Since some computer scientists use category theory, I was wondering if there are any programming languages that use it extensively.

3$\begingroup$ Strangely enough, I'm actually doing work on this subject at the moment, but for most likely a VERY different reason than why you'd have interest. I'm looking into the subject for educational reasons, trying to show that a mathematics background can be an asset in learning programming, and that computer programming can be an inroad to learning mathematics. $\endgroup$ – Michael Hoffman Nov 2 '09 at 3:00

$\begingroup$ You might like my explanation of matrix math with Python: stackoverflow.com/questions/1016284/… $\endgroup$ – Ilya Nikokoshev Nov 3 '09 at 19:59

2$\begingroup$ Haskell is a canonical example of a functional programming language, I have a reference to a "categorical programming language" below, Hagino. $\endgroup$ – Michael Hoffman Nov 2 '09 at 2:07

3$\begingroup$ Relevant: "Hask is not a category" by Andrej Bauer math.andrej.com/2016/08/06/haskisnotacategory $\endgroup$ – j.c. Mar 28 '19 at 19:44

$\begingroup$ IMHO Charity (and Squiggol) have a slightly stronger claim to being "a categorical programming language" than Haskell. $\endgroup$ – Adam Jan 18 '10 at 23:35

1$\begingroup$ Seen on the Charity website: "All Charity computations terminate (up to user input).". Is Charity Turingcomplete? $\endgroup$ – Samuele Giraudo Mar 6 '13 at 20:55

1$\begingroup$ @SamueleGiraudo Technically it is not, however you could always supply nontermination primitives using similar mechanisms as for I/o (e.g. a nontermination monad). So the language alone may not be turing complete but the overall system might be. $\endgroup$ – Dobes Vandermeer May 4 '15 at 3:29
Haskell is a purely functional language. However sideeffects are (almost by definition) difficult to incorporate into a functional language. This is an important problem since I/O is a very important sideeffect for most computer programs. Haskell's method of incorporating sideeffects is to use monads.
One of the simplest ways to get a monad is from a pair of adjoint functors.
For more on monads see:
(1) Embûches tissues blog: Monads in Mathematics 1: examples
(2) A series of lectures on youtube by TheCatsters:
Pairs of adjoint functors are fairly common and I've found they provide a useful way for seeing part of the "bigpicture" in many different branches of mathematics.
Here is one of several introductions to pairs of adjoint functors from the Concrete Nonsense blog.
ML is used in
Apparently there exists at least one "categorical programming language", namely Hagino
A Categorical Programming Language
(Hagino's Thesis)

$\begingroup$ Oh, and I'm reading both of these currently, so I can say that they are at least initially quite readable, even at my level (advanced undergraduate) $\endgroup$ – Michael Hoffman Nov 2 '09 at 3:09

1$\begingroup$ That second link is broken, at present. $\endgroup$ – David Roberts Jun 18 '15 at 11:29
All of the answers so far are based on Cartesian closed categories. There are a few languages based on Cartesian categories, which reject use of higherorder functions in the base language:
 The key example are Joseph Goguen's OBJ languages for programming with algebras, based on ordersorted Cartesian categories. Goguen's rejection of higherorder functions is explained in his Higherorder functions considered unnecessary for higherorder programming.
 The programming language Maude, based on rewriting logic, is probably the fittest successor of OBJ. Cf. Goguen's OBJ Family page, which summarises the programming languages in this family.
 The mathematical payoff to rejecting the "closed" part of Cartesian closed categories is that this opens the possibility of easier modelling of colimit constructions; I'd love to see a good reference for this point, one that explores that practical difficulties higherorder functions cause, but the general issue is easy enough to see. This has been important for approaches to programming languages that are based on various pushout constructions for specifying rewrites. Cf. Gibbons (2002) Towards a colimitbased semantics for visual programming.
CAML by definition is Categorical Abstract Machine Language,
however I am not cetain that you can say that an language explicitly uses category theory. Perhaps you are asking "Are there languages that allow Category Theory Concepts to be easily represented?" or perhaps you are asking if the compilation or interpretation of a particular programming language uses Category Theory in its implementation?
While technically, all Turingcomplete capable languages should be equivalently able to express the same set of computations, some languages do so more elegantly than others, allowing the programmer or mathematician to be more eloquent.
I would say LISP and SCHEME, even though based on lambdacalculus, are more connected to the spirit of category theory in concept. While the numbers and integers are conceptually defined as atomic and can be built up from primitives in concept and in theory; in practice, the implementations of SCHEME and LISP and (CLU) tend to take shortcuts to speed up implementation.
The hierarchical ability to pass functions and functions of functions (etc.) as firstclass parameters to functions in LISP and SCHEME let you be able to emulate the actions or morphisms of category theory better in that language than others. You just have to start from the ground up, as I have not yet seen a library or package in LISP or SCHEME for category theory.
I'd say every statically typed functional language is such. Why? There's a wellknown relation between propositions and types, or more precisely between certain theories of logic and certain type theories going by the name of CurryHoward correspondence. This relation has a less wellknown cousin known as CurryHowardLambek correspondence that extends it to categories. Thus, because every reasonable statically typed functional language is based on type theory, it has direct connection to category theory as well. You simply cannot escape it, try as you might. Bob Harper likes to call this The Holy Trinity  categories, languages and logic.
The list of such languages follows: Haskell, ML, OCaml, Idris, Coq, Agda, etc.
Of course, you have to take all of this with a grain of salt. Safe for Coq and Agda  both of which are foremost theorem provers  these languages contain many industrydriven quirks that make them less susceptible to the formal reasoning mentioned in the first paragraph.
In a bit different direction, because type theory is so expressive (for example, it can replace set theory as foundations of mathematics), it's fairly straightforward to express many algebraic structures in it. All of the above languages use this feature profoundly, with e.g. monads in Haskell playing a very prominent role.
Here's (internet archive link) another one based on Hagino's thesis, asides charity. Implemented both in Haskell and Ruby.

1$\begingroup$ The Haskell code survives on github github.com/msakai/cpl $\endgroup$ – j.c. Apr 17 '18 at 20:31
See also the answers to Resources for learning practical category theory and What is λcalculus related to?
From the Categorical abstract machine entry on wikipedia:
"The notion of categorical abstract machine, or CAM arose in the mid1980s and in computer science takes a place of a kind of theory of computation for programmers. In a theory CAM is represented by Cartesian closed category (c.c.c.) and embedded into the combinatory logic."
Caml (aka the basis for Microsoft's F#) is an acronym for Categorical Abstract Machine Language.
Other interesting reading is A Categorical Manifesto by Joseph Goguen.
His language project is the OBJ family of languages.
The U.S. government has sponsored reseach in using category theorybased languages (e.g. https://www.kestrel.edu/home/projects/) with the hope specifying systems as the composition of theories using category theory to combine and refine these theories into executable robust code. The language used on some of the projects listed at the link above use the specification language Metaslang which has a subset which is executable. One side effect of this reseach led to our supporting the development of CRYPTOL, a good example of specifying a domain specific language using the functional language Haskell. In my use of Metaslang and the development environment (emacs/Specware), it is difficult to get "normal" programmers to use algebraic specification and functional programming. Despite being some of the nicest people I have met, Haskell programmers are definitely NOT normal. (;)

$\begingroup$ Research in pure functional programming cannot have sideeffects! $\endgroup$ – Alex M. May 30 '19 at 19:09

2
See A HigherOrder Calculus for Categories by by Glynn Winskel and his student Mario Jose Cáccamo.
And also my related question.
https://www.npmjs.com/package/cte It's a javascript Library about Category Theory

2$\begingroup$ This looks like implementing categorytheoretical ideas, rather than using category theory itself (e.g. like Haskell, as described in other answers). $\endgroup$ – user44191 Mar 28 '19 at 15:16
I have no horse in this race, as I program mostly in R, but a very good friend was very enthusiastic about F sharp as a categorical language. He claimed that it was based on category theory. I would google "F sharp category theory' for more information.

2$\begingroup$ F sharp is essentially ocaml + Microsoft's .NET framework. $\endgroup$ – Najib Idrissi Mar 28 '19 at 14:57