Rather than merely being run as a slave process to accomplish specific tasks, a special-purpose interpreted language can become the core of an entire architecture; we'll consider the advantages and disadvantages of this approach in Chapter13. troff requests were an early example; today, the Emacs editor is one of the best-known and most powerful modern ones. It's built around a dialect of Lisp with primitives for both describing actions on editing buffers and controlling slave processes.

The fact that Emacs is built around a powerful language for describing editing actions or front ends for other programs means that it can be used for many other things besides ordinary editing. We'll examine the applications of Emacs's task-specific intelligence for day-to-day program development (compilation, debugging, version control) in Chapter15. Emacs ‘modes’ are user-defined libraries — programs written in Emacs Lisp that specialize the editor for a particular job — usually, but not necessarily, one related to editing.

Thus there are specialized modes that know the syntax of a large number of programming languages, and of markup languages like SGML, XML, and HTML. But many people also use Emacs modes to send and receive email (these use Unix system mail utilities as slaves) or Usenet news. Emacs can browse the web, or act as a front-end for various chat programs. There is also a calendaring package, Emacs's own calculator program, and even a fairly wide selection of games written as Emacs Lisp modes (including a descendant of the famous ELIZA program that simulates a Rogersian psychiatrist).^[93]