Editing Metaprogramming (section)

=== Code generation ===
A simple example of a metaprogram is this [[Unix shell|POSIX Shell]] [[script (computer programming)|script]], which is an example of [[generative programming]]:

<syntaxhighlight lang="bash">
#!/bin/sh
# metaprogram
echo '#!/bin/sh' > program
for i in $(seq 992)
do
    echo "echo $i" >> program
done
chmod +x program
</syntaxhighlight>

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 over 1000 lines of code in that amount of time.

A [[Quine (computing)|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 compiling is available (such as in [[C Sharp (programming language)|C#]], [[Forth (programming language)|Forth]], [[Frink (programming language)|Frink]], [[Groovy (programming language)|Groovy]], [[JavaScript]], [[Lisp (programming language)|Lisp]], [[Elixir (programming language)|Elixir]], [[Lua (programming language)|Lua]], [[Nim (programming language)|Nim]], [[Perl]], [[PHP]], [[Python (programming language)|Python]], [[Rebol]], [[Ruby (programming language)|Ruby]], [[Rust (programming language)|Rust]], [[R (programming language)|R]], [[SAS (software)|SAS]], [[Smalltalk]], and [[Tcl]]), then techniques can be used to perform metaprogramming without generating source code.

One style of generative approach is to employ [[domain-specific language]]s (DSLs). A fairly common example of using DSLs involves generative metaprogramming: [[Lex programming tool|lex]] and [[yacc]], two tools used to generate [[lexical analysis|lexical analyser]]s and [[parsing|parser]]s, let the user describe the language using [[regular expression]]s and [[context-free grammar]]s, and embed the complex algorithms required to efficiently parse the language.