In the original Lisp, S-expressions consisted only of symbols, integers, and the list constructors
( xi... ) and
(x . y). Later Lisps, culminating in Common Lisp, added literals for floating-point, complex, and rational numbers, strings, and constructors for vectors.
The reader is responsible for parsing list structure, interning symbols, converting numbers to internal form, and calling read macros.
The reader is controlled by the
readtable, which defines the meaning of each character.
Unlike most programming languages, Lisp supports parse-time execution of programs, called "read macros" or "reader macros". These are used to extend the syntax either in universal or program-specific ways. For example, the quoted form
(quote x) operator can be abbreviated as
' operator can be defined as a read macro which reads the following list and wraps it with
quote. Similarly, the backquote operator (` ) can be defined as a read macro.