Macro (computer science)

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 automated. Separate programs for creating these macros are called macro recorders.

During the 1980s, macro programs – originally SmartKey, then SuperKey, KeyWorks, Prokey – were very popular, first as a means to automatically format screenplays, then for a variety of user input tasks. These programs were based on the TSR (terminate and stay resident) 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 processors and spreadsheets, making it possible to create application-sensitive keyboard macros.

Keyboard macros can be used in massively multiplayer online role-playing games (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 of most MMORPGs, and their administrators spend considerable effort to suppress them.[4]

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.

As a simple example, in the C programming language, 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 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 functions.

The parameterized macros used in languages such as Lisp, PL/I and Scheme, 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.

Languages such as C and some assembly languages have rudimentary macro systems, implemented as preprocessors to the compiler or assembler. C preprocessor 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 (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 and its derivatives, where most of the functionality is based on macros.

MacroML is an experimental system that seeks to reconcile static typing and macro systems. Nemerle 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
• Macro Extension TAL, accompanying Template Attribute Language
• SMX: for web pages
• ML/1 (Macro Language One)
• The General Purpose Macrogenerator is a contextual pattern-matching macro processor, which could be described as a combination of regular expressions, EBNF and AWK
• troff and nroff: for typesetting and formatting Unix manpages.
• CMS EXEC: for command-line macros and application macros
• EXEC 2 in Conversational Monitor System (CMS): for command-line macros and application macros
• CLIST in IBM's Time Sharing Option (TSO): for command-line macros and application macros
• REXX: for command-line macros and application macros in, e.g., AmigaOS, CMS, OS/2, TSO
• SCRIPT: for formatting documents
• Various shells for, e.g., Linux

Some major applications have been written as text macro invoked by other applications, e.g., by XEDIT in CMS.

Macros in the PL/I language are written in a subset of PL/I itself: the compiler executes "preprocessor 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 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 subroutines.

Most assembly languages have less powerful procedural macro facilities, for example allowing a block of code to be repeated N times for loop unrolling; but these have a completely different syntax from the actual assembly language.

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 trees, 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-expressions). 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,[6] Erlang,[7] Dylan,[8] Scala,[9] Nemerle,[10] Rust,[11] Elixir,[12] Nim,[13] Haxe,[14] and Julia.[15] They are also available as third-party extensions to JavaScript,[16] C#[17] and Python.[18]

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).[27][28] 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.

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 (including macros such as OPEN, CLOSE, READ and WRITE)
• operating system functions such as ATTACH, WAIT and POST for subtask creation and synchronization.[29] 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[30]—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, e.g., OS/360. 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.

In the mid-1950s, when assembly language programming was commonly used to write programs for digital computers, 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.[31] Macro instructions were effectively a middle step between assembly language programming and the high-level programming languages that followed, such as FORTRAN and COBOL. 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 instructions.[32]

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.[32]

In 1959, Douglas E. Eastwood and Douglas McIlroy of Bell Labs introduced conditional and recursive macros into the popular SAP assembler,[33] creating what is known as Macro SAP.[34] McIlroy's 1960 paper was seminal in the area of extending any (including high-level) programming languages through macro processors.[35][33]

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.

• Anaphoric macros
• Assembly language § Macros (the origin of the concept of macros)
• Extensible programming
• Hygienic macros
• Programming by demonstration – Technique for teaching a computer or a robot new behaviors
• String interpolation – Replacing placeholders in a string with values
• Computer science and engineering
• Computer science

• How to write Macro Instructions
• Rochester Institute of Technology, Professors Powerpoint