Lisp (historically LISP) is a family of

programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming l ...

s with a long history and a distinctive, fully parenthesized

prefix notation Polish notation (PN), also known as normal Polish notation (NPN), Łukasiewicz notation, Warsaw notation, Polish prefix notation or simply prefix notation, is a mathematical notation in which operators ''precede'' their operands, in contrast t ...

. Originally specified in 1960, Lisp is the second-oldest high-level programming language still in common use, after Fortran. Lisp has changed since its early days, and many dialects have existed over its history. Today, the best-known general-purpose Lisp dialects are

Common Lisp Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived f ...

, Scheme, Racket and Clojure. Lisp was originally created as a practical

mathematical notation Mathematical notation consists of using symbols for representing operations, unspecified numbers, relations and any other mathematical objects, and assembling them into expressions and formulas. Mathematical notation is widely used in mathema ...

for computer programs, influenced by (though not originally derived from) the notation of

Alonzo Church Alonzo Church (June 14, 1903 – August 11, 1995) was an American mathematician, computer scientist, logician, philosopher, professor and editor who made major contributions to mathematical logic and the foundations of theoretical computer scienc ...

's lambda calculus. It quickly became a favored programming language for

artificial intelligence Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech rec ...

(AI) research. As one of the earliest programming languages, Lisp pioneered many ideas in computer science, including tree data structures, automatic storage management,

dynamic typing In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every "term" (a word, phrase, or other set of symbols). Usually the terms are various constructs of a computer pro ...

conditionals Conditional (if then) may refer to: * Causal conditional, if X then Y, where X is a cause of Y *Conditional probability, the probability of an event A given that another event B has occurred *Conditional proof, in logic: a proof that asserts a c ...

, higher-order functions, recursion, the self-hosting compiler, and the read–eval–print loop. The name ''LISP'' derives from "LISt Processor". Linked lists are one of Lisp's major

data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for efficient access to data. More precisely, a data structure is a collection of data values, the relationships among them, a ...

s, and Lisp

source code In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the ...

is made of lists. Thus, Lisp programs can manipulate source code as a data structure, giving rise to the macro systems that allow programmers to create new syntax or new domain-specific languages embedded in Lisp. The interchangeability of code and data gives Lisp its instantly recognizable syntax. All program code is written as '' s-expressions'', or parenthesized lists. A function call or syntactic form is written as a list with the function or operator's name first, and the arguments following; for instance, a function that takes three arguments would be called as .

History

John McCarthy began developing Lisp in 1958 while he was at the

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...

(MIT). McCarthy published its design in a paper in '' Communications of the ACM'' in April 1960, entitled "Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I". He showed that with a few simple operators and a notation for anonymous functions borrowed from Church, one can build a

Turing-complete In computability theory, a system of data-manipulation rules (such as a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be used to simulate any ...

language for algorithms.

Information Processing Language Information Processing Language (IPL) is a programming language created by Allen Newell, Cliff Shaw, and Herbert A. Simon at RAND Corporation and the Carnegie Institute of Technology about 1956. Newell had the job of language specifier-applica ...

was the first AI language, from 1955 or 1956, and already included many of the concepts, such as list-processing and recursion, which came to be used in Lisp. McCarthy's original notation used bracketed " M-expressions" that would be translated into S-expressions. As an example, the M-expression is equivalent to the S-expression . Once Lisp was implemented, programmers rapidly chose to use S-expressions, and M-expressions were abandoned. M-expressions surfaced again with short-lived attempts of MLisp by Horace Enea and CGOL by Vaughan Pratt. Lisp was first implemented by Steve Russell on an IBM 704 computer using punched cards. Russell had read McCarthy's paper and realized (to McCarthy's surprise) that the Lisp ''eval'' function could be implemented in machine code. According to McCarthy: The result was a working Lisp interpreter which could be used to run Lisp programs, or more properly, "evaluate Lisp expressions". Two assembly language macros for the IBM 704 became the primitive operations for decomposing lists: (''Contents of the Address part of Register'' number) and (''Contents of the Decrement part of Register'' number), where "register" refers to registers of the computer's central processing unit (CPU). Lisp dialects still use and ( and ) for the operations that return the first item in a list and the rest of the list, respectively. The first complete Lisp compiler, written in Lisp, was implemented in 1962 by Tim Hart and Mike Levin at MIT, and could be compiled by simply having an existing LISP interpreter interpret the compiler code, producing machine code output able to be executed at a 40-fold improvement in speed over that of the interpreter. This compiler introduced the Lisp model of incremental compilation, in which compiled and interpreted functions can intermix freely. The language used in Hart and Levin's memo is much closer to modern Lisp style than McCarthy's earlier code. Garbage collection routines were developed by MIT graduate student Daniel Edwards, prior to 1962. During the 1980s and 1990s, a great effort was made to unify the work on new Lisp dialects (mostly successors to Maclisp such as ZetaLisp and NIL (New Implementation of Lisp) into a single language. The new language,

, was somewhat compatible with the dialects it replaced (the book '' Common Lisp the Language'' notes the compatibility of various constructs). In 1994, ANSI published the Common Lisp standard, "ANSI X3.226-1994 Information Technology Programming Language Common Lisp".

Timeline

Connection to artificial intelligence

Since inception, Lisp was closely connected with the

research community, especially on PDP-10The 36-bit word size of the PDP-6/ PDP-10 was influenced by the usefulness of having two Lisp 18-bit pointers in a single word. systems. Lisp was used as the implementation of the language Micro Planner, which was used in the famous AI system

SHRDLU SHRDLU was an early natural-language understanding computer program, developed by Terry Winograd at MIT in 1968–1970. In the program, the user carries on a conversation with the computer, moving objects, naming collections and querying the s ...

. In the 1970s, as AI research spawned commercial offshoots, the performance of existing Lisp systems became a growing issue, as programmers needed to be familiar with the performance ramifications of the various techniques and choices involved in the implementation of Lisp.

Genealogy and variants

Over its sixty-year history, Lisp has spawned many variations on the core theme of an S-expression language. Moreover, each given dialect may have several implementations—for instance, there are more than a dozen implementations of

. Differences between dialects may be quite visible—for instance, Common Lisp uses the keyword defun to name a function, but Scheme uses define.Common Lisp: (defun f (x) x)
Scheme: (define f (lambda (x) x)) or (define (f x) x) Within a dialect that is standardized, however, conforming implementations support the same core language, but with different extensions and libraries.

Historically significant dialects

* LISP 1 Accessed May 11, 2010. – First implementation. * LISP 1.5 – First widely distributed version, developed by McCarthy and others at MIT. So named because it contained several improvements on the original "LISP 1" interpreter, but was not a major restructuring as the planned

LISP 2 LISP 2 was a programming language proposed in the 1960s as the successor to Lisp. It had largely Lisp-like semantics and Algol 60-like syntax. Today it is mostly remembered for its syntax, but in fact it had many features beyond those of early Lisp ...

would be. * Stanford LISP 1.6 – This was a successor to LISP 1.5 developed at the

Stanford AI Lab Stanford University has many centers and institutes dedicated to the study of various specific topics. These centers and institutes may be within a department, within a school but across departments, an independent laboratory, institute or center ...

, and widely distributed to PDP-10 systems running the

TOPS-10 TOPS-10 System (''Timesharing / Total Operating System-10'') is a discontinued operating system from Digital Equipment Corporation (DEC) for the PDP-10 (or DECsystem-10) mainframe computer family. Launched in 1967, TOPS-10 evolved from the earli ...

operating system. It was rendered obsolete by Maclisp and InterLisp. * MACLISP – developed for MIT's Project MAC, MACLISP is a direct descendant of LISP 1.5. It ran on the PDP-10 and Multics systems. MACLISP would later come to be called Maclisp, and is often referred to as MacLisp. The "MAC" in MACLISP is related neither to Apple's

Macintosh The Mac (known as Macintosh until 1999) is a family of personal computers designed and marketed by Apple Inc. Macs are known for their ease of use and minimalist designs, and are popular among students, creative professionals, and software en ...

nor to McCarthy. *

Interlisp Interlisp (also seen with a variety of capitalizations) is a programming environment built around a version of the programming language Lisp. Interlisp development began in 1966 at Bolt, Beranek and Newman (renamed BBN Technologies) in Cambridge, ...

– developed at BBN Technologies for PDP-10 systems running the TENEX operating system, later adopted as a "West coast" Lisp for the Xerox Lisp machines as

InterLisp-D Interlisp (also seen with a variety of capitalizations) is a programming environment built around a version of the programming language Lisp. Interlisp development began in 1966 at Bolt, Beranek and Newman (renamed BBN Technologies) in Cambridge, ...

. A small version called "InterLISP 65" was published for the

6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small te ...

-based Atari 8-bit family computer line. For quite some time, Maclisp and InterLisp were strong competitors. *

Franz Lisp In computer programming, Franz Lisp is a discontinued Lisp programming language system written at the University of California, Berkeley (UC Berkeley, UCB) by Professor Richard Fateman and several students, based largely on Maclisp and distrib ...

– originally a

University of California, Berkeley The University of California, Berkeley (UC Berkeley, Berkeley, Cal, or California) is a public land-grant research university in Berkeley, California. Established in 1868 as the University of California, it is the state's first land-grant uni ...

project; later developed by Franz Inc. The name is a humorous deformation of the name " Franz Liszt", and does not refer to Allegro Common Lisp, the dialect of Common Lisp sold by Franz Inc., in more recent years. *

XLISP XLISP is a family of Lisp implementations written by David Betz and first released in 1983. The first version was a Lisp with object-oriented extensions for computers with limited power. The second version (XLISP 2.0) moved toward Common Lisp, bu ...

, which AutoLISP was based on. * Standard Lisp and

Portable Standard Lisp Portable Standard Lisp (PSL) is a programming language, a dialect of the language Lisp. PSL was inspired by its predecessor, ''Standard Lisp'' and the Portable Lisp Compiler. It is tail-recursive, late binding (or dynamically bound), and was devel ...

were widely used and ported, especially with the Computer Algebra System REDUCE. * ZetaLisp, also termed Lisp Machine Lisp – used on the Lisp machines, direct descendant of Maclisp. ZetaLisp had a big influence on Common Lisp. *

LeLisp Le Lisp (also Le_Lisp and Le-Lisp) is a programming language, a dialect of the language Lisp. It was developed at the French Institute for Research in Computer Science and Automation (INRIA), to be an implementation language for a very large scal ...

is a French Lisp dialect. One of the first Interface Builders (called SOS InterfaceOutils de generation d'interfaces : etat de l'art et classification by H. El Mrabet
/ref>) was written in LeLisp. * Scheme (1975). *

(1984), as described by '' Common Lisp the Language'' – a consolidation of several divergent attempts (ZetaLisp,

Spice Lisp Spice Lisp (Scientific Personal Integrated Computing Environment) is a programming language, a dialect of Lisp. Its implementation, originally written by Carnegie Mellon University's (CMU) Spice Lisp Group, targeted the microcode of the 16-bit work ...

, NIL, and S-1 Lisp) to create successor dialects to Maclisp, with substantive influences from the Scheme dialect as well. This version of Common Lisp was available for wide-ranging platforms and was accepted by many as a de facto standard until the publication of ANSI Common Lisp (ANSI X3.226-1994). Among the most widespread sub-dialects of Common Lisp are Steel Bank Common Lisp (SBCL), CMU Common Lisp (CMU-CL), Clozure OpenMCL (not to be confused with Clojure!), GNU CLisp, and later versions of Franz Lisp; all of them adhere to the later ANSI CL standard (see below). * Dylan was in its first version a mix of Scheme with the Common Lisp Object System. *

EuLisp EuLisp is a statically and dynamically scoped Lisp dialect developed by a loose formation of industrial and academic Lisp users and developers from around Europe. The standardizers intended to create a new Lisp "less encumbered by the past" (c ...

– attempt to develop a new efficient and cleaned-up Lisp. *

ISLISP ISLISP (also capitalized as ISLisp) is a programming language in the Lisp family standardized by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) joint working group ISO/IEC JTC 1/SC 22/W ...

– attempt to develop a new efficient and cleaned-up Lisp. Standardized as ISO/IEC 13816:1997 and later revised as ISO/IEC 13816:2007: ''Information technology – Programming languages, their environments and system software interfaces – Programming language ISLISP''. * IEEE Scheme – IEEE standard, 1178–1990 (R1995). * ANSI

– an

American National Standards Institute The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...

(ANSI)

standard Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object ...

for Common Lisp, created by subcommittee

X3J13 X3J13 is the name of a technical committee which was part of the International Committee for Information Technology Standards (INCITS, then named ''X3''). The X3J13 committee was formed in 1986 to draw up an American National Standards Institute ( ...

, chartered to begin with ''Common Lisp: The Language'' as a base document and to work through a public consensus process to find solutions to shared issues of portability of programs and

compatibility Compatibility may refer to: Computing * Backward compatibility, in which newer devices can understand data generated by older devices * Compatibility card, an expansion card for hardware emulation of another device * Compatibility layer, compo ...

of Common Lisp implementations. Although formally an ANSI standard, the implementation, sale, use, and influence of ANSI Common Lisp has been and continues to be seen worldwide. *

ACL2 ACL2 ("A Computational Logic for Applicative Common Lisp") is a software system consisting of a programming language, created by Timothy Still it was an extensible theory in a first-order logic, and an automated theorem prover. ACL2 is designed t ...

or "A Computational Logic for Applicative Common Lisp", an applicative (side-effect free) variant of Common LISP. ACL2 is both a programming language which can model computer systems, and a tool to help proving properties of those models. * Clojure, a recent dialect of Lisp which compiles to the

Java virtual machine A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describ ...

and has a particular focus on concurrency. * Game Oriented Assembly Lisp (or GOAL) is a video game programming language developed by Andy Gavin at

Naughty Dog Naughty Dog, LLC (formerly JAM Software, Inc.) is an American first-party video game developer based in Santa Monica, California. Founded by Andy Gavin and Jason Rubin in 1984, the studio was acquired by Sony Computer Entertainment in 2001. G ...

. It was written using Allegro Common Lisp and used in the development of the entire Jak and Daxter series of games developed by Naughty Dog.

2000 to present

After having declined somewhat in the 1990s, Lisp has experienced a resurgence of interest after 2000. Most new activity has been focused around implementations of

, Scheme,

Emacs Lisp Emacs Lisp is a dialect of the Lisp programming language used as a scripting language by Emacs (a text editor family most commonly associated with GNU Emacs and XEmacs). It is used for implementing most of the editing functionality built into ...

, Clojure, and Racket, and includes development of new portable libraries and applications. Many new Lisp programmers were inspired by writers such as Paul Graham and Eric S. Raymond to pursue a language others considered antiquated. New Lisp programmers often describe the language as an eye-opening experience and claim to be substantially more productive than in other languages. This increase in awareness may be contrasted to the "

AI winter In the history of artificial intelligence, an AI winter is a period of reduced funding and interest in artificial intelligence research. , there were eleven actively maintained Common Lisp implementations. Scieneer Common Lisp is a new commercial implementation forked from CMUCL with a first release in 2002. The

open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...

community has created new supporting infrastructure: CLiki is a wiki that collects Common Lisp related information, the Common Lisp directory lists resources, #lisp is a popular IRC channel and allows the sharing and commenting of code snippets (with support by lisppaste, an IRC bot written in Lisp), Planet Lisp collects the contents of various Lisp-related blogs, on LispForum users discuss Lisp topics, Lispjobs is a service for announcing job offers and there is a weekly news service, '' Weekly Lisp News''. ''Common-lisp.net'' is a hosting site for open source Common Lisp projects. Quicklisp is a library manager for Common Lisp. Fifty years of Lisp (1958–2008) was celebrated at LISP50@OOPSLA. There are regular local user meetings in Boston, Vancouver, and Hamburg. Other events include the European Common Lisp Meeting, the European Lisp Symposium and an International Lisp Conference. The Scheme community actively maintains over twenty implementations. Several significant new implementations (Chicken, Gambit, Gauche, Ikarus, Larceny, Ypsilon) have been developed in the 2000s (decade). The Revised⁵ Report on the Algorithmic Language SchemeDocuments: Standards: R5RS
schemers.org (2012-01-11). Retrieved on 2013-07-17. standard of Scheme was widely accepted in the Scheme community. The

Scheme Requests for Implementation Scheme Requests for Implementation (SRFI) is an effort to coordinate libraries and extensions of standard Scheme programming language, necessitated by Scheme's minimalist design, and particularly the lack of a standard library before Scheme R6RS. ...

process has created a lot of quasi standard libraries and extensions for Scheme. User communities of individual Scheme implementations continue to grow. A new language standardization process was started in 2003 and led to the R⁶RS Scheme standard in 2007. Academic use of Scheme for teaching computer science seems to have declined somewhat. Some universities are no longer using Scheme in their computer science introductory courses; MIT now uses Python instead of Scheme for its undergraduate computer science program and MITx massive open online course. There are several new dialects of Lisp: Arc, Hy, Nu, Liskell, and LFE (Lisp Flavored Erlang). The parser for

Julia Julia is usually a feminine given name. It is a Latinate feminine form of the name Julio and Julius. (For further details on etymology, see the Wiktionary entry "Julius".) The given name ''Julia'' had been in use throughout Late Antiquity (e. ...

is implemented in Femtolisp, a dialect of Scheme (Julia is inspired by Scheme, which in turn is a Lisp dialect). In October 2019, Paul Graham release
a specification for Bel
"a new dialect of Lisp."

Major dialects

and Scheme represent two major streams of Lisp development. These languages embody significantly different design choices.

is a successor to Maclisp. The primary influences were

Lisp Machine Lisp Lisp Machine Lisp is a programming language, a dialect of the language Lisp. A direct descendant of Maclisp, it was initially developed in the mid to late 1970s as the system programming language for the Massachusetts Institute of Technology (M ...

, Maclisp, NIL, S-1 Lisp,

, and Scheme.Chapter 1.1.2, History, ANSI CL Standard It has many of the features of Lisp Machine Lisp (a large Lisp dialect used to program Lisp Machines), but was designed to be efficiently implementable on any personal computer or workstation. Common Lisp is a general-purpose programming language and thus has a large language standard including many built-in data types, functions, macros and other language elements, and an object system (

Common Lisp Object System The Common Lisp Object System (CLOS) is the facility for object-oriented programming which is part of ANSI Common Lisp. CLOS is a powerful dynamic object system which differs radically from the OOP facilities found in more static languages such ...

). Common Lisp also borrowed certain features from Scheme such as lexical scoping and lexical closures. Common Lisp implementations are available for targeting different platforms such as the LLVM,
Clasp is a Common Lisp implementation that interoperates with C++ and uses LLVM for just-in-time compilation (JIT) to native code. the

,
"Armed Bear Common Lisp (ABCL) is a full implementation of the Common Lisp language featuring both an interpreter and a compiler, running in the JVM" x86-64, PowerPC, Alpha, ARM, Motorola 68000, and MIPS,
Common Lisp Implementations: A Survey and operating systems such as Windows, macOS, Linux, Solaris, FreeBSD, NetBSD, OpenBSD, Dragonfly BSD, and Heroku.
Comparison of actively developed Common Lisp implementations Scheme is a statically scoped and properly tail-recursive dialect of the Lisp programming language invented by Guy L. Steele, Jr. and

Gerald Jay Sussman Gerald Jay Sussman (born February 8, 1947) is the Panasonic Professor of Electrical Engineering at the Massachusetts Institute of Technology (MIT). He received his S.B. and Ph.D. degrees in mathematics from MIT in 1968 and 1973 respectively. H ...

. It was designed to have exceptionally clear and simple semantics and few different ways to form expressions. Designed about a decade earlier than Common Lisp, Scheme is a more minimalist design. It has a much smaller set of standard features but with certain implementation features (such as tail-call optimization and full continuations) not specified in Common Lisp. A wide variety of programming paradigms, including imperative, functional, and message passing styles, find convenient expression in Scheme. Scheme continues to evolve with a series of standards (Revisedⁿ Report on the Algorithmic Language Scheme) and a series of

. Clojure is a recent dialect of Lisp that targets mainly the

, and the

Common Language Runtime The Common Language Runtime (CLR), the virtual machine component of Microsoft .NET Framework, manages the execution of .NET programs. Just-in-time compilation converts the managed code (compiled intermediate language code) into machine inst ...

(CLR), the Python VM, the Ruby VM YARV, and compiling to

JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, often ...

. It is designed to be a pragmatic general-purpose language. Clojure draws considerable influences from Haskell and places a very strong emphasis on immutability.An In-Depth Look at Clojure Collections
Retrieved 2012-06-24 Clojure provides access to Java frameworks and libraries, with optional type hints and

type inference Type inference refers to the automatic detection of the type of an expression in a formal language. These include programming languages and mathematical type systems, but also natural languages in some branches of computer science and linguistic ...

, so that calls to Java can avoid reflection and enable fast primitive operations. Clojure is not designed to be backwards compatible with other Lisp dialects. Further, Lisp dialects are used as

scripting language A scripting language or script language is a programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. The ...

s in many applications, with the best-known being

in the Emacs editor, AutoLISP and later Visual Lisp in AutoCAD, Nyquist in Audacity, and Scheme in LilyPond. The potential small size of a useful Scheme interpreter makes it particularly popular for embedded scripting. Examples include

SIOD Scheme In One Defun, or humorously Scheme In One Day (SIOD) is a programming language, a dialect of the language Lisp, a small-size implementation of the dialect Scheme, written in C and designed to be embedded inside C programs. It is notable ...

and TinyScheme, both of which have been successfully embedded in the GIMP image processor under the generic name "Script-fu".Script-fu In GIMP 2.4
Retrieved 2009-10-29 LIBREP, a Lisp interpreter by John Harper originally based on the

language, has been embedded in the

Sawfish Sawfish, also known as carpenter sharks, are a family of rays characterized by a long, narrow, flattened rostrum, or nose extension, lined with sharp transverse teeth, arranged in a way that resembles a saw. They are among the largest fish ...

window manager A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface. Most window managers are designed to help provide a desktop environment. They work in conjunctio ...

.librep
at Sawfish Wikia, retrieved 2009-10-29

Standardized dialects

Lisp has officially standardized dialects: R6RS Scheme, R7RS Scheme, IEEE Scheme, ANSI Common Lisp and ISO

Language innovations

Paul Graham identifies nine important aspects of Lisp that distinguished it from existing languages like Fortran: *

Conditionals Conditional (if then) may refer to: * Causal conditional, if X then Y, where X is a cause of Y *Conditional probability, the probability of an event A given that another event B has occurred *Conditional proof, in logic: a proof that asserts a c ...

not limited to goto *

First-class function In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning them as the values fro ...

s * Recursion * Treating variables uniformly as

pointers Pointer may refer to: Places * Pointer, Kentucky * Pointers, New Jersey * Pointers Airport, Wasco County, Oregon, United States * The Pointers, a pair of rocks off Antarctica People with the name * Pointer (surname), a surname (including a ...

, leaving types to values * Garbage collection * Programs made entirely of expressions with no statements * The

symbol A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different conce ...

data type, distinct from the

string String or strings may refer to: * String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian ani ...

data type * Notation for code made of trees of symbols (using many parentheses) * Full language available at

load time In computer systems a loader is the part of an operating system that is responsible for loading programs and libraries. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them ...

compile time In computer science, compile time (or compile-time) describes the time window during which a computer program is compiled. The term is used as an adjective to describe concepts related to the context of program compilation, as opposed to concep ...

, and run time Lisp was the first language where the structure of program code is represented faithfully and directly in a standard data structure—a quality much later dubbed "

homoiconicity In computer programming, homoiconicity (from the Greek words ''homo-'' meaning "the same" and ''icon'' meaning "representation") is a property of some programming languages. A language is homoiconic if a program written in it can be manipulated as ...

". Thus, Lisp functions can be manipulated, altered or even created within a Lisp program without lower-level manipulations. This is generally considered one of the main advantages of the language with regard to its expressive power, and makes the language suitable for syntactic macros and meta-circular evaluation. A conditional using an '' if–then–else'' syntax was invented by McCarthy for a chess program written in Fortran. He proposed its inclusion in

ALGOL ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...

, but it was not made part of the Algol 58 specification. For Lisp, McCarthy used the more general ''cond''-structure. Algol 60 took up ''if–then–else'' and popularized it. Lisp deeply influenced Alan Kay, the leader of the research team that developed

Smalltalk Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by A ...

Xerox PARC PARC (Palo Alto Research Center; formerly Xerox PARC) is a research and development company in Palo Alto, California. Founded in 1969 by Jacob E. "Jack" Goldman, chief scientist of Xerox Corporation, the company was originally a division of Xer ...

; and in turn Lisp was influenced by Smalltalk, with later dialects adopting object-oriented programming features (inheritance classes, encapsulating instances, message passing, etc.) in the 1970s. The

Flavors Flavor or flavour is either the sensory perception of taste or smell, or a flavoring in food that produces such perception. Flavor or flavour may also refer to: Science * Flavors (programming language), an early object-oriented extension to Li ...

object system introduced the concept of multiple inheritance and the

mixin In object-oriented programming languages, a mixin (or mix-in) is a class that contains methods for use by other classes without having to be the parent class of those other classes. How those other classes gain access to the mixin's methods depe ...

. The

provides multiple inheritance, multimethods with multiple dispatch, and first-class generic functions, yielding a flexible and powerful form of

dynamic dispatch In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented ...

. It has served as the template for many subsequent Lisp (including Scheme) object systems, which are often implemented via a metaobject protocol, a

reflective Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. T ...

meta-circular design in which the object system is defined in terms of itself: Lisp was only the second language after Smalltalk (and is still one of the very few languages) to possess such a metaobject system. Many years later, Alan Kay suggested that as a result of the confluence of these features, only Smalltalk and Lisp could be regarded as properly conceived object-oriented programming systems. Lisp introduced the concept of automatic garbage collection, in which the system walks the heap looking for unused memory. Progress in modern sophisticated garbage collection algorithms such as generational garbage collection was stimulated by its use in Lisp. Edsger W. Dijkstra in his 1972

Turing Award The ACM A. M. Turing Award is an annual prize given by the Association for Computing Machinery (ACM) for contributions of lasting and major technical importance to computer science. It is generally recognized as the highest distinction in compu ...

lecture said, Largely because of its resource requirements with respect to early computing hardware (including early microprocessors), Lisp did not become as popular outside of the AI community as Fortran and the

-descended C language. Because of its suitability to complex and dynamic applications, Lisp enjoyed some resurgence of popular interest in the 2010s.

Syntax and semantics

:''Note: This article's examples are written in

(though most are also valid in Scheme).''

Symbolic expressions (S-expressions)

Lisp is an expression oriented language. Unlike most other languages, no distinction is made between "expressions" and "statements"; all code and data are written as expressions. When an expression is ''evaluated'', it produces a value (in Common Lisp, possibly multiple values), which can then be embedded into other expressions. Each value can be any data type. McCarthy's 1958 paper introduced two types of syntax: ''Symbolic expressions'' ( S-expressions, sexps), which mirror the internal representation of code and data; and ''Meta expressions'' ( M-expressions), which express functions of S-expressions. M-expressions never found favor, and almost all Lisps today use S-expressions to manipulate both code and data. The use of parentheses is Lisp's most immediately obvious difference from other programming language families. As a result, students have long given Lisp nicknames such as ''Lost In Stupid Parentheses'', or ''Lots of Irritating Superfluous Parentheses''. However, the S-expression syntax is also responsible for much of Lisp's power: the syntax is simple and consistent, which facilitates manipulation by computer. However, the syntax of Lisp is not limited to traditional parentheses notation. It can be extended to include alternative notations. For example, XMLisp is a Common Lisp extension that employs the metaobject protocol to integrate S-expressions with the Extensible Markup Language (

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. ...

). The reliance on expressions gives the language great flexibility. Because Lisp functions are written as lists, they can be processed exactly like data. This allows easy writing of programs which manipulate other programs (

metaprogramming Metaprogramming is a programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyze or transform other programs, and even modify itsel ...

). Many Lisp dialects exploit this feature using macro systems, which enables extension of the language almost without limit.

Lists

A Lisp list is written with its elements separated by

whitespace White space or whitespace may refer to: Technology * Whitespace characters, characters in computing that represent horizontal or vertical space * White spaces (radio), allocated but locally unused radio frequencies * TV White Space Database, a ...

, and surrounded by parentheses. For example, is a list whose elements are the three ''atoms'' , , and . These values are implicitly typed: they are respectively two integers and a Lisp-specific data type called a "symbol", and do not have to be declared as such. The empty list is also represented as the special atom . This is the only entity in Lisp which is both an atom and a list. Expressions are written as lists, using

. The first element in the list is the name of a function, the name of a macro, a lambda expression or the name of a "special operator" (see below). The remainder of the list are the arguments. For example, the function returns its arguments as a list, so the expression (list 1 2 (quote foo)) evaluates to the list . The "quote" before the in the preceding example is a "special operator" which returns its argument without evaluating it. Any unquoted expressions are recursively evaluated before the enclosing expression is evaluated. For example, (list 1 2 (list 3 4)) evaluates to the list . Note that the third argument is a list; lists can be nested.

Operators

Arithmetic operators are treated similarly. The expression (+ 1 2 3 4) evaluates to 10. The equivalent under

infix notation Infix notation is the notation commonly used in arithmetical and logical formulae and statements. It is characterized by the placement of operators between operands—"infixed operators"—such as the plus sign in . Usage Binary relations are o ...

would be "". Lisp has no notion of operators as implemented in Algol-derived languages. Arithmetic operators in Lisp are

variadic function In mathematics and in computer programming, a variadic function is a function of indefinite arity, i.e., one which accepts a variable number of arguments. Support for variadic functions differs widely among programming languages. The term ''va ...

s (or ''n-ary''), able to take any number of arguments. A C-style '++' increment operator is sometimes implemented under the name incf giving syntax (incf x) equivalent to (setq x (+ x 1)), returning the new value of x. "Special operators" (sometimes called "special forms") provide Lisp's control structure. For example, the special operator takes three arguments. If the first argument is non-nil, it evaluates to the second argument; otherwise, it evaluates to the third argument. Thus, the expression (if nil (list 1 2 "foo") (list 3 4 "bar")) evaluates to . Of course, this would be more useful if a non-trivial expression had been substituted in place of . Lisp also provides logical operators and, or and not. The and and or operators do

short-circuit evaluation A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...

and will return their first nil and non-nil argument respectively. (or (and "zero" nil "never") "James" 'task 'time) will evaluate to "James".

Lambda expressions and function definition

Another special operator, , is used to bind variables to values which are then evaluated within an expression. This operator is also used to create functions: the arguments to are a list of arguments, and the expression or expressions to which the function evaluates (the returned value is the value of the last expression that is evaluated). The expression (lambda (arg) (+ arg 1)) evaluates to a function that, when applied, takes one argument, binds it to and returns the number one greater than that argument. Lambda expressions are treated no differently from named functions; they are invoked the same way. Therefore, the expression ((lambda (arg) (+ arg 1)) 5) evaluates to . Here, we're doing a function application: we execute the

anonymous function In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed t ...

by passing to it the value 5. Named functions are created by storing a lambda expression in a symbol using the defun macro. (defun foo (a b c d) (+ a b c d)) defines a new function named in the global environment. It is conceptually similar to the expression: (setf (fdefinition 'f) #'(lambda (a) (block f b...))) where is a macro used to set the value of the first argument to a new function object. is a global function definition for the function named . is an abbreviation for special operator, returning a function object.

Atoms

In the original LISP there were two fundamental

data type In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most pro ...

s: atoms and lists. A list was a finite ordered sequence of elements, where each element is either an atom or a list, and an atom was a

number A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers ca ...

or a symbol. A symbol was essentially a unique named item, written as an

alphanumeric Alphanumericals or alphanumeric characters are a combination of alphabetical and numerical characters. More specifically, they are the collection of Latin letters and Arabic digits. An alphanumeric code is an identifier made of alphanumeric ...

string in

, and used either as a variable name or as a data item in symbolic processing. For example, the list contains three elements: the symbol , the list , and the number 2. The essential difference between atoms and lists was that atoms were immutable and unique. Two atoms that appeared in different places in source code but were written in exactly the same way represented the same object, whereas each list was a separate object that could be altered independently of other lists and could be distinguished from other lists by comparison operators. As more data types were introduced in later Lisp dialects, and

programming style Programming style, also known as code style, is a set of rules or guidelines used when writing the source code for a computer program. It is often claimed that following a particular programming style will help programmers read and understand s ...

s evolved, the concept of an atom lost importance. Many dialects still retained the predicate ''atom'' for legacy compatibility, defining it true for any object which is not a cons.

Conses and lists

A Lisp list is implemented as a singly linked list. Each cell of this list is called a ''cons'' (in Scheme, a ''pair'') and is composed of two

, called the ''car'' and ''cdr''. These are respectively equivalent to the and fields discussed in the article '' linked list''. Of the many data structures that can be built out of cons cells, one of the most basic is called a ''proper list''. A proper list is either the special (empty list) symbol, or a cons in which the points to a datum (which may be another cons structure, such as a list), and the points to another proper list. If a given cons is taken to be the head of a linked list, then its car points to the first element of the list, and its cdr points to the rest of the list. For this reason, the and functions are also called and when referring to conses which are part of a linked list (rather than, say, a tree). Thus, a Lisp list is not an atomic object, as an instance of a container class in C++ or Java would be. A list is nothing more than an aggregate of linked conses. A variable that refers to a given list is simply a pointer to the first cons in the list. Traversal of a list can be done by ''cdring down'' the list; that is, taking successive cdrs to visit each cons of the list; or by using any of several higher-order functions to map a function over a list. Because conses and lists are so universal in Lisp systems, it is a common misconception that they are Lisp's only data structures. In fact, all but the most simplistic Lisps have other data structures, such as vectors (

arrays An array is a systematic arrangement of similar objects, usually in rows and columns. Things called an array include: {{TOC right Music * In twelve-tone and serial composition, the presentation of simultaneous twelve-tone sets such that the ...

), hash tables, structures, and so forth.

S-expressions represent lists

Parenthesized S-expressions represent linked list structures. There are several ways to represent the same list as an S-expression. A cons can be written in ''dotted-pair notation'' as , where is the car and the cdr. A longer proper list might be written in dotted-pair notation. This is conventionally abbreviated as in ''list notation''. An improper listNB: a so-called "dotted list" is only one kind of "improper list". The other kind is the "circular list" where the cons cells form a loop. Typically this is represented using #n=(...) to represent the target cons cell that will have multiple references, and #n# is used to refer to this cons. For instance, (#1=(a b) . #1#) would normally be printed as ((a b) a b) (without circular structure printing enabled), but makes the reuse of the cons cell clear. #1=(a . #1#) cannot normally be printed as it is circular, although (a...) is sometimes displayed, the CDR of the cons cell defined by #1= is itself. may be written in a combination of the two – as for the list of three conses whose last cdr is (i.e., the list in fully specified form).

List-processing procedures

Lisp provides many built-in procedures for accessing and controlling lists. Lists can be created directly with the procedure, which takes any number of arguments, and returns the list of these arguments. (list 1 2 'a 3) ;Output: (1 2 a 3) (list 1 '(2 3) 4) ;Output: (1 (2 3) 4) Because of the way that lists are constructed from cons pairs, the procedure can be used to add an element to the front of a list. Note that the procedure is asymmetric in how it handles list arguments, because of how lists are constructed. (cons 1 '(2 3)) ;Output: (1 2 3) (cons '(1 2) '(3 4)) ;Output: ((1 2) 3 4) The procedure appends two (or more) lists to one another. Because Lisp lists are linked lists, appending two lists has asymptotic time complexity

O(n)

(append '(1 2) '(3 4)) ;Output: (1 2 3 4) (append '(1 2 3) '() '(a) '(5 6)) ;Output: (1 2 3 a 5 6)

Shared structure

Lisp lists, being simple linked lists, can share structure with one another. That is to say, two lists can have the same ''tail'', or final sequence of conses. For instance, after the execution of the following Common Lisp code: (setf foo (list 'a 'b 'c)) (setf bar (cons 'x (cdr foo))) the lists and are and respectively. However, the tail is the same structure in both lists. It is not a copy; the cons cells pointing to and are in the same memory locations for both lists. Sharing structure rather than copying can give a dramatic performance improvement. However, this technique can interact in undesired ways with functions that alter lists passed to them as arguments. Altering one list, such as by replacing the with a , will affect the other: (setf (third foo) 'goose) This changes to , but thereby also changes to – a possibly unexpected result. This can be a source of bugs, and functions which alter their arguments are documented as ''destructive'' for this very reason. Aficionados of functional programming avoid destructive functions. In the Scheme dialect, which favors the functional style, the names of destructive functions are marked with a cautionary exclamation point, or "bang"—such as (read ''set car bang''), which replaces the car of a cons. In the Common Lisp dialect, destructive functions are commonplace; the equivalent of is named for "replace car". This function is rarely seen, however, as Common Lisp includes a special facility, , to make it easier to define and use destructive functions. A frequent style in Common Lisp is to write code functionally (without destructive calls) when prototyping, then to add destructive calls as an optimization where it is safe to do so.

Self-evaluating forms and quoting

Lisp evaluates expressions which are entered by the user. Symbols and lists evaluate to some other (usually, simpler) expression – for instance, a symbol evaluates to the value of the variable it names; evaluates to . However, most other forms evaluate to themselves: if entering into Lisp, it returns . Any expression can also be marked to prevent it from being evaluated (as is necessary for symbols and lists). This is the role of the special operator, or its abbreviation (one quotation mark). For instance, usually if entering the symbol , it returns the value of the corresponding variable (or an error, if there is no such variable). To refer to the literal symbol, enter or, usually, . Both Common Lisp and Scheme also support the ''backquote'' operator (termed '' quasiquote'' in Scheme), entered with the character ( grave accent). This is almost the same as the plain quote, except it allows expressions to be evaluated and their values interpolated into a quoted list with the comma ''unquote'' and comma-at ''splice'' operators. If the variable has the value then evaluates to , while evaluates to . The backquote is most often used in defining macro expansions.Quasiquotation in Lisp
, Alan Bawden Self-evaluating forms and quoted forms are Lisp's equivalent of literals. It may be possible to modify the values of (mutable) literals in program code. For instance, if a function returns a quoted form, and the code that calls the function modifies the form, this may alter the behavior of the function on subsequent invocations. (defun should-be-constant () '(one two three)) (let ((stuff (should-be-constant))) (setf (third stuff) 'bizarre)) ; bad! (should-be-constant) ; returns (one two bizarre) Modifying a quoted form like this is generally considered bad style, and is defined by ANSI Common Lisp as erroneous (resulting in "undefined" behavior in compiled files, because the file-compiler can coalesce similar constants, put them in write-protected memory, etc.). Lisp's formalization of quotation has been noted by

Douglas Hofstadter Douglas Richard Hofstadter (born February 15, 1945) is an American scholar of cognitive science, physics, and comparative literature whose research includes concepts such as the sense of self in relation to the external world, consciousness, an ...

(in '' Gödel, Escher, Bach'') and others as an example of the philosophical idea of

self-reference Self-reference occurs in natural or formal languages when a sentence, idea or formula refers to itself. The reference may be expressed either directly—through some intermediate sentence or formula—or by means of some encoding. In philosop ...

Scope and closure

The Lisp family splits over the use of

dynamic Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' " power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical d ...

or static (a.k.a. lexical)

scope Scope or scopes may refer to: People with the surname * Jamie Scope (born 1986), English footballer * John T. Scopes (1900–1970), central figure in the Scopes Trial regarding the teaching of evolution Arts, media, and entertainment * Cinem ...

. Clojure, Common Lisp and Scheme make use of static scoping by default, while

newLISP newLISP is a scripting language which is a dialect of the Lisp family of programming languages. It was designed and developed by Lutz Mueller. newLISP is free and open-source software released under the GNU General Public License, version 3 or ...

Picolisp PicoLisp is a programming language, a dialect of the language Lisp. It runs on operating systems including Linux and others that are ''Portable Operating System Interface'' (POSIX) compliant. Its most prominent features are simplicity and minimalis ...

and the embedded languages in Emacs and AutoCAD use dynamic scoping. Since version 24.1, Emacs uses both dynamic and lexical scoping.

List structure of program code; exploitation by macros and compilers

A fundamental distinction between Lisp and other languages is that in Lisp, the textual representation of a program is simply a human-readable description of the same internal data structures (linked lists, symbols, number, characters, etc.) as would be used by the underlying Lisp system. Lisp uses this to implement a very powerful macro system. Like other macro languages such as the one defined by the

C preprocessor The C preprocessor is the macro preprocessor for the C, Objective-C and C++ computer programming languages. The preprocessor provides the ability for the inclusion of header files, macro expansions, conditional compilation, and line control ...

(the macro preprocessor for the C, Objective-C and

C++ C, or c, is the third Letter (alphabet), letter in the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is English alphabet#Le ...

programming languages), a macro returns code that can then be compiled. However, unlike C preprocessor macros, the macros are Lisp functions and so can exploit the full power of Lisp. Further, because Lisp code has the same structure as lists, macros can be built with any of the list-processing functions in the language. In short, anything that Lisp can do to a data structure, Lisp macros can do to code. In contrast, in most other languages, the parser's output is purely internal to the language implementation and cannot be manipulated by the programmer. This feature makes it easy to develop ''efficient'' languages within languages. For example, the Common Lisp Object System can be implemented cleanly as a language extension using macros. This means that if an application needs a different inheritance mechanism, it can use a different object system. This is in stark contrast to most other languages; for example, Java does not support multiple inheritance and there is no reasonable way to add it. In simplistic Lisp implementations, this list structure is directly interpreted to run the program; a function is literally a piece of list structure which is traversed by the interpreter in executing it. However, most substantial Lisp systems also include a compiler. The compiler translates list structure into machine code or bytecode for execution. This code can run as fast as code compiled in conventional languages such as C. Macros expand before the compilation step, and thus offer some interesting options. If a program needs a precomputed table, then a macro might create the table at compile time, so the compiler need only output the table and need not call code to create the table at run time. Some Lisp implementations even have a mechanism, eval-when, that allows code to be present during compile time (when a macro would need it), but not present in the emitted module.Time of Evaluation - Common Lisp Extensions
Gnu.org. Retrieved on 2013-07-17.

Evaluation and the read–eval–print loop

Lisp languages are often used with an interactive

command line A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive command (computing), commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invokin ...

, which may be combined with an

integrated development environment An integrated development environment (IDE) is a software application that provides comprehensive facilities to computer programmers for software development. An IDE normally consists of at least a source code editor, build automation tools an ...

(IDE). The user types in expressions at the command line, or directs the IDE to transmit them to the Lisp system. Lisp ''reads'' the entered expressions, ''evaluates'' them, and ''prints'' the result. For this reason, the Lisp command line is called a '' read–eval–print loop'' ( REPL). The basic operation of the REPL is as follows. This is a simplistic description which omits many elements of a real Lisp, such as quoting and macros. The function accepts textual S-expressions as input, and parses them into an internal data structure. For instance, if you type the text at the prompt, translates this into a linked list with three elements: the symbol , the number 1, and the number 2. It so happens that this list is also a valid piece of Lisp code; that is, it can be evaluated. This is because the car of the list names a function—the addition operation. Note that a will be read as a single symbol. will be read as the number one hundred and twenty-three. will be read as the string "123". The function evaluates the data, returning zero or more other Lisp data as a result. Evaluation does not have to mean interpretation; some Lisp systems compile every expression to native machine code. It is simple, however, to describe evaluation as interpretation: To evaluate a list whose car names a function, first evaluates each of the arguments given in its cdr, then applies the function to the arguments. In this case, the function is addition, and applying it to the argument list yields the answer . This is the result of the evaluation. The symbol evaluates to the value of the symbol foo. Data like the string "123" evaluates to the same string. The list evaluates to the list (1 2 3). It is the job of the function to represent output to the user. For a simple result such as this is trivial. An expression which evaluated to a piece of list structure would require that traverse the list and print it out as an S-expression. To implement a Lisp REPL, it is necessary only to implement these three functions and an infinite-loop function. (Naturally, the implementation of will be complex, since it must also implement all special operators like or .) This done, a basic REPL is one line of code: . The Lisp REPL typically also provides input editing, an input history, error handling and an interface to the debugger. Lisp is usually evaluated eagerly. In

, arguments are evaluated in applicative order ('leftmost innermost'), while in Scheme order of arguments is undefined, leaving room for optimization by a compiler.

Control structures

Lisp originally had very few control structures, but many more were added during the language's evolution. (Lisp's original conditional operator, , is the precursor to later structures.) Programmers in the Scheme dialect often express loops using

tail recursion In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion. Tail recur ...

. Scheme's commonality in academic computer science has led some students to believe that tail recursion is the only, or the most common, way to write iterations in Lisp, but this is incorrect. All oft-seen Lisp dialects have imperative-style iteration constructs, from Scheme's loop to

's complex expressions. Moreover, the key issue that makes this an objective rather than subjective matter is that Scheme makes specific requirements for the handling of

tail call In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion. Tail recu ...

s, and thus the reason that the use of tail recursion is generally encouraged for Scheme is that the practice is expressly supported by the language definition. By contrast, ANSI Common Lisp does not require3.2.2.3 Semantic Constraints
i

/ref> the optimization commonly termed a tail call elimination. Thus, the fact that tail recursive style as a casual replacement for the use of more traditional

iteration Iteration is the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. ...

constructs (such as , or ) is discouraged4.3. Control Abstraction (Recursion vs. Iteration) i
Tutorial on Good Lisp Programming Style
by

Kent Pitman Kent M. Pitman (KMP) is a programmer who has been involved for many years in the design, implementation, and use of systems based on the programming languages Lisp and Scheme. , he has been President of HyperMeta, Inc. Pitman was chair of the ad ...

and Peter Norvig, August, 1993. in Common Lisp is not just a matter of stylistic preference, but potentially one of efficiency (since an apparent tail call in Common Lisp may not compile as a simple

jump Jumping is a form of locomotion or movement in which an organism or non-living (e.g., robotic) mechanical system propels itself through the air along a ballistic trajectory. Jump or Jumping also may refer to: Places * Jump, Kentucky or Jump S ...

) and program correctness (since tail recursion may increase stack use in Common Lisp, risking

stack overflow In software, a stack overflow occurs if the call stack pointer exceeds the stack bound. The call stack may consist of a limited amount of address space, often determined at the start of the program. The size of the call stack depends on many fac ...

). Some Lisp control structures are ''special operators'', equivalent to other languages' syntactic keywords. Expressions using these operators have the same surface appearance as function calls, but differ in that the arguments are not necessarily evaluated—or, in the case of an iteration expression, may be evaluated more than once. In contrast to most other major programming languages, Lisp allows implementing control structures using the language. Several control structures are implemented as Lisp macros, and can even be macro-expanded by the programmer who wants to know how they work. Both Common Lisp and Scheme have operators for non-local control flow. The differences in these operators are some of the deepest differences between the two dialects. Scheme supports ''re-entrant continuations'' using the procedure, which allows a program to save (and later restore) a particular place in execution. Common Lisp does not support re-entrant continuations, but does support several ways of handling escape continuations. Often, the same algorithm can be expressed in Lisp in either an imperative or a functional style. As noted above, Scheme tends to favor the functional style, using tail recursion and continuations to express control flow. However, imperative style is still quite possible. The style preferred by many Common Lisp programmers may seem more familiar to programmers used to structured languages such as C, while that preferred by Schemers more closely resembles pure-functional languages such as Haskell. Because of Lisp's early heritage in list processing, it has a wide array of higher-order functions relating to iteration over sequences. In many cases where an explicit loop would be needed in other languages (like a loop in C) in Lisp the same task can be accomplished with a higher-order function. (The same is true of many functional programming languages.) A good example is a function which in Scheme is called and in Common Lisp is called . Given a function and one or more lists, applies the function successively to the lists' elements in order, collecting the results in a new list: (mapcar #'+ '(1 2 3 4 5) '(10 20 30 40 50)) This applies the function to each corresponding pair of list elements, yielding the result .

Examples

Here are examples of Common Lisp code. The basic " Hello, World!" program: (print "Hello, World!") Lisp syntax lends itself naturally to recursion. Mathematical problems such as the enumeration of recursively defined sets are simple to express in this notation. For example, to evaluate a number's factorial: (defun factorial (n) (if (zerop n) 1 (* n (factorial (1- n))))) An alternative implementation takes less stack space than the previous version if the underlying Lisp system optimizes

: (defun factorial (n &optional (acc 1)) (if (zerop n) acc (factorial (1- n) (* acc n)))) Contrast the examples above with an iterative version which uses

's macro: (defun factorial (n) (loop for i from 1 to n for fac = 1 then (* fac i) finally (return fac))) The following function reverses a list. (Lisp's built-in ''reverse'' function does the same thing.) (defun -reverse (list) (let ((return-value)) (dolist (e list) (push e return-value)) return-value))

Object systems

Various object systems and models have been built on top of, alongside, or into Lisp, including: * The

, CLOS, is an integral part of ANSI Common Lisp. CLOS descended from New Flavors and CommonLOOPS. ANSI Common Lisp was the first standardized object-oriented programming language (1994, ANSI X3J13). * ObjectLisppg 17 of Bobrow 1986 or Object Lisp, used by Lisp Machines Incorporated and early versions of Macintosh Common Lisp * LOOPS (Lisp Object-Oriented Programming System) and the later

CommonLoops CommonLoops (the Common Lisp Object-Oriented Programming System; an acronym reminiscent of the earlier Lisp OO system "Loops" for the Interlisp-D system) is an early programming language which extended Common Lisp to include Object-oriented program ...

, built at MIT, and its descendant New Flavors (developed by

Symbolics Symbolics was a computer manufacturer Symbolics, Inc., and a privately held company that acquired the assets of the former company and continues to sell and maintain the Open Genera Lisp system and the Macsyma computer algebra system.

). * KR (short for Knowledge Representation), a constraints-based object system developed to aid the writing of Garnet, a GUI library for

. *

Knowledge Engineering Environment Knowledge Engineering Environment (KEE) is a frame-based development tool for expert systems. It was developed and sold by IntelliCorp, and first released in 1983. It ran on Lisp machines, and was later ported to Lucid Common Lisp with the CL ...

(KEE) used an object system named UNITS and integrated it with an

inference engine In the field of artificial intelligence, an inference engine is a component of the system that applies logical rules to the knowledge base to deduce new information. The first inference engines were components of expert systems. The typical expert ...

Veitch, p 108, 1988 and a truth maintenance system (ATMS).

Operating systems

Several

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also ...

s, including language-based systems, are based on Lisp (use Lisp features, conventions, methods, data structures, etc.), or are written in Lisp, including:

Genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomencl ...

, renamed Open Genera, by

; Medley, written in Interlisp, originally a family of graphical operating systems that ran on

Xerox Xerox Holdings Corporation (; also known simply as Xerox) is an American corporation that sells print and digital document products and services in more than 160 countries. Xerox is headquartered in Norwalk, Connecticut (having moved from St ...

's later

Star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Eart ...

workstation A workstation is a special computer designed for technical or scientific applications. Intended primarily to be used by a single user, they are commonly connected to a local area network and run multi-user operating systems. The term ''works ...

s; Mezzano; Interim; ChrysaLisp, by developers of Tao Systems' TAOS.

References

External links

;History
History of Lisp
– John McCarthy's history of 12 February 1979
Lisp History
– Herbert Stoyan's history compiled from the documents (acknowledged by McCarthy as more complete than his own, see
McCarthy's history links

History of LISP at the Computer History Museum
* about the use of LISP software on NASA robots. * ;Associations and meetings
Association of Lisp Users

European Common Lisp Meeting

European Lisp Symposium

International Lisp Conference
;Books and tutorials *

', a comic-book style introductory tutorial *

', a free book by Paul Graham *
Practical Common Lisp
', freeware edition by Peter Seibel
Lisp for the web

Land of Lisp

Let over Lambda
;Interviews
Oral history interview with John McCarthy
at Charles Babbage Institute, University of Minnesota, Minneapolis. McCarthy discusses his role in the development of time-sharing at the Massachusetts Institute of Technology. He also describes his work in artificial intelligence (AI) funded by the Advanced Research Projects Agency, including logic-based AI (LISP) and robotics.
Interview
with Richard P. Gabriel (Podcast) ;Resources
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* {{DEFAULTSORT:Lisp Programming Language Academic programming languages American inventions Articles with example Lisp (programming language) code Dynamically typed programming languages Extensible syntax programming languages Functional languages Programming languages Programming languages created in 1958