1 The Extensible Language

Is Lisp AI?

Not long ago, if you asked what Lisp was for, many people would have answered “for artificial intelligence.” In fact, the association between Lisp and AI is just an accident of history.

Lisp was invented by John McCarthy, who also invented the term “artificial intelligence.” His students and colleagues wrote their programs in Lisp, and so it began to be spoken of as an AI language. This line was taken up and repeated so often during the brief AI boom in the 1980s that it became almost an institution.

Word has begun to spread that AI is not what Lisp is all about. Recent advances in hardware and software have made Lisp commercially viable: it is now used in

• Gnu Emacs
• Autocad
• Interleaf

What is Lisp Best At?

If Lisp is not the language of AI, what is it? Instead of judging Lisp by the company it keeps, let’s look at the language itself. What can you do in Lisp that you can’t do in other languages?

One of the most distinctive qualities of Lisp is the way it can be tailored to suit the program being written in it.

• Lisp itself is a Lisp program, and
• Lisp programs can be expressed as lists, which are Lisp data structures.

Together, these two principles mean that any user can add operators to Lisp which are indistinguishable from the ones that come built-in.