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5.1 Defining a macro

The normal way to define or redefine macros is to use the builtin define:

— Builtin: define (name, [expansion])

Defines name to expand to expansion. If expansion is not given, it is taken to be empty.

The expansion of define is void. The macro define is recognized only with parameters.

The following example defines the macro foo to expand to the text ‘Hello World.’.

     define(`foo', `Hello world.')
     ⇒
     foo
     ⇒Hello world.

The empty line in the output is there because the newline is not a part of the macro definition, and it is consequently copied to the output. This can be avoided by use of the macro dnl. See Dnl, for details.

The first argument to define should be quoted; otherwise, if the macro is already defined, you will be defining a different macro. This example shows the problems with underquoting, since we did not want to redefine one:

     define(foo, one)
     ⇒
     define(foo, two)
     ⇒
     one
     ⇒two

GNU m4 normally replaces only the topmost definition of a macro if it has several definitions from pushdef (see Pushdef). Some other implementations of m4 replace all definitions of a macro with define. See Incompatibilities, for more details.

As a GNU extension, the first argument to define does not have to be a simple word. It can be any text string, even the empty string. A macro with a non-standard name cannot be invoked in the normal way, as the name is not recognized. It can only be referenced by the builtins indir (see Indir) and defn (see Defn).

Arrays and associative arrays can be simulated by using non-standard macro names.

— Composite: array (index)
— Composite: array_set (index, [value])

Provide access to entries within an array. array reads the entry at location index, and array_set assigns value to location index.

     define(`array', `defn(format(``array[%d]'', `$1'))')
     ⇒
     define(`array_set', `define(format(``array[%d]'', `$1'), `$2')')
     ⇒
     array_set(`4', `array element no. 4')
     ⇒
     array_set(`17', `array element no. 17')
     ⇒
     array(`4')
     ⇒array element no. 4
     array(eval(`10 + 7'))
     ⇒array element no. 17

Change the ‘%d’ to ‘%s’ and it is an associative array.