The m4 macro language supports conditionals and recursion. The combination can be used to implement loops, and this was intended; m4 is deliberately Turing-complete. But actually trying to use m4 as a general-purpose language would be deeply perverse.

The m4 macro processor is usually employed as a preprocessor for minilanguages that lack a built-in notion of named procedures or a built-in file-inclusion feature. It's an easy way to extend the syntax of the base language so the combination with m4 supports both these features.

One well-known use of m4 has been to clean up (or at least effectively hide) another minilanguage design that was called out as a bad example earlier in this chapter. Most system administrators now generate their sendmail.cf configuration files using an m4 macro package supplied with the sendmail distribution. The macros start from feature names (or name/value pairs) and generate the corresponding (much uglier) strings in the sendmail configuration language.