Metaprogramming

Metaprogramming is the writing of computer programs that write or manipulate other programs (or themselves) as their data, or that do part of the work at compile time that would otherwise be done at runtime. In many cases, this allows programmers to get more done in the same amount of time as they would take to write all the code manually[citation needed], or it gives programs greater flexibility to efficiently handle new situations without recompilation.

The language in which the metaprogram is written is called the metalanguage. The language of the programs that are manipulated is called the object language. The ability of a programming language to be its own metalanguage is called reflection or reflexivity.

Reflection is a valuable language feature to facilitate metaprogramming. Having the programming language itself as a first-class data type (as in Lisp, Forth or Rebol) is also very useful. Generic programming invokes a metaprogramming facility within a language, in those languages supporting it.

Metaprogramming usually works in one of three ways[citation needed]. The first way is to expose the internals of the run-time engine to the programming code through application programming interfaces (APIs). The second approach is dynamic execution of string expressions that contain programming commands. Thus, "programs can write programs". Although both approaches can be used in the same language, most languages tend to lean toward one or the other.

The third way is to step outside the language entirely. General purpose program transformation systems, which accept language descriptions and can carry out arbitrary transformations on those languages, are direct implementations of general metaprogramming. This allows metaprogramming to be applied to virtually any target language without regard to whether that target language has any metaprogramming abilities of its own.

Contents

Approaches

In statically typed functional languages

Template meta-programming

  • C++ Templates

Staged meta-programming

Macro systems

IBM/360 assembler

The IBM/360 and derivatives had powerful Assembler macro facilities that were often used to generate complete programs[citation needed] or sections of programs (for different operating systems for instance). Macros provided with CICS transaction processing system had Assembler macros that generated COBOL statements as a pre-processing step.

Examples

A simple example of a metaprogram is this bash script, which is an example of generative programming:

#!/bin/bash
# metaprogram
echo '#!/bin/bash' >program
for ((I=1; I<=992; I++)) do
    echo "echo $I" >>program
done
chmod +x program

This script (or program) generates a new 993-line program that prints out the numbers 1–992. This is only an illustration of how to use code to write more code; it is not the most efficient way to print out a list of numbers. Nonetheless, a programmer can write and execute this metaprogram in just a couple of minutes, and will have generated exactly 1000 lines of code in that amount of time.

A quine is a special kind of metaprogram that produces its own source code as its output.

Not all metaprogramming involves generative programming. If programs are modifiable at runtime or if an incremental compilation is available (such as in C#, Forth, Frink, Groovy, JavaScript, Lisp, Lua, Perl, PHP, Python, REBOL, Ruby, Smalltalk, and Tcl), then techniques can be used to perform metaprogramming without actually generating source code.

Lisp is probably the quintessential language with metaprogramming facilities, both because of its historical precedence and because of the simplicity and power of its metaprogramming. In Lisp metaprogramming, the quasiquote operator (typically a comma) introduces code that is evaluated at program definition time rather than at run time. The metaprogramming language is thus identical to the host programming language, and existing Lisp routines can be directly reused for metaprogramming, if desired.

This approach has been implemented in other languages by incorporating an interpreter in the program, which works directly with the program’s data. There are implementations of this kind for some common high-level languages, such as RemObject’s Pascal Script for Object Pascal.

One style of metaprogramming is to employ domain-specific programming languages (DSLs). A fairly common example of using DSLs involves generative metaprogramming: lex and yacc, two tools used to generate lexical analyzers and parsers, let the user describe the language using regular expressions and context-free grammars, and embed the complex algorithms required to efficiently parse the language.

Implementations

See also

External links


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