## Cartesian closed categories and the λ-calculus

#### September 28, 2006

Categorical semantics was born in Lawvere's celebrated 1963 thesis on algebraic theories:

Algebraic theories are a simple formalism for reasoning about operations that satisfy equations. For example, since the concept of a "group" involves only some operations (multiplication, inverses...) satisfying equations, this concept can be formalized using an algebraic theory called Th(Grp).

The role of semantics enters when we consider "models" of an algebraic theory. Loosely speaking, a model is just one of the things the theory seeks to describe. For example, a "model" of Th(Grp) is just a group. Technically, an algebraic theory T is a category with finite products, and a model is a functor that preserves finite products:

Z: T → Set

from T to the category of sets. The basic idea is simple: if for example T = Th(Grp), then Z maps the abstract concept of "group" to a specific set, the abstract concept of "multiplication" to a specific multiplication on the chosen set, and so on, thus picking out a specific group.

Dual to the concept of semantics is the concept of syntax, which deals with symbol manipulation. Just as semantics deals with models, syntax deals with "proofs". For example, starting from Th(Grp) we can prove consequences of the group axioms merely by juggling equations. In the case of algebraic theories, the syntax often goes by the name of universal algebra:

In fact, universal algebra was around long before Lawvere introduced algebraic theories; he just modernized it with the realization that a model was a functor - hence his thesis title, "Functorial Semantics".

The relevance of all this to computer science becomes visible when we note that a proof in Th(Grp), or indeed in any algebraic theory, can be seen as a rudimentary form of computation. The "input" of the computation is a set of assumptions, while the "output" is the equation to be proved.

Treating proofs as computations may seem strained, but it becomes less so when we move to richer formalisms which allow for more complex logical reasoning. One of the most well-known is the lambda calculus, invented by Church and Kleene in the 1930s as a model of computation. Any function computable by the lambda calculus is also computable by a Turing machine, and according to the Church-Turing thesis these are all the functions computable by any sort of systematic process. Moreover, computations in the lambda calculus can actually be seen as proofs.

The aptness of this way of thinking was brought out in Landin's classic paper:

• P. Landin, A correspondence between ALGOL 60 and Church's lambda-notation, Comm. ACM 8 (1965), 89-101, 158-165. Also available from the ACM site if you have a subscription.
This began a long and fruitful line of research - see for example this:
• H. Barendregt, The Lambda Calculus, its Syntax and Semantics, North-Holland, 1984.
The power of the lambda calculus is evident in the textbook developed for MIT's introductory course in computer science, which is available online:
It cites pioneers like Haskell Curry, and it even has a big λ on the cover! Fans of Oz and the Wizard will be pleased to hear that students call it "the wizard book", for the obvious reason. It's used at over 100 colleges and universities, and it has spawned a secret society called The Knights of the Lambda Calculus, whose emblem celebrates the ability of the λ-calculus to do recursion: In 1980, Lambek made a great discovery:
• Joachim Lambek, From lambda calculus to Cartesian closed categories, in To H. B. Curry: Essays on Combinatory Logic, Lambda Calculus and Formalism, eds. J. P. Seldin and J. Hindley, Academic Press, 1980, pp. 376-402.
He showed that just as algebraic theories can be regarded as certain special categories, so can theories formulated in the lambda calculus: to be precise, these correspond to cartesian closed categories or CCCs for short. These are categories with finite products such that the operation

a |→ x × a

has a right adjoint, the internal hom

b |→ hom(x,b)

In other words, we have a natural isomorphism

HOM(x × a, b) ≅ HOM(a, hom(x,b))

Here HOM(a,b) denotes the usual set of morphisms from one object to another, while hom(a,b) is the "internal hom", which is itself an object in our category.

If this is befuddling, take our category to be Set and take HOM = hom; then the above equation says:

"a function from x × a to b is the same as a function from x to the set of functions from a to b."

CCCs just generalize this idea to other categories! In the category Set we often write the object hom(a,b) as an exponential ba, and people also do this for other CCCs.

Lambek's discovery introduced a semantics for the lambda calculus, since it lets us to speak of models of theories formulated in the lambda calculus:

Z: T → Set

just as we could for algebraic theories. These are again just functors that preserve finite products. In computer programming, the importance of a model is that it gives a picture of what a program actually accomplishes. A model Z sends any program to an actual function between sets.

There's no way to list all the interesting references to CCCs and the λ-calculus, but here are some great online places to get started, starting out easy and working up to the harder ones:

and here's a classic:
• Joachim Lambek and Phil J. Scott, Introduction to Higher Order Categorical Logic, volume 7 of Cambridge Studies in Advanced Mathematics, Cambridge U. Press, 1986.

For more discussion of these concepts visit the n-Category Café and check out my course notes.