In LISP (and Scheme, and all of the other variants) an atom is one of the two major categories into which all S-expressions fall - the other being the dotted pair. This is the first thing that you'll learn upon picking up a copy of The Little LISPer or Schemer. At its simplest, an atom is an alphanumeric string which doesn't contain a parenthesis or the comment operator (usually the semicolon ( ; ) or double solidus ( // )). Consider the following examples:
n
- is an atom, because it's a character.
42
- is an atom, because it's a number.
abc
- is an atom, because it's a string.
ATOMNUMBER75
- is an atom, because it's a string of letters and numbers.
(+ 3 5)
- is not an atom, because it's a list.
(chemical . compound)
- is not an atom, because it's a dotted pair.
(atom? S)
This operator tests whether or not its input is an atom, although it is not standard in some
interpreters. It's functionally the same as
(not (pair? S))
so it can easily be synthesized if you don't have it.
Atoms, being absolutely fundamental to the language, have been with us since the first LISP interpreter was made in 1959.