Category theory

Haskell theoretical foundations General: Mathematics - Category theory Research - Curry/Howard/Lambek Lambda calculus: Alpha conversion - Beta reduction Eta conversion - Lambda abstraction Other: Recursion - Combinatory logic Chaitin's construction - Turing machine Relational algebra

Category theory can be helpful in understanding Haskell's type system. There exists a "Haskell category", of which the objects are Haskell types, and the morphisms from types a to b are Haskell functions of type a -> b. Various other Haskell structures can be used to make it a Cartesian closed category.

The Haskell wikibooks has an introduction to Category theory, written specifically with Haskell programmers in mind.

Defintion of a category

A category ${\mathcal {C}}$consists of two collections:

Ob$({\mathcal {C}})$, the objects of ${\mathcal {C}}$

Ar$({\mathcal {C}})$, the arrows of ${\mathcal {C}}$ (which are not the same as Arrows defined in GHC)

Each arrow $f$ in Ar$({\mathcal {C}})$ has a domain, dom $f$, and a codomain, cod $f$, each chosen from Ob$({\mathcal {C}})$. The notation $f\colon A\to B$ means $f$ is an arrow with domain $A$ and codomain $B$. Further, there is a function $\circ$ called composition, such that $g\circ f$ is defined only when the codomain of $f$ is the domain of $g$, and in this case, $g\circ f$ has the domain of $f$ and the codomain of $g$.

In symbols, if $f\colon A\to B$ and $g\colon B\to C$, then $g\circ f\colon A\to C$.

Also, for each object $A$, there is an arrow $\mathrm {id} _{A}\colon A\to A$, (often simply denoted as $1$ or $\mathrm {id}$, when there is no chance of confusion).

Axioms

The following axioms must hold for ${\mathcal {C}}$ to be a category:

1. If $f\colon A\to B$ then $f\circ \mathrm {id} _{A}=\mathrm {id} _{B}\circ f=f$ (left and right identity)
2. If $f\colon A\to B$ and $g\colon B\to C$ and $h\colon C\to D$, then $h\circ (g\circ f)=(h\circ g)\circ f$ (associativity)

Examples of categories

• Set, the category of sets and set functions.
• Mon, the category of monoids and monoid morphisms.
• Monoids are themselves one-object categories.
• Grp, the category of groups and group morphisms.
• Rng, the category of rings and ring morphisms.
• Grph, the category of graphs and graph morphisms.
• Top, the category of topological spaces and continuous maps.
• Preord, the category of preorders and order preserving maps.
• CPO, the category of complete partial orders and continuous functions.
• Cat, the category of categories and functors.

• the category of data types and functions on data structures
• the category of functions and data flows (~ data flow diagram)
• the category of stateful objects and dependencies (~ object diagram)
• the category of values and value constructors
• the category of states and messages (~ state diagram)

Further definitions

With examples in Haskell at:

• Category theory/Functor
• Category theory/Natural transformation
• Category theory/Monads

Categorical programming

Catamorphisms and related concepts, categorical approach to functional programming, categorical programming. Many materials cited here refer to category theory, so as an introduction to this discipline see the #See also section.

• Erik Meijer, Maarten Fokkinga, Ross Paterson: Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire. See also related documents (in the CiteSeer page). Understanding the article does not require knowledge of category theory—the paper is self-contained with regard to understanding catamorphisms, anamorphisms and other related concepts.
• Varmo Vene and Tarmo Uustalu: Functional Programming with Apomorphisms / Corecursion
• Varmo Vene: Categorical Programming with Inductive and Coinductive Types. The book gives Haskell examples to illustrate the deep categorical theory topic.
• Tatsuya Hagino: A Categorical Programming Language
• Charity, a categorical programming language implementation.
• Deeply uncurried products, as categorists might like them article mentions a conjecture: relatedness to Combinatory logic

Haskell libraries and tools

• Category extras by David Menendez: libraries for e.g. comonads, infinite data types.

See also

• Michael Barr and Charles Wells have a paper that presents category theory from a computer-science perspective, assuming no prior knowledge of categories.
• Michael Barr and Charles Wells: Toposes, Triples and Theories. The online, freely available book is both an introductory and a detailed description of category theory. It also contains a category-theoretical description of the concept of monad (but calling it a triple instead of monad).
• A Gentle Introduction to Category Theory - the calculational approach written by Maarten M Fokkinga.
• Wikipedia has a good collection of category-theory articles, although, as is typical of Wikipedia articles, they are rather dense.