M4
{{language |exec=interpreted |safety=safe |untyped=yes |tag=m4 }} '''m4''' reads text input, expands macros in the text, and prints the result. It can be used as a front-end to a compiler or assembler, or for general purpose expanding etc.
Various builtin macros can do integer arithmetic, run shell commands, access files, divert output to temporary buffers for re-ordering, etc. define creates new macros.
define(`foo', `blah blah: $1')
foo(123)
=>
blah blah: 123
Control flow is limited to ifelse, but it's easy to construct loops by recursion. GNU m4 includes examples of macros implementing various general-purpose loops.
Quoting data values against premature or unwanted expansion can be a little tricky. The default quote characters are </code> and <code>'</code>. If they would occur in text too often then <code>changequote()</code> can set something else. Autoconf changes to <code>[</code> and <code>]</code> since <code>' occur often in its [[Bourne Shell]] output.
When a macro expands, its value (with $1 etc parameters substituted) is re-read as input. This is how macro definitions can contain further macros to expand.
define(`foo', `bar(`$1',x,`$2')')
Various m4 implementations, including BSD, have a fixed limit on the amount "push-back" text to be re-read. GNU m4 has no limit except available memory. A limit restricts the size of macro values and the data they might operate on. Cutting data into pieces can keep expansions to a reasonable size.
The simple text re-reading means that macro calls are "properly tail recursive". If an expansion ends with another macro call then that call can re-expand recursively or by co-routining endlessly. But a tail call must be the very last thing, no newline or other fixed text after. See [[Factorial#M4|Factorial]] for an example of such recursion.
One implementation of this Unix macro processor m4 is the GNU m4
- [[wp:m4 (computer language)|m4 on wikipedia]]
- [http://www.gnu.org/software/m4/ GNU m4]