(m4.info)Define


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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'.  Note: 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' (Note: Pushdef).  Some
other implementations of `m4' replace all definitions of a macro with
`define'.  Note: 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' (Note: Indir) and `defn' (Note: 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.


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