Metaprogramming

Approaches

Metaprogramming enables developers to write programs and develop code that falls under the generic programming paradigm. Having the programming language itself as a first-class data type (as in Lisp, Prolog, SNOBOL, or Rebol) is also very useful; this is known as homoiconicity. Generic programming invokes a metaprogramming facility within a language by allowing one to write code without the concern of specifying data types since they can be supplied as parameters when used.

Metaprogramming usually works in one of three ways.

1. The first approach is to expose the internals of the run-time engine to the programming code through application programming interfaces (APIs) like that for the .NET IL emitter.
2. The second approach is dynamic execution of expressions that contain programming commands, often composed from strings, but can also be from other methods using arguments or context, like Javascript. 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.
3. The third approach is to step outside the language entirely. General purpose program transformation systems such as compilers, which accept language descriptions and 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. One can see this at work with scheme programming language and how it allows to tackle some limitations faced in the C language by using constructs that were part of the Scheme language itself to extend C. Art of Metaprogramming

Examples

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

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 less than a minute, 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. Quines are generally of recreational or theoretical interest only.

Not all metaprogramming involves generative programming. If programs are modifiable at runtime or if 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 unquote operator (typically a comma) introduces code that is evaluated at program definition time rather than at run time; see Self-evaluating forms and quoting in Lisp. 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 RemObjects’ Pascal Script for Object Pascal.

One style of metaprogramming is to employ domain-specific 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.

Support and challenges

One of the benefits of metaprogramming is that it increases the productivity of developers once they get past the convention over configuration phase in the learning phase. Some argue that there is a sharp learning curve to make complete use of the metaprogramming features and to take of advantage of it. Since metaprogramming gives more flexibility and configurability at runtime, misuse or wrong use of the metaprogramming can result in unwarranted and unexpected errors that can be extremely difficult to debug to an average developer. It can introduce risks in the system and make it more vulnerable if not used with care. Some of the common problems which can occur due to wrong use of metaprogramming are inability of the compiler to identify missing configuration parameters, invalid or incorrect data can result in unknown exception or different results. Due to this, some critics believe that only high-skilled developers should work on developing features to support metaprogramming in a language or platform and an average developer must learn how to use these features as part of convention.

Macro systems

Macro assemblers

The IBM/360 and derivatives had powerful macro assembler facilities that were often used to generate complete assembly language programs 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.

Other assemblers, such as MASM, also support macros.

Metaclasses

Metaclasses are provided by the following programming languages:

Template metaprogramming

Staged metaprogramming

With dependent types