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In
computer programming Computer programming is the process of performing a particular computation (or more generally, accomplishing a specific computing result), usually by designing and building an executable computer program. Programming involves tasks such as ana ...
, a macro (short for "macro instruction"; ) is a rule or
pattern A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated l ...
that specifies how a certain input should be mapped to a replacement output. Applying a macro to an input is known as macro expansion. The input and output may be a sequence of
lexical token In computer science, lexical analysis, lexing or tokenization is the process of converting a sequence of characters (such as in a computer program or web page) into a sequence of ''lexical tokens'' ( strings with an assigned and thus identified ...
s or
characters Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
, or a
syntax tree Syntax tree may refer to: * Abstract syntax tree, used in computer science * Concrete syntax tree A parse tree or parsing tree or derivation tree or concrete syntax tree is an ordered, rooted tree that represents the syntactic structure of a str ...
. Character macros are supported in
software application Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consists ...
s to make it easy to invoke common
command Command may refer to: Computing * Command (computing), a statement in a computer language * COMMAND.COM, the default operating system shell and command-line interpreter for DOS * Command key, a modifier key on Apple Macintosh computer keyboards * ...
sequences. Token and tree macros are supported in some
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 ...
s to enable
code reuse In software development (and computer programming in general), code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software, following the reusability principles. Code reuse may be achie ...
or to extend the language, sometimes for
domain-specific languages A domain-specific language (DSL) is a computer language specialized to a particular application Domain (software engineering), domain. This is in contrast to a general-purpose language (GPL), which is broadly applicable across domains. There are a ...
. Macros are used to make a sequence of computing instructions available to the programmer as a single
program statement In computer programming, a statement is a syntactic unit of an imperative programming language that expresses some action to be carried out. A program written in such a language is formed by a sequence of one or more statements. A statement may ha ...
, making the programming task less tedious and less error-prone. (Thus, they are called "macros" because a "big" block of code can be expanded from a "small" sequence of characters.) Macros often allow positional or keyword parameters that dictate what the conditional assembler program generates and have been used to create entire programs or program suites according to such variables as
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 in ...
,
platform Platform may refer to: Technology * Computing platform, a framework on which applications may be run * Platform game, a genre of video games * Car platform, a set of components shared by several vehicle models * Weapons platform, a system or ...
or other factors. The term derives from "macro instruction", and such expansions were originally used in generating
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
code.


Keyboard and mouse macros

Keyboard macros and mouse macros allow short sequences of keystrokes and mouse actions to transform into other, usually more time-consuming, sequences of keystrokes and mouse actions. In this way, frequently used or repetitive sequences of keystrokes and mouse movements can be
automate Automation describes a wide range of technologies that reduce human intervention in processes, namely by predetermining decision criteria, subprocess relationships, and related actions, as well as embodying those predeterminations in machines ...
d. Separate programs for creating these macros are called
macro recorder A macro recorder is software that records macros for playback at a later time. The main advantage of using a macro recorder is that it allows a user to easily perform complex operations much faster and with less effort without requiring custom co ...
s. During the 1980s, macro programs – originally
SmartKey SmartKey was the first macro processing program of its type, and the first TSR or ''terminate and stay resident'' program for PCs and CP/M microcomputers, their eight bit predecessors. Smartkey's "keyboard definitions" were first used with the ...
, then SuperKey, KeyWorks, Prokey – were very popular, first as a means to automatically format
screenplay ''ScreenPlay'' is a television drama anthology series broadcast on BBC2 between 9 July 1986 and 27 October 1993. Background After single-play anthology series went off the air, the BBC introduced several showcases for made-for-television, fe ...
s, then for a variety of user input tasks. These programs were based on the TSR (
terminate and stay resident A terminate-and-stay-resident program (commonly TSR) is a computer program running under DOS that uses a system call to return control to DOS as though it has finished, but remains in computer memory so it can be reactivated later. This technique ...
) mode of operation and applied to all keyboard input, no matter in which context it occurred. They have to some extent fallen into obsolescence following the advent of mouse-driven user interfaces and the availability of keyboard and mouse macros in applications such as
word processor A word processor (WP) is a device or computer program that provides for input, editing, formatting, and output of text, often with some additional features. Word processor (electronic device), Early word processors were stand-alone devices ded ...
s and
spreadsheet A spreadsheet is a computer application for computation, organization, analysis and storage of data in tabular form. Spreadsheets were developed as computerized analogs of paper accounting worksheets. The program operates on data entered in cel ...
s, making it possible to create application-sensitive keyboard macros. Keyboard macros can be used in
massively multiplayer online role-playing game A massively multiplayer online role-playing game (MMORPG) is a video game that combines aspects of a role-playing video game and a massively multiplayer online game. As in role-playing games (RPGs), the player assumes the role of a Player charac ...
s (MMORPGs) to perform repetitive, but lucrative tasks, thus accumulating resources. As this is done without human effort, it can skew the economy of the game. For this reason, use of macros is a violation of the TOS or
EULA An end-user license agreement or EULA () is a legal contract between a software supplier and a customer or end-user, generally made available to the customer via a retailer acting as an intermediary. A EULA specifies in detail the rights and restr ...
of most MMORPGs, and their administrators spend considerable effort to suppress them.


Application macros and scripting

Keyboard and mouse macros that are created using an application's built-in macro features are sometimes called application macros. They are created by carrying out the sequence once and letting the application record the actions. An underlying macro programming language, most commonly a
scripting language A scripting language or script language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. Scripting languages are usually interpreted at runtime rather than compiled. A scripting ...
, with direct access to the features of the application may also exist. The programmers' text editor, Emacs, (short for "editing macros") follows this idea to a conclusion. In effect, most of the editor is made of macros. Emacs was originally devised as a set of macros in the editing language TECO; it was later ported to dialects of Lisp. Another programmers' text editor, Vim (a descendant of vi), also has an implementation of keyboard macros. It can record into a register (macro) what a person types on the keyboard and it can be replayed or edited just like VBA macros for Microsoft Office. Vim also has a scripting language called Vimscript to create macros.
Visual Basic for Applications Visual Basic for Applications (VBA) is an implementation of Microsoft's event-driven programming language Visual Basic 6.0 built into most desktop Microsoft Office applications. Although based on pre-.NET Visual Basic, which is no longer support ...
(VBA) is a programming language included in
Microsoft Office Microsoft Office, or simply Office, is the former name of a family of client software, server software, and services developed by Microsoft. It was first announced by Bill Gates on August 1, 1988, at COMDEX in Las Vegas. Initially a marketin ...
from Office 97 through Office 2019 (although it was available in some components of Office prior to Office 97). However, its function has evolved from and replaced the macro languages that were originally included in some of these applications.
XEDIT XEDIT is a visual editor for VM/CMS using block mode IBM 3270 terminals. (Line-mode terminals are also supported.) XEDIT is much more line-oriented than modern PC and Unix editors. For example, XEDIT supports automatic line numbers, and ma ...
, running on the
Conversational Monitor System The Conversational Monitor System (CMS – originally: "Cambridge Monitor System") is a simple interactive computing, interactive single-user operating system. CMS was originally developed as part of IBM's CP/CMS operating system, which went ...
(CMS) component of VM, supports macros written in
EXEC Exec or EXEC may refer to: * Executive officer, a person responsible for running an organization * Executive producer, provides finance and guidance for the making of a commercial entertainment product * A family of kit helicopters produced by Rot ...
,
EXEC2 EXEC 2 is an interpreted, command procedure control, computer scripting language used by the EXEC 2 Processor originally supplied with the CMS component of the IBM Virtual Machine/System Product (VM/SP) operating system. Relation to EXEC EXEC 2 ...
and
REXX Rexx (Restructured Extended Executor) is a programming language that can be interpreted or compiled. It was developed at IBM by Mike Cowlishaw. It is a structured, high-level programming language designed for ease of learning and reading. ...
, and some CMS commands were actually wrappers around XEDIT macros.
The Hessling Editor XEDIT is a visual editor for VM/CMS using block mode IBM 3270 terminals. (Line-mode terminals are also supported.) XEDIT is much more line-oriented than modern PC and Unix editors. For example, XEDIT supports automatic line numbers, and ma ...
(THE), a partial clone of XEDIT, supports Rexx macros using Regina and Open Object REXX (oorexx). Many common applications, and some on PCs, use Rexx as a scripting language.


Macro virus

VBA has access to most Microsoft Windows system calls and executes when documents are opened. This makes it relatively easy to write
computer virus A computer virus is a type of computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code. If this replication succeeds, the affected areas are then said to be "infected" with a compu ...
es in VBA, commonly known as
macro virus In computing terminology, a macro virus is a virus that is written in a macro language: a programming language which is embedded inside a software application (e.g., word processors and spreadsheet applications). Some applications, such as Micr ...
es. In the mid-to-late 1990s, this became one of the most common types of computer virus. However, during the late 1990s and to date,
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
has been patching and updating its programs. In addition, current anti-virus programs immediately counteract such attacks.


Parameterized macro

A parameterized macro is a macro that is able to insert given objects into its expansion. This gives the macro some of the power of a
function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...
. As a simple example, in the
C programming language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
, this is a typical macro that is ''not'' a parameterized macro: #define PI 3.14159 This causes PI to always be replaced with 3.14159 wherever it occurs. An example of a parameterized macro, on the other hand, is this: #define pred(x) ((x)-1) What this macro expands to depends on what
argument An argument is a statement or group of statements called premises intended to determine the degree of truth or acceptability of another statement called conclusion. Arguments can be studied from three main perspectives: the logical, the dialectic ...
''x'' is passed to it. Here are some possible expansions: pred(2) → ((2) -1) pred(y+2) → ((y+2) -1) pred(f(5)) → ((f(5))-1) Parameterized macros are a useful source-level mechanism for performing in-line expansion, but in languages such as C where they use simple textual substitution, they have a number of severe disadvantages over other mechanisms for performing in-line expansion, such as
inline function In the C and C++ programming languages, an inline function is one qualified with the keyword inline; this serves two purposes: # It serves as a compiler directive that suggests (but does not require) that the compiler substitute the body of ...
s. The parameterized macros used in languages such as Lisp,
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. I ...
and
Scheme A scheme is a systematic plan for the implementation of a certain idea. Scheme or schemer may refer to: Arts and entertainment * ''The Scheme'' (TV series), a BBC Scotland documentary series * The Scheme (band), an English pop band * ''The Schem ...
, on the other hand, are much more powerful, able to make decisions about what code to produce based on their arguments; thus, they can effectively be used to perform
run-time code generation In computer science, self-modifying code (SMC) is code that alters its own instructions while it is executing – usually to reduce the instruction path length and improve performance or simply to reduce otherwise repetitively similar code, ...
.


Text-substitution macros

Languages such as C and some
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
s have rudimentary macro systems, implemented as
preprocessor In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which is often used by so ...
s to the compiler or assembler.
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 ...
macros work by simple textual substitution at the token, rather than the character level. However, the macro facilities of more sophisticated assemblers, e.g.,
IBM High Level Assembler High Level Assembler or HLASM is IBM's current assembler programming language for its z/OS, z/VSE, z/VM and z/TPF operating systems on z/Architecture mainframe computers. There is also a version that runs on Linux, primarily intended for sys ...
(HLASM) can't be implemented with a preprocessor; the code for assembling instructions and data is interspersed with the code for assembling macro invocations. A classic use of macros is in the computer typesetting system
TeX Tex may refer to: People and fictional characters * Tex (nickname), a list of people and fictional characters with the nickname * Joe Tex (1933–1982), stage name of American soul singer Joseph Arrington Jr. Entertainment * ''Tex'', the Italian ...
and its derivatives, where most of the functionality is based on macros. MacroML is an experimental system that seeks to reconcile
static 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 progra ...
and macro systems.
Nemerle Nemerle is a general-purpose, high-level, statically typed programming language designed for platforms using the Common Language Infrastructure ( .NET/Mono). It offers functional, object-oriented, aspect-oriented, reflective and imperative fea ...
has typed syntax macros, and one productive way to think of these syntax macros is as a multi-stage computation. Other examples: * m4 is a sophisticated stand-alone macro processor. *
TRAC Trac is an open-source, web-based project management and bug tracking system. It has been adopted by a variety of organizations for use as a bug tracking system for both free and open-source software and proprietary projects and products. Tra ...
* Macro Extension TAL, accompanying
Template Attribute Language The Template Attribute Language (TAL) is a templating language used to generate dynamic HTML and XML pages. Its main goal is to simplify the collaboration between programmers and designers. This is achieved by embedding TAL statements inside valid ...
* SMX: for web pages * ML/1 (Macro Language One) * The
General Purpose Macrogenerator A general-purpose macro processor or general purpose preprocessor is a macro processor that is not tied to or integrated with a particular language or piece of software. A macro processor is a program that copies a stream of text from one place ...
is a contextual pattern-matching macro processor, which could be described as a combination of
regular expression A regular expression (shortened as regex or regexp; sometimes referred to as rational expression) is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for "find" ...
s, EBNF and
AWK AWK (''awk'') is a domain-specific language designed for text processing and typically used as a data extraction and reporting tool. Like sed and grep, it is a filter, and is a standard feature of most Unix-like operating systems. The AWK lang ...
*
troff troff (), short for "typesetter roff", is the major component of a document processing system developed by Bell Labs for the Unix operating system. troff and the related nroff were both developed from the original roff. While nroff was inte ...
and
nroff nroff (short for "new roff") is a text-formatting computer program, program on Unix and Unix-like operating systems. It produces output suitable for simple fixed-width computer printer, printers and computer terminal, terminal windows. It is an ...
: for typesetting and formatting Unix manpages. *
CMS EXEC CMS EXEC, or EXEC, is an interpreted, command procedure control, computer scripting language used by the CMS EXEC Processor supplied with the IBM Virtual Machine/Conversational Monitor System (VM/CMS) operating system. EXEC was written in 1966 b ...
: for command-line macros and application macros *
EXEC 2 EXEC 2 is an interpreted, command procedure control, computer scripting language used by the EXEC 2 Processor originally supplied with the CMS component of the IBM Virtual Machine/System Product ( VM/SP) operating system. Relation to EXEC EXEC 2 ...
in
Conversational Monitor System The Conversational Monitor System (CMS – originally: "Cambridge Monitor System") is a simple interactive computing, interactive single-user operating system. CMS was originally developed as part of IBM's CP/CMS operating system, which went ...
(CMS): for command-line macros and application macros *
CLIST CLIST (Command List) (pronounced "C-List") is a procedural programming language for TSO in MVS systems. It originated in OS/360 Release 20 and has assumed a secondary role since the availability of Rexx in TSO/E Version 2. The term CLIST is als ...
in IBM's
Time Sharing Option Time Sharing Option (TSO) is an interactive time-sharing environment for IBM mainframe operating systems, including OS/360 MVT, OS/VS2 (SVS), MVS, OS/390, and z/OS. Use In computing, time-sharing is a design technique that allows many peop ...
(TSO): for command-line macros and application macros *
REXX Rexx (Restructured Extended Executor) is a programming language that can be interpreted or compiled. It was developed at IBM by Mike Cowlishaw. It is a structured, high-level programming language designed for ease of learning and reading. ...
: for command-line macros and application macros in, e.g.,
AmigaOS AmigaOS is a family of proprietary native operating systems of the Amiga and AmigaOne personal computers. It was developed first by Commodore International and introduced with the launch of the first Amiga, the Amiga 1000, in 1985. Early versions ...
, CMS,
OS/2 OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
, TSO *
SCRIPT Script may refer to: Writing systems * Script, a distinctive writing system, based on a repertoire of specific elements or symbols, or that repertoire * Script (styles of handwriting) ** Script typeface, a typeface with characteristics of handw ...
: for formatting documents * Various shells for, e.g.,
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...
Some major applications have been written as text macro invoked by other applications, e.g., by
XEDIT XEDIT is a visual editor for VM/CMS using block mode IBM 3270 terminals. (Line-mode terminals are also supported.) XEDIT is much more line-oriented than modern PC and Unix editors. For example, XEDIT supports automatic line numbers, and ma ...
in CMS.


Embeddable languages

Some languages, such as
PHP PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group. ...
, can be embedded in free-format text, or the source code of other languages. The mechanism by which the code fragments are recognised (for instance, being bracketed by and ?>) is similar to a textual macro language, but they are much more powerful, fully featured languages.


Procedural macros

Macros in the
PL/I PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. I ...
language are written in a subset of PL/I itself: the compiler executes "
preprocessor In computer science, a preprocessor (or precompiler) is a program that processes its input data to produce output that is used as input in another program. The output is said to be a preprocessed form of the input data, which is often used by so ...
statements" at compilation time, and the output of this execution forms part of the code that is compiled. The ability to use a familiar
procedural language Procedural programming is a programming paradigm, derived from imperative programming, based on the concept of the ''procedure call''. Procedures (a type of routine or subroutine) simply contain a series of computational steps to be carried ...
as the macro language gives power much greater than that of text substitution macros, at the expense of a larger and slower compiler. Frame technology's frame macros have their own command syntax but can also contain text in any language. Each frame is both a generic component in a hierarchy of nested subassemblies, and a procedure for integrating itself with its subassembly frames (a recursive process that resolves integration conflicts in favor of higher level subassemblies). The outputs are custom documents, typically compilable source modules. Frame technology can avoid the proliferation of similar but subtly different components, an issue that has plagued software development since the invention of macros and
subroutine In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Functions may ...
s. Most assembly languages have less powerful procedural macro facilities, for example allowing a block of code to be repeated N times for
loop unrolling Loop unrolling, also known as loop unwinding, is a loop transformation technique that attempts to optimize a program's execution speed at the expense of its binary size, which is an approach known as space–time tradeoff. The transformation ...
; but these have a completely different syntax from the actual assembly language.


Syntactic macros

Macro systems—such as the C preprocessor described earlier—that work at the level of lexical tokens cannot preserve the lexical structure reliably. Syntactic macro systems work instead at the level of
abstract syntax tree In computer science, an abstract syntax tree (AST), or just syntax tree, is a tree representation of the abstract syntactic structure of text (often source code) written in a formal language. Each node of the tree denotes a construct occurring ...
s, and preserve the lexical structure of the original program. The most widely used implementations of syntactic macro systems are found in Lisp-like languages. These languages are especially suited for this style of macro due to their uniform, parenthesized syntax (known as
S-expression In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested list (tree-structured) data. S-expressions were invented for and popularized by the programming la ...
s). In particular, uniform syntax makes it easier to determine the invocations of macros. Lisp macros transform the program structure itself, with the full language available to express such transformations. While syntactic macros are often found in Lisp-like languages, they are also available in other languages such as
Prolog Prolog is a logic programming language associated with artificial intelligence and computational linguistics. Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is intended primarily ...
, Erlang, Dylan, Scala,
Nemerle Nemerle is a general-purpose, high-level, statically typed programming language designed for platforms using the Common Language Infrastructure ( .NET/Mono). It offers functional, object-oriented, aspect-oriented, reflective and imperative fea ...
,
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH ...
,
Elixir ELIXIR (the European life-sciences Infrastructure for biological Information) is an initiative that will allow life science laboratories across Europe to share and store their research data as part of an organised network. Its goal is to bring t ...
, Nim,
Haxe Haxe is an open source high-level cross-platform programming language and compiler that can produce applications and source code, for many different computing platforms from one code-base. It is free and open-source software, released under the ...
, and
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.g ...
. They are also available as third-party extensions 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 Website, websites use JavaScript on the Client (computing), client side ...
, C# and
Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...
.


Early Lisp macros

Before Lisp had macros, it had so-called FEXPRs, function-like operators whose inputs were not the values computed by the arguments but rather the syntactic forms of the arguments, and whose output were values to be used in the computation. In other words, FEXPRs were implemented at the same level as EVAL, and provided a window into the meta-evaluation layer. This was generally found to be a difficult model to reason about effectively. In 1963, Timothy Hart proposed adding macros to Lisp 1.5 in
AI Memo AI is artificial intelligence, intellectual ability in machines and robots. Ai, AI or A.I. may also refer to: Animals * Ai (chimpanzee), an individual experimental subject in Japan * Ai (sloth) or the pale-throated sloth, northern Amazonian ma ...
57: MACRO Definitions for LISP.


Anaphoric macros

An anaphoric macro is a type of programming macro that deliberately captures some form supplied to the macro which may be referred to by an anaphor (an expression referring to another). Anaphoric macros first appeared in Paul Graham's On Lisp and their name is a reference to linguistic anaphora—the use of words as a substitute for preceding words.


Hygienic macros

In the mid-eighties, a number of papers introduced the notion of
hygienic macro Hygienic macros are macros whose expansion is guaranteed not to cause the accidental capture of identifiers. They are a feature of programming languages such as Scheme, Dylan, Rust, Nim, and Julia. The general problem of accidental capture was w ...
expansion (syntax-rules), a pattern-based system where the syntactic environments of the macro definition and the macro use are distinct, allowing macro definers and users not to worry about inadvertent variable capture (cf.
referential transparency In computer science, referential transparency and referential opacity are properties of parts of computer programs. An expression is called ''referentially transparent'' if it can be replaced with its corresponding value (and vice-versa) withou ...
). Hygienic macros have been standardized for Scheme in the R5RS,
R6RS Scheme is a dialect of the Lisp family of programming languages. Scheme was created during the 1970s at the MIT AI Lab and released by its developers, Guy L. Steele and Gerald Jay Sussman, via a series of memos now known as the Lambda Papers. It ...
, and R7RS standards. A number of competing implementations of hygienic macros exist such as syntax-rules, syntax-case, explicit renaming, and syntactic closures. Both syntax-rules and syntax-case have been standardized in the Scheme standards. Recently, Racket has combined the notions of hygienic macros with a " tower of evaluators", so that the syntactic expansion time of one macro system is the ordinary runtime of another block of code, and showed how to apply interleaved expansion and parsing in a non-parenthesized language. A number of languages other than Scheme either implement hygienic macros or implement partially hygienic systems. Examples include Scala,
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH ...
,
Elixir ELIXIR (the European life-sciences Infrastructure for biological Information) is an initiative that will allow life science laboratories across Europe to share and store their research data as part of an organised network. Its goal is to bring t ...
,
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.g ...
, Dylan, Nim, and
Nemerle Nemerle is a general-purpose, high-level, statically typed programming language designed for platforms using the Common Language Infrastructure ( .NET/Mono). It offers functional, object-oriented, aspect-oriented, reflective and imperative fea ...
.


Applications

; Evaluation order :Macro systems have a range of uses. Being able to choose the order of evaluation (see
lazy evaluation In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an expression until its value is needed (non-strict evaluation) and which also avoids repeated evaluations (sharing). The b ...
and non-strict functions) enables the creation of new syntactic constructs (e.g.
control structures In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imp ...
) indistinguishable from those built into the language. For instance, in a Lisp dialect that has cond but lacks if, it is possible to define the latter in terms of the former using macros. For example, Scheme has both
continuation In computer science, a continuation is an abstract representation of the control state of a computer program. A continuation implements ( reifies) the program control state, i.e. the continuation is a data structure that represents the computati ...
s and hygienic macros, which enables a programmer to design their own control abstractions, such as looping and early exit constructs, without the need to build them into the language. ; Data sub-languages and domain-specific languages : Next, macros make it possible to define data languages that are immediately compiled into code, which means that constructs such as state machines can be implemented in a way that is both natural and efficient. ; Binding constructs :Macros can also be used to introduce new binding constructs. The most well-known example is the transformation of let into the application of a function to a set of arguments. Felleisen conjectures
Matthias Felleisen, LL1 mailing list posting
that these three categories make up the primary legitimate uses of macros in such a system. Others have proposed alternative uses of macros, such as
anaphoric macro An anaphoric macro is a type of programming macro that deliberately captures some form supplied to the macro which may be referred to by an ''anaphor'' (an expression referring to another). Anaphoric macros first appeared in Paul Graham's '' On Li ...
s in macro systems that are unhygienic or allow selective unhygienic transformation. The interaction of macros and other language features has been a productive area of research. For example,
component Circuit Component may refer to: •Are devices that perform functions when they are connected in a circuit.   In engineering, science, and technology Generic systems * System components, an entity with discrete structure, such as an assem ...
s and
module Module, modular and modularity may refer to the concept of modularity. They may also refer to: Computing and engineering * Modular design, the engineering discipline of designing complex devices using separately designed sub-components * Mo ...
s are useful for large-scale programming, but the interaction of macros and these other constructs must be defined for their use together. Module and component-systems that can interact with macros have been proposed for Scheme and other languages with macros. For example, the Racket language extends the notion of a macro system to a syntactic tower, where macros can be written in languages including macros, using hygiene to ensure that syntactic layers are distinct and allowing modules to export macros to other modules.


Macros for machine-independent software

Macros are normally used to map a short string (macro invocation) to a longer sequence of instructions. Another, less common, use of macros is to do the reverse: to map a sequence of instructions to a macro string. This was the approach taken by the STAGE2 Mobile Programming System, which used a rudimentary macro compiler (called SIMCMP) to map the specific instruction set of a given computer into ''machine-independent'' macros. Applications (notably compilers) written in these machine-independent macros can then be run without change on any computer equipped with the rudimentary macro compiler. The first application run in such a context is a more sophisticated and powerful macro compiler, written in the machine-independent macro language. This macro compiler is applied to itself, in a bootstrap fashion, to produce a compiled and much more efficient version of itself. The advantage of this approach is that complex applications can be ported from one computer to a very different computer with very little effort (for each target machine architecture, just the writing of the rudimentary macro compiler). The advent of modern programming languages, notably C, for which compilers are available on virtually all computers, has rendered such an approach superfluous. This was, however, one of the first instances (if not the first) of compiler bootstrapping.


Assembly language

While ''macro instructions'' can be defined by a programmer for any set of native assembler program instructions, typically macros are associated with macro libraries delivered with the operating system allowing access to operating system functions such as * peripheral access by
access methods An access method is a function of a mainframe operating system that enables access to data on disk, tape or other external devices. Access methods were present in several mainframe operating systems since the late 1950s, under a variety of name ...
(including macros such as OPEN, CLOSE, READ and WRITE) * operating system functions such as ATTACH, WAIT and POST for subtask creation and synchronization. Typically such macros expand into executable code, e.g., for the EXIT macroinstruction, * a list of ''define constant'' instructions, e.g., for the DCB macro—DTF (Define The File) for
DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicat ...
—or a combination of code and constants, with the details of the expansion depending on the parameters of the macro instruction (such as a reference to a file and a data area for a READ instruction); * the executable code often terminated in either a ''branch and link register'' instruction to call a routine, or a supervisor call instruction to call an operating system function directly. * Generating a ''Stage 2'' job stream for
system generation In computing system generation or sysgen is the process of creating a particular unique instance of an operating system by combining user-specified options and parameters with manufacturer-supplied general-purpose program code to produce an oper ...
in, e.g.,
OS/360 OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB ...
. Unlike typical macros, sysgen stage 1 macros do not generate data or code to be loaded into storage, but rather use the PUNCH statement to output JCL and associated data. In older operating systems such as those used on IBM mainframes, full operating system functionality was only available to assembler language programs, not to high level language programs (unless assembly language subroutines were used, of course), as the standard macro instructions did not always have counterparts in routines available to high-level languages.


History

In the mid-1950s, when
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
programming was commonly used to write programs for
digital computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These pro ...
s, the use of macro instructions was initiated for two main purposes: to reduce the amount of program coding that had to be written by generating several assembly language statements from one macro instruction and to enforce program writing standards, e.g. specifying input/output commands in standard ways. Macro instructions were effectively a middle step between assembly language programming and the
high-level programming languages In computer science, a high-level programming language is a programming language with strong abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language ''elements'', be easier to use, ...
that followed, such as FORTRAN and
COBOL COBOL (; an acronym for "common business-oriented language") is a compiled English-like computer programming language designed for business use. It is an imperative, procedural and, since 2002, object-oriented language. COBOL is primarily us ...
. Two of the earliest programming installations to develop "macro languages" for the IBM 705 computer were at Dow Chemical Corp. in Delaware and the Air Material Command, Ballistics Missile Logistics Office in California. A macro instruction written in the format of the target assembly language would be processed by a macro compiler, which was a pre-processor to the assembler, to generate one or more assembly language instructions to be processed next by the assembler program that would translate the assembly language instructions into
machine language In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
instructions. By the late 1950s the macro language was followed by the Macro Assemblers. This was a combination of both where one program served both functions, that of a macro pre-processor and an assembler in the same package. In 1959, Douglas E. Eastwood and
Douglas McIlroy Malcolm Douglas McIlroy (born 1932) is a mathematician, engineer, and programmer. As of 2019 he is an Adjunct Professor of Computer Science at Dartmouth College. McIlroy is best known for having originally proposed Unix pipelines and developed se ...
of
Bell Labs Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984), then AT&T Bell Laboratories (1984–1996) and Bell Labs Innovations (1996–2007), is an American industrial research and scientific development company owned by mult ...
introduced conditional and recursive macros into the popular
SAP Sap is a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant. Sap is distinct from latex, resin, or cell sap; it is a separ ...
assembler, creating what is known as Macro SAP. McIlroy's 1960 paper was seminal in the area of extending any (including
high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific goals of a systematic operation; and are applied in a wide range of contexts, such as, for instance, in domains as widely varied as computer scienc ...
) programming languages through macro processors. Macro Assemblers allowed assembly language programmers to implement their own macro-language and allowed limited portability of code between two machines running the same CPU but different operating systems, for example, early versions of MSDOS and CPM-86. The macro library would need to be written for each target machine but not the overall assembly language program. Note that more powerful macro assemblers allowed use of conditional assembly constructs in macro instructions that could generate different code on different machines or different operating systems, reducing the need for multiple libraries. In the 1980s and early 1990s, desktop PCs were only running at a few MHz and assembly language routines were commonly used to speed up programs written in C, Fortran, Pascal and others. These languages, at the time, used different calling conventions. Macros could be used to interface routines written in assembly language to the front end of applications written in almost any language. Again, the basic assembly language code remained the same, only the macro libraries needed to be written for each target language. In modern operating systems such as Unix and its derivatives, operating system access is provided through subroutines, usually provided by dynamic libraries. High-level languages such as C offer comprehensive access to operating system functions, obviating the need for assembler language programs for such functionality.


See also

* s * (the origin of the concept of macros) * * s * * *
Computer science and engineering Computer Science and Engineering (CSE) is an academic program at many universities which comprises scientific and engineering aspects of computing. CSE is also a term often used in Europe to translate the name of engineering informatics academic ...
*
Computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...


References


External links


How to write Macro Instructions Rochester Institute of Technology, Professors Powerpoint
{{Authority control Programming constructs Source code Automation software