The prefix “meta” is thrown around frequently nowadays, and often incorrectly, so we will have to settle on an exact definition for our discussion.
For our purposes, the prefix “meta” signifies that something acts upon the world in a solely self-referential way.
So, for example, meta-magic is a spell acting upon magic, magically. Meta-text is text that discusses text, itself. And meta-programs are programs that act upon programs, programmatically.
Metaprogramming is a programming technique in which computer programs have the ability to treat other programs as their data. It means that a program can be designed to read, generate, analyze or transform other programs, and even modify itself while running. In some cases, this allows programmers to minimize the number of lines of code to express a solution, in turn reducing development time. It also allows programs greater flexibility to efficiently handle new situations without recompilation.
Metaprogramming can be used to move computations from run-time to compile-time, to generate code using compile time computations, and to enable self-modifying code. The language in which the metaprogram is written is called the metalanguage. The language of the programs that are manipulated is called the attribute-oriented programming 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.
Metaprogramming was popular in the 1970s and 1980s using list processing languages such as LISP. LISP hardware machines were popular in the 1980s and enabled applications that could process code. They were frequently used for artificial intelligence applications.
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. Some argue that there is a sharp learning curve to make complete use of metaprogramming features. Since metaprogramming gives more flexibility and configurability at runtime, misuse or incorrect 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 believe that only high-skilled developers should work on developing features which exercise metaprogramming in a language or platform and average developers must learn how to use these features as part of convention.
Metaprogramming refers to a variety of ways a program has knowledge of itself or can manipulate itself.
In languages like C#, reflection is a form of metaprogramming since the program can examine information about itself. For example returning a list of all the properties of an object.
In languages like ActionScript, you can evaluate functions at runtime to create new programs such as eval(“x” + i). DoSomething() would affect an object called x1 when i is 1 and x2 when i is 2.
Finally, another common form of metaprogramming is when the program can change itself in non-trivial fashions. LISP is well known for this and is something Paul Graham championed about a decade ago. I’ll have to look up some of his specific essays. But the idea is that the program would change another part of the program based on its state. This allows a level of flexibility to make decisions at runtime that is very difficult in most popular languages today.
It is also worth noting that back in the good ol’ days of programming in straight assembly, programs that altered themselves at runtime were necessary and very commonplace.
Many languages have something called a macro. But Lisp macros are unique. And believe it or not, what they do is related to the parentheses. The designers of Lisp didn’t put all those parentheses in the language just to be different. To the Blub programmer, Lisp code looks weird. But those parentheses are there for a reason. They are the outward evidence of a fundamental difference between Lisp and other languages.
#!/bin/sh # metaprogram echo '#!/bin/sh' > program for i in $(seq 992) 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 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.
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.