Arc is a dialect of the Lisp programming language developed by Paul Graham and Robert Morris.
In 2001, Paul Graham announced that he was working on a new dialect of Lisp named "Arc". Over the years since, he has written several essays describing features or goals of the language, and some internal projects at Y Combinator have been written in Arc, most notably the Hacker News web forum and news aggregator program. Arc itself is written in Racket.
In the essay Being Popular Graham describes a few of his goals for the language. While many of the goals are very general ("Arc should be hackable," "there should be good libraries"), he did give some specifics. For instance, he believes that it is important for a language to be terse:
It would not be far from the truth to say that a hacker about to write a program decides what language to use, at least subconsciously, based on the total number of characters he'll have to type. If this isn't precisely how hackers think, a language designer would do well to act as if it were.
He also stated that it is better for a language to only implement a small number of "axioms", even when that means the language may not have features that large organizations want, such as object-orientation (OO). In fact, Graham feels that OO is not useful as its methods and patterns are just "good design", and he sees the language features used to implement OO as partially mistaken. At Arc's introduction in 2008, Graham stated one of its benefits was its brevity.
A controversy among Lisp programmers is whether, and how much, the S-expressions of the language should be complemented by other forms of syntax. Graham feels that additional syntax should be used in situations where pure S-expressions would be overly verbose, saying, "I don't think we should be religiously opposed to introducing syntax into Lisp." Graham also feels that efficiency problems should be solved by giving the programmer a good profiler.
Hello world in Arc :
A program used by Paul Graham to illustrate Arc's terseness. It produces a form with one field at the url "/said". When the form is submitted, it leads to a page with a link that says "click here", which itself leads to a page with the value of the original input field.
The first publicly released version of Arc was made available on 29 January 2008, implemented on top of Racket ("PLT-Scheme" at that time). The release comes in the form of a .tar archive, containing the Racket source code for Arc. A tutorial and a discussion forum are also available. The forum uses the same program that Hacker News does, and is itself written in Arc.
Due to the slow development of the official Arc branch, some members of the Arc community started unofficial repositories with unofficial emendations, extensions and libraries. One version, Anarki, permitted anyone to submit changes to the project. The community-managed wiki is a better source of information than the official site at this point.
Rainbow is an implementation of Arc in Java.
Arcadia is an implementation of Arc in C.