Features
m4 offers these facilities:
- a free-form syntax, rather than line based syntax
- a high degree of macro expansion (arguments get expanded during scan and again during interpolation)
- text replacement
- parameter substitution
- file inclusion
- string manipulation
- conditional evaluation
- arithmetic expressions
- system interface
- programmer diagnostics
- programming language independent
- human language independent
- provides programming language capabilities
Unlike most earlier macro processors, m4 does not target any particular computer or human language; historically, however, its development originated for supporting the Ratfor dialect of Fortran. Unlike some other macro processors, m4 is Turing-complete as well as a practical programming language.
Unquoted identifiers which match defined macros are replaced with their definitions. Placing identifiers in quotes suppresses expansion until possibly later, such as when a quoted string is expanded as part of macro replacement. Unlike most languages, strings in m4 are quoted using the backtick (`) as the starting delimiter, and apostrophe (') as the ending delimiter. The use of separate starting and ending delimiters allows for the arbitrary nesting of quotation marks in strings, allowing a fine degree of control of how and when macro expansion takes place in different parts of a string.
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