2.2 Data structures

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 bitThis article is about the unit of information, see Bit (disambiguation) for other meanings. A bit (abbreviated b is the most basic information unit used in computing and information theory. A single bit (short for b inary dig it is a zero or a one, or a ts.

Hash tableIn computer science, a hash table is a data structure that speeds up searching for information by a particular aspect of that information, called a key''. If the information is a set of customer records, for example, the keys might include first name, lass 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 namespaceIn general, a namespace is an abstract container, which is or could be filled by names, or technical terms, or words, and these represent (stand for) real-world things. As a rule, names in a namespace cannot have more than one meaning, that is, two or mors. A package may export some symbols, marking them as part of a public interface.

Structures, similar in use to C structs and Pascal records, represent arbitrary complex data structures with any number and type of fields (called slots).

2.3 Functions

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:

1. A special operator (easily checked against a fixed list)
2. A macro operator (must have been defined previously)
3. The name of a function (default), which may either be a symbol, or a sub-form beginning with the symbol lambda.

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.

2.3.1 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 defun, flet, and labels.

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 would be illegal in Scheme as they clash with 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. [2]

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.

2.4 Other types

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.

Common Lisp also includes a toolkit for object-oriented programming, the Common Lisp Object System or CLOS.

2.5 Macros

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.

2.5.1 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 gensym s -- guaranteed-unique symbols which can be used in a macroexpansion 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")))

The 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 the 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.

3 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 only dynamically-scoped variables. Scheme introduced lexically-scoped variables to Lisp, which were 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".

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 lis, lst, or 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.

4 Implementations

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.

Most Unix-based implementations, such as CLISP, can be used as script interpreters; that is, invoked by the system transparently in the way that a Perl or Unix shell interpreter is.

4.1 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." [3] 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]), 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." [4] 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 a Java programming language Virtual Machine. It includes a compiler to Java programming language bytecodes, and allows access to Java libraries from Common Lisp.

Commercial implementations are available from Franz, Inc., Xanalys Corp., Digitool, Inc., Corman Technologies and Scieneer Pty Ltd..

5 Applications

Common Lisp is used in many successful commercial applications, the most famous (no doubt due to Paul Graham's promotion) being the Yahoo! Store web-commerce site. Several notable examples are:

• 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:

• acl2 : 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 it's 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.

6 External links

• 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.
• 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.
• The Two Towers: Face to Face With Gollum, an interesting article on how Mirai was used in making Gollum's face.

LISP dialects LISP programming language family Functional languages Major programming languages Object-oriented programming languages

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