# What are κappa-categories?

Hey There,

i have a simple question:

What are $\kappa$-categories?

Do you have something related for further reading?

thx and greetings,

frosch03

edit: There is actually a paper called "Closed Freyd- and kappa-categories" by A. John Power and Hayo Thielecke (portal.acm.org/citation.cfm?id=646229.681558)

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In what context?... –  Harry Gindi Aug 27 '10 at 9:37
If the $\kappa$ were a $k$, a commutative field/ring, this is the usual name for categories enriched over $k$-modules, that is, whose $\hom$-spaces are $k$-modules in such a way that composition is bilinear, &c. –  Mariano Suárez-Alvarez Aug 27 '10 at 12:05
oh, sorry, in the context of category-theory ... There is actually a paper called "Closed Freyd- and kappa-categories" by A. John Power and Hayo Thielecke (portal.acm.org/citation.cfm?id=646229.681558) I wonder if this is the only one, that mentions these kappa-categories... –  frosch03 Aug 27 '10 at 13:09
Please remember that this forum is largely aimed at researchers, and as such we hope to maintain some amount of professionalization. As such, internet slang like "thx" is not so appropriate --- in person, you would not approach a colleague (I hope) with the same amount of slang. See mathoverflow.net/faq , and in particular mathoverflow.net/faq#pants , and also mathoverflow.net/howtoask for more on how to better-ize the question in revision. –  Theo Johnson-Freyd Aug 27 '10 at 16:16
@frosch03: if you edit the question to include the reference you give in that last comment, and also to include a little more detail (eg include the definition, in case someone knows them but under a different name), then (a) you'll be much more likely to get a useful answer, and (b) no-one will mind so much if you end it with “kthxbai” or anything else. It's a good rule of thumb on MO (& similar sites) that a little effort putting details, background etc. into a question will pay off a lot in getting good answers: it makes it easier for people to answer, and shows respect for their time. –  Peter LeFanu Lumsdaine Aug 28 '10 at 15:00

The intuitive explanation is that $\kappa$-categories are to first-order functions what cartesian closed categories are to higher-order functions.
This all started with Lambek's work on polynomial categories; the best reference for that is J. Lambek. Functional completeness of cartesian categories. Annals of Mathematical Logic, 6:251–292, 1973. That paper introduced the choice of the letter $\kappa$. A polynomial category is what you get when you take a category with a terminal object, pick an object $X$, and then freely adjoin a new morphism $f:1\to X$ and close under composition. This is very much like the ring of polynomials $R[X]$ over a ring $R$ arrived at by adjoining an indeterminate element and closing under the ring operations. Lambek shows that this property can be stated in universal terms -- as the unique category admitting a particular kind of functor from the original category. This is formally more satisfying but not the best route for beginners.
Later, Hasegawa developed this idea much further in M. Hasegawa. Decomposing typed lambda calculus into a couple of categorical programming languages. Lecture Notes in Computer Science, 953, 1995. He showed that just as the $\lambda$-calculus can be used as a "syntax" for specifying morphisms in a cartesian closed category, so too can the $\kappa$-calculus -- roughly the $\lambda$-calculus without first-class functions -- be used as a "syntax" for specifying morphisms. I recommend studying the figure immediately after the first paragraph of section 3 in his paper (very carefully). It conveys both the essence of these categories and their relevance to the study of programming languages.
To wrap up, cartesian closed categories have been an immensely useful tool in understanding programming languages with first-class functions. Unfortunately they can only be used to study languages with the property that for every pair of types $B$ and $C$ there is also a type of functions from $B\Rightarrow C$ and the ways of getting a $B\Rightarrow C$ from an $A$ are in one-to-one-correspondence with the ways of getting a $C$ from a (cartesian) pair of an $A$ and a $B$. Lambek-Hasegawa $\kappa$-categories are an elegant way to extend these techniques to languages in which this assumption does not hold.