In the programming language
Lisp
Lisp (historically LISP, an abbreviation of "list processing") is a family of programming languages with a long history and a distinctive, fully parenthesized Polish notation#Explanation, prefix notation.
Originally specified in the late 1950s, ...
, the reader or
read function is the
parser
Parsing, syntax analysis, or syntactic analysis is a process of analyzing a string of symbols, either in natural language, computer languages or data structures, conforming to the rules of a formal grammar by breaking it into parts. The term '' ...
which converts the textual form of Lisp objects to the corresponding internal object structure.
In the original Lisp, S-expressions consisted only of
symbol
A symbol is a mark, Sign (semiotics), sign, or word that indicates, signifies, or is understood as representing an idea, physical object, object, or wikt:relationship, relationship. Symbols allow people to go beyond what is known or seen by cr ...
s, integers, and the list constructors
( ''xi...'' ) and
(''x'' . ''y''). Later Lisps, culminating in
Common Lisp
Common Lisp (CL) is a dialect of the Lisp programming language, published in American National Standards Institute (ANSI) standard document ''ANSI INCITS 226-1994 (S2018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperli ...
, 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.
Read table
The reader is controlled by the
readtable, which defines the meaning of each
character.
Read macros
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
''x''. The
' 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.
References
{{Reflist
Bibliography
*
John McCarthy ''et al.'', ''LISP 1.5 Programmer's Manual'', MIT Press, 1962.
*
David A. Moon, ''MACLISP Reference Manual'', 1974.
*
Guy Steele
Guy Lewis Steele Jr. (; born October 2, 1954) is an American computer scientist who has played an important role in designing and documenting several computer programming languages and technical standards.
Biography
Steele was born in Missouri ...
, ''Common LISP: The Language'', Second Edition, 1990.
Lisp (programming language)
Parsing