The LISP language is designed primarily for symbolic data processing. It has been used for symbolic calculations in differential and integral calculus, electrical circuit theory, mathematical logic, game playing, and other fields of artificial intelligence.
LISP is a formal mathematical language. It is therefore possible to give a concise yet complete description of it. Such is the purpose of this first section of the manual. Other sections will describe ways of using LISP to advantage and will explain extensions of the language which make it a convenient programming system.
LISP differs from most programming languages in three important ways. The first way is in the nature of the data. In the LISP language, all data are in the form of symbolic expressions usually referred to as S-expressions. S-expressions are of indefinite length and have a branching tree type of structure, so that significant subexpressions can be readily isolated. In the LISP programming system, the bulk of available memory is used for storing S-expressions in the form of list structures. This type of memory organization frees the programmer from the necessity of allocating storage for the different sections of his program.
The second important part of the LISP language is the source language itself which specifies in what way the S-expressions are to be processed This consists of recursive functions of S-expressions. Since the notation for the writing of recursive functions of S-expressions is itself outside the S-expression notation, it will be called the meta language. These expressions will therefore be called M-expressions.
Third, LISP can interpret and execute programs written in the form of S-expressions. Thus, like machine language, and unlike most other higher level languages, it can be used to generate programs for further execution.