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Essay on Introduction into Data Structures
LISP data structures are called "S-expressions." The S stands for "symbolic." In this text, the terms "S-expression" and "expression" are used interchangeably. An S-expression is
1. a number, e.g., 15, written as an optional plus or minus sign, followed by one or more digits.
2. a symbol, e.g., F00, written as a letter followed by zero or more letters or digits.
3. a string, e.g., "This is a string", written as a double quote, followed by zero or more characters, followed by another double quote.
4. a character, e.g., #∖9, written as a sharp sign, followed by a backslash, followed by a character. (Numbers, symbols, strings, and characters are called atoms. There are other kinds of atoms. We will see them in later chapters.)
5. a list of S-expressions, e.g., (A B) or (IS TALL (FATHER BILL)), written as a left parenthesis, followed by zero or more S-expressions, followed by a right parenthesis.
Parentheses are more significant in LISP than they are in most other programming languages. Parentheses are virtually the only punctuation marks available in LISP programs. They are used to indicate the structure of Sexpressions. For example, (A) is a list of one element, the symbol A. ((A)) is also a list of one element, which is in turn a list of one element, which is the symbol A. Notice also that the left and right parentheses must balance. That is, a well-formed S-expression has a right parenthesis to close off each left parenthesis.
When LISP was first invented, no one had ever heard of "Structured, Programming," the movement among programmers to discipline themselves (and each other) to write cleaner, more modular code (see Dahl et al. [23]). Many LISP programmers continue to write code as if the valuable insights from this movement had never been gained, and the impression has caught on that LISP is reactionary....
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