S-expression

An S-expression is a convention for representing data or an expression in a computer program in a text form. S-expressions are used in the Lisp programming language and Lisp-derived languages such as Scheme. The details of the syntax and supported data types vary in the different languages, but the most common feature is the extensive use of prefix notation with explicit use of brackets.

S-expressions are used for both code and data in Lisp. S-expressions were originally intended only as machine representations of human-readable M-expressions, but Lisp programmers soon started using S-expressions as the default notation.

S-expressions can either be single objects such as numbers, Lisp atoms including the special atoms nil and t, or cons pairs, written as (x . y). Longer lists are made up of nested cons pairs, for example (1 . (2 . (3 . nil))) which can also be written as the easier to read and write (1 2 3).

Program code can be written in S-expressions, using prefix notation. An extra piece of syntactic sugar for writing Lisp programs is that the common expression (quote x) can be written with the abbreviation 'x.

Example:

(defun factorial (x)
   (cond 
      ((eq x 1)  1)
      (t (* x (factorial (- x 1))))))