
3. It has natural ‘projections’ (πᵢ : X₁ ⊗ X₂ ⟶ Xᵢ) with (π₁ : I ⊗ I ≅ I : λ˘).

If in addition it is symmetric with (natural involution) (σ : X ⊗ Y ⟶ Y ⊗ X) and has a natural ‘diagonal’ (Δ_X : X ⟶ X ⊗ X) such that the obvious maps (X ⟶ X) coincide — i.e., (λ ∘ (! ⊗ Id) ∘ Δ = Id = ρ ∘ (Id ⊗ !) ∘ Δ), “duplicating data, then deleting a copy, is the same as doing nothing”— then it is necessairly cartesian!

\room

An exponential for Y is characterised by the following adjoint isomorphism that is natural in Y and Z:

\room

• Note that ⌊_⌋ generalises currying, and ⌈_⌉ generalises uncurrying.
• The counit (\eval_Z = ⌈\Id_{Y ➩ z}⌉ : (Y ➩ Z) ⊗ Y → Z) is called the evaluation morphism.

\room When exponentials always exists, one refers to (_{}➩_{} : 𝒞 × 𝒞^{op} → 𝒞) as the internal hom and says (𝒞, ⊗, I, α, λ, ρ, ➩) is a closed monoidal category.

\room In the cartesian case, the entire collection of morphisms (X → Y) is encoded by the single object (X ➩ Y). That is, (X → Y \quad≅\quad 1 → (X ➩ Y)) in 𝒮ℯ𝓉.

\room

\room More generally, a closed category is a category 𝒞 with a bifunctor (_{}➩_{}) and two ‘coherent’ transformations as above.

\room

It is common to notate (X ➩ Y, ⌊f⌋) by (Y^X, \transpose{f}).

# Enrichment & Internal Algebraic Structures

A Category 𝒳 enriched in a monoidal category 𝒱 or a 𝒱-category is essentially a category but its hom-types are objects of 𝒱. Formally, there is a collection Obj 𝒳 and for each pair A, B of such ‘objects’ there is a ‘hom-object’ 𝒳(A, B)` in 𝒱, and there are two 𝒱-morphisms:

1. Composition: (μ_{A, B, C} : 𝒳(B, C) ⊗ 𝒳(A, B) ⟶ 𝒳(A, C))
• Associativity: The two obvious ways ((𝒳(C, D) ⊗ 𝒳(B, C)) ⊗ 𝒳(A, B) ⟶ 𝒳(A, D)) coincide.
2. Identities: (η_A : I ⟶ 𝒳(A, A)).
• Unity: The obvious maps (I ⊗ 𝒳(A, B) ⟶ 𝒳(A, B)) coincide, as do the obvious maps (𝒳(A, B) ⊗ I ⟶ 𝒳(A, B)).

\room A usual category is just a 𝒮ℯ𝓉-category.

\room A monoid in 𝒱 is an object (M) along with two morphisms (μ : M ⊗ M ⟶ M, η : I ⟶ M) such that the former is associative and has the latter as unit. Notice that monoids in 𝒱 are ‘untyped’ analogues of 𝒱-categories. A ‘monad’ is a monoid in a category of endofunctors with composition as tensor.

• If M is a monoid in 𝒱 and F : 𝒱 ⟶ 𝒲 is a monoidal functor, then F M is a monoid in 𝒲. Woah!

\room In a monoidal category with natural transformations ‘discard’ (!_X : X ⟶̇ I) and ‘duplicate’ (Δ_X : X ⟶̇ X ⊗ X), such as Cartesian monoidal categories, a group is like a monoid but with an additional morphism (i : M ⟶ M) such that the inverse axioms hold; e.g., (e = x · x^{-1}) takes the point-free shape (η ∘ !_M = μ ∘ (\Id ⊗ i) ∘ Δ). In an arbitrary monoidal category, a Hopf algebra is like a group where (Δ_M) and (!_M) exist for our specific (M). These generalise groups.

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