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Common Lisp, commonly abbreviated CL (not to be confused with Combinatory logic which is also abbreviated CL), is a dialect of Lisp, standardised by ANSI X3.226-1994. Developed to standardize the divergent variants of Lisp which predated it, it is not an implementation but rather a language specification to which most Lisp implementations conform.
Common Lisp is a general-purpose programming language, in contrast to Lisp variants such as Emacs Lisp and AutoLISP which are embedded extension languages in particular products. Unlike many earlier Lisps, but like Scheme, Common Lisp uses lexical scoping for variables.
Common Lisp is a multi-paradigm programming language that:
- Supports programming techniques such as imperative, functional and object-oriented programming.
- Is dynamically typed, but with optional type declarations that can improve efficiency or safety.
- Is extensible through standard features such as Lisp macros (compile-time code rearrangement accomplished by the program itself) and reader macros (extension of syntax to give special meaning to characters reserved for users for this purpose).
Common Lisp is a Lisp; it uses S-expressions to denote both code and data structure. Function and macro calls are written as lists, with the name of the function first, as in these examples:
(+ 2 2) ; adds 2 and 2, yielding 4
(setq p 3.1415) ; sets the variable "p" equal to 3.1415
;Define a function that squares a number: (defun square (x) (* x x)) ;Execute the function: (square 3) ; Returns "9"
Common Lisp has a plethora of data types, more than many languages.
Number types include integers, ratios, floating-point numbers, and complex numbers. Common Lisp uses bignums to represent numerical values of arbitrary size and precision. The ratio type represents fractions exactly, a facility not available in many languages. Common Lisp automatically coerces numeric values among these types as appropriate.
The symbol type is common to Lisp languages, but largely unknown outside them. A symbol is a unique, named data object. Symbols in Lisp are similar to identifiers in other languages, in that they are used as variables to hold values; however, they are more general and can be used for themselves as well. Normally, when a symbol is evaluated, its value as a variable is returned. Exceptions exist: keyword symbols such as :foo evaluate to themselves, and Boolean values in Common Lisp are represented by the reserved symbols T and NIL.
Sequence types in Common Lisp include lists, vectors, bit-vectors, and strings.
As in any other Lisp, lists in Common Lisp are composed of conses, sometimes called cons cells or pairs. A cons is a data structure with two slots, called its car and cdr. A list is a linked chain of conses. Each cons's car refers to a member of the list (possibly another list). Each cons's cdr refers to the next cons -- except for the last cons, whose cdr refers to the nil value. Conses can also easily be used to implement trees and other complex data structures; though it is usually advised to use structure or class instances instead.
Common Lisp supports multidimensional arrays, and can dynamically resize arrays if required. Multidimensional arrays can be used for matrix mathematics. A vector is a one-dimensional array. Arrays can carry any type as members (even mixed types in the same array) or can be specialized to contain a specific type of members, as in a vector of integers. Many implementations can optimize array functions when the array used is type-specialized. Two type-specialized array types are standard: a string is a vector of characters, while a bit-vector is a vector of bits.
Hash tables store associations between data objects. Any object may be used as key or value. Hash tables, like arrays, are automatically resized as needed.
Packages are collections of symbols, used chiefly to separate the parts of a program into namespaces. A package may export some symbols, marking them as part of a public interface.
Class instances, similar to structures, but created by the object system, CLOS.
In Common Lisp, the type of functions is a data type. For instance, it is possible to write functions that take other functions as arguments or return functions as well. This makes it possible to describe very general operations.
The Common Lisp library relies heavily on such higher-order functions. For example, the
sort function takes a comparison operator as an argument. This can be used not only to sort any type of data, but also to sort data structures according to a key.
(sort (list 5 2 6 3 1 4) #'>) ; Sorts the list using the > function as the comparison operator. ; Returns (6 5 4 3 2 1). (sort (list '(9 a) '(3 b) '(4 c)) #'(lambda (x y) (< (car x) (car y)))) ; Sorts the list according to the first element (car) of each sub-list. ; Returns ((3 b) (4 c) (9 a)).
The evaluation model for functions is very simple. When the evaluator encounters a form
(F A1 A2...) then it is to assume that the symbol named F is one of the following:
- A special operator (easily checked against a fixed list)
- A macro operator (must have been defined previously)
- The name of a function (default), which may either be a symbol, or a sub-form beginning with the symbol
If F is the name of a function, then the arguments A1, A2, ..., An are evaluated in left-to-right order, and the function is found and invoked with those values supplied as parameters.
The function namespace
There is a key difference between Common Lisp and Scheme here, however. In CL, the function name is looked up in a namespace that is separate from the namespace for data variables; it is called the function namespace. Operators which define names in the function namespace include
To pass a function by name as an argument to another function, one must use the
function special operator, commonly abbreviated as #'. The first
sort example above refers to the function named by the symbol
> in the function namespace, with the code
Scheme's evaluation model is simpler: there is only one namespace, and all positions in the form are evaluated (in any order) -- not just the arguments. Code written in one dialect is therefore sometimes confusing to programmers more experienced in the other. For instance, many CL programmers like to use descriptive variable names such as list or string which could cause problems in Scheme as they would locally shadow function names.
Whether a separate namespace for functions is an advantage is a source of contention in the Lisp community. It is usually referred to as the Lisp-1 vs. Lisp-2 debate. These names were coined in a 1988 paper by Richard P. Gabriel , which extensively compares the two approaches. 
Finally, while a function definition (a
defun form) is a list, functions are not generally internally represented as lists. Most Common Lisp systems compile functions individually to bytecode or machine code.
Other data types in Common Lisp include:
- Pathnames represent files and directories in the filesystem. Because historically Lisp was separate from Unix, The Common Lisp pathname facility is more general than most operating systems' file naming conventions, making Lisp programs' access to files broadly portable across diverse systems.
- Input and output streams represent sources and sinks of binary or textual data, such as the terminal or open files.
- Common Lisp has a built-in pseudo-random number generator. Random state objects represent reusable sources of pseudo-random numbers, allowing the user to seed the PRNG or cause it to replay a sequence.
- Conditions are a special type used to represent errors, exceptions, and other "interesting" events to which a program may respond.
- Classes are first-class objects, and are themselves instances of classes called metaclasses.
A macro in Lisp superficially resembles a function in usage. However, rather than representing an expression which is evaluated, it represents a transformation of the program source code.
Macros allow Lisp programmers to create new syntactic forms in the language. For instance, this macro provides the
until loop form, which may be familiar from languages such as Perl:
(defmacro until (test &body body) `(do () (, test) ,@body)) ;; example (until (= (random 10) 0) (write-line "Hello"))
All macros must be expanded before the source code containing them can be evaluated or compiled normally. Macros can be considered functions that accept and return abstract syntax trees (Lisp S-expressions). These functions are invoked before the evaluator or compiler to produce the final source code. Macros are written in normal Common Lisp, and may use any Common Lisp (or third-party) operator available. The backquote notation used above is provided by Common Lisp specifically to simplify the common case of substitution into a code template.
Variable capture and shadowing
Common Lisp macros are capable of variable capture, a situation in which symbols in the macro-expansion body coincide with those in the calling context. Variable capture is sometimes a desired effect: it allows the programmer to create macros wherein various symbols have special meaning. However, it can also introduce unexpected and unusual errors.
Some Lisp systems, such as Scheme, avoid variable capture by using macro syntax — so-called "hygienic macros" — that does not allow it. In Common Lisp, one can avoid unwanted capture by using gensyms : guaranteed-unique symbols which can be used in a macro-expansion without threat of capture.
Another issue is the inadvertant shadowing of operators used in a macroexpansion. For example, consider the following (incorrect) code:
(macrolet ((do (...) ... something else ...)) (until (= (random 10) 0) (write-line "Hello")))
UNTIL macro will expand into a form which calls
DO which is intended to refer to the built-in special form
DO. However, in this context,
DO may have a completely different meaning.
Common Lisp ameliorates the problem of operator shadowing by forbidding the redefinition of built-in operators, such as
DO in this example. Moreover, users may separate their own code into packages. Built-in symbols are found in the
COMMON-LISP package, which will not be shadowed by a symbol in a user package.
Comparison with other Lisps
Common Lisp is most frequently compared with, and contrasted to, Scheme—if only because they are the two most popular Lisp dialects. Scheme antedates CL, and comes not only from the same Lisp tradition but from some of the same engineers -- Guy L. Steele, who with Gerald Jay Sussman designed Scheme, chaired the standards committee for Common Lisp.
Most of the Lisp systems whose designs contributed to Common Lisp -- such as Zetalisp and Franz Lisp -- used dynamically-scoped variables in their interpreters and lexically-scoped variables in their compilers. Scheme introduced the sole use of lexically-scoped variables to Lisp (an inspiration from Algol68), which was widely recognized as a good idea and adopted into CL. CL supports dynamically-scoped variables as well, but they must be explicitly declared as "special". There are no differences in scoping between ANSI CL interpreters and compilers.
Common Lisp is sometimes termed a Lisp-2 and Scheme a Lisp-1, referring to CL's use of separate namespaces for functions and variables. (In fact, CL has many namespaces, such as those for go tags, block names, and
loop keywords.) There is a long-standing controversy between CL and Scheme advocates over the tradeoffs involved in multiple namespaces. In Scheme, it is (broadly) necessary to avoid giving variables names which clash with functions; Scheme functions frequently have arguments named
lyst so as not to conflict with the system function
list. However, in CL it is necessary to explicitly refer to the function namespace when passing a function as an argument -- which is also a common occurrence, as in the
sort example above.
CL also differs from Scheme in its handling of boolean values. Scheme uses the special values #t and #f to represent truth and falsity. CL follows the older Lisp convention of using the symbols T and NIL, with NIL standing also for the empty list.
Lastly, the Scheme standards documents require tail-call optimization, which the CL standard does not. Most CL implementations do offer tail-call optimization, although often only when the programmer uses an optimization directive. Nonetheless, common CL coding style does not favor the ubiquitous use of recursion that Scheme style prefers -- what a Scheme programmer would express with tail recursion, a CL user would usually express with an iterative expression in
loop, or (more recently) with the
Common Lisp is defined by a specification (like Ada and C) rather than by a single implementation (like Perl). There are many implementations, and the standard spells out areas in which they may validly differ.
In addition, implementations tend to come with library packages, which provide functionality not covered in the standard. Free Software libraries have been created to support such features in a portable way, most notably the Common Lisp Open Code Collection project.
Common Lisp has been designed to be implemented by incremental compilers. Standard declarations to optimize compilation (such as function inlining) are proposed in the language specification. Most Common Lisp implementations compile functions to native machine code. Others compile to bytecode, which reduces speed but eases binary-code portability. The misconception that Lisp is a purely-interpreted language is most likely due to the fact that Common Lisp environments provide an interactive prompt and that functions are compiled one-by-one, in an incremental way.
List of implementations
Freely redistributable implementations include:
- CMUCL, originally from Carnegie Mellon University, now maintained as Free Software by a group of volunteers. CMUCL uses a fast native-code compiler. It is available on Linux and BSD for Intel x86; Linux for Alpha; and Solaris, IRIX, and HP-UX on their native platforms.
- GNU CLISP, a bytecode-compiling implementation. It is portable and runs on a number of Unix and Unix-like systems (including Mac OS X), as well as Microsoft Windows and several other systems.
- Steel Bank Common Lisp (SBCL), a branch from CMUCL. "Broadly speaking, SBCL is distinguished from CMU CL by a greater emphasis on maintainability."  SBCL runs on the platforms CMUCL does, except HP/UX; in addition, it runs on Linux for PowerPC, SPARC, and MIPS, and on Mac OS X. SBCL does not use an interpreter; all expressions are compiled to native code.
- GNU Common Lisp (GCL), the GNU Project's Lisp compiler. Not yet fully ANSI-compliant, GCL is however the implementation of choice for several large projects including the mathematical tools Maxima and ACL2. GCL runs under GNU/Linux on eleven different architectures, and also under Windows, Solaris, and FreeBSD.
- Embeddable Common Lisp (ECLS), designed to be embedded in C applications;
- OpenMCL, an open source branch of Macintosh Common Lisp. As the name implies, OpenMCL is native to the Macintosh; it runs on Mac OS X, Darwin, and Linux for PowerPC.
- Movitz implements a Lisp environment for x86 computers without relying on any underlying OS.
- Armed Bear Common Lisp Armed Bear Lisp is a Common Lisp implementation that runs on the Java Virtual Machine. It includes a compiler to Java byte codes, and allows access to Java libraries from Common Lisp.
- Jatha is a Java library that implements a fairly large subset of Common LISP
- Orbitz, a major travel booking site powered by Lisp.
- Mirai, Izware LLC's fully integrated 2d/3d computer graphics content creation suite that features what is almost universally regarded as the best polygonal modeler in the industry, an advanced IK/FK and non-linear animation system (later popularized by such products as Sega's Animanium and Softimage XSI, respectively), and advanced 2d and 3d painting. It is used in major motion pictures (most famously in New Line Cinema's Lord of the Rings), video games and military simulations.
- Xanalys Corp.'s line of investigation software, used by police, security and fraud prevention services worldwide.
- Knowledge Technologies International ICAD mechanical design software, one of the leading products in the field.
There also exist successful open-source applications written in Common Lisp, such as:
- Applicative Common Lisp, a full-featured theorem prover for a subset of Common Lisp.
- Maxima, a sophisticated computer algebra system.
- Compo, a language allowing complex musical structures to be described in a natural way.
- Lisa, a production-rule system to build "intelligent" software agents.
As well, Common Lisp is used by many government and non-profit institutions. Examples of its use in NASA include:
- SPIKE, the Hubble Space Telescope planning and scheduling system.
- Remote Agent, winner of the 1999 NASA Software of the Year Award.
- The Common Lisp HyperSpec, a hypertext version of the Common Lisp standard.
- The CLiki, a Wiki for Free Software Common Lisp systems running on Unix-like systems.
- The Association of Lisp Users.
- The quick guide to starting with Common Lisp.
- Common Lisp the Language, 2nd Edition, known as "CLtL2". Guy Steele's book on Common Lisp, which served as the basis for the ANSI Common Lisp standard.
- The Common Lisp Cookbook, a collection of useful programming methods.
- Paul Graham's page about Lisp. If you follow the links you can find his book On Lisp (available online), the book on macro design in Common Lisp.
- Common Lisp: A Gentle Introduction to Symbolic Computation by David S. Touretsky, available online and aimed at beginners.
- Peter Norvig's page containing many interesting resources about Common Lisp.
- Practical Common Lisp
- Casting Spells in Lisp A cartoon introduction to Common Lisp
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