Writing GNU Emacs Extensions

Lisp

Various forms of Lisp have been around since the 1950s. It is traditionally associated with artificial intelligence applications, for which Lisp is well-suited because it permits symbolic computation, can treat code as data, and simplifies building very complicated data structures. But Lisp is much more than just an AI language. It is applicable to a wide range of problems, a fact that is frequently overlooked by computer scientists but which is well known to Emacs users. Among the features that distinguish Lisp from other programming languages are:

Fully-parenthesized prefix notation

All expressions and function calls in Lisp are surrounded by parentheses,^[1] and the function name always precedes the arguments to the function. So whereas in other languages you may be able to write:

x + y

to apply the + function to the arguments x and y, in Lisp you write

(+ x y)

"Prefix notation" means that the operator precedes the operands. When the operator is between the operands, it's called "infix notation."

Though unfamiliar, prefix notation has some benefits over infix notation. In infix languages, to write the sum of five variables you need four plus signs:

a + b + c + d + e

Lisp is more concise:

(+ a b c d e)

Also, questions of operator precedence do not arise. For example, is the value of

3 + 4 * 5

35 or 23? It depends on whether * has higher precedence than +. But in Lisp, the confusion vanishes:

(+ 3 (* 4 5))      ;result is 23
(* (+ 3 4) 5)      ;result is 35

(Comments in Lisp are introduced with a semicolon ...