m4: Pseudo Arguments
5.3 Special arguments to macros
===============================
There is a special notation for the number of actual arguments supplied,
and for all the actual arguments.
The number of actual arguments in a macro call is denoted by '$#' in
the expansion text.
-- Composite: nargs (...)
Expands to a count of the number of arguments supplied.
define(`nargs', `$#')
=>
nargs
=>0
nargs()
=>1
nargs(`arg1', `arg2', `arg3')
=>3
nargs(`commas can be quoted, like this')
=>1
nargs(arg1#inside comments, commas do not separate arguments
still arg1)
=>1
nargs((unquoted parentheses, like this, group arguments))
=>1
Remember that '#' defaults to the comment character; if you forget
quotes to inhibit the comment behavior, your macro definition may not
end where you expected.
dnl Attempt to define a macro to just `$#'
define(underquoted, $#)
oops)
=>
underquoted
=>0)
=>oops
The notation '$*' can be used in the expansion text to denote all the
actual arguments, unquoted, with commas in between. For example
define(`echo', `$*')
=>
echo(arg1, arg2, arg3 , arg4)
=>arg1,arg2,arg3 ,arg4
Often each argument should be quoted, and the notation '$@' handles
that. It is just like '$*', except that it quotes each argument. A
simple example of that is:
define(`echo', `$@')
=>
echo(arg1, arg2, arg3 , arg4)
=>arg1,arg2,arg3 ,arg4
Where did the quotes go? Of course, they were eaten, when the
expanded text were reread by 'm4'. To show the difference, try
define(`echo1', `$*')
=>
define(`echo2', `$@')
=>
define(`foo', `This is macro `foo'.')
=>
echo1(foo)
=>This is macro This is macro foo..
echo1(`foo')
=>This is macro foo.
echo2(foo)
=>This is macro foo.
echo2(`foo')
=>foo
Trace, if you do not understand this. As another example of the
difference, remember that comments encountered in arguments are passed
untouched to the macro, and that quoting disables comments.
define(`echo1', `$*')
=>
define(`echo2', `$@')
=>
define(`foo', `bar')
=>
echo1(#foo'foo
foo)
=>#foo'foo
=>bar
echo2(#foo'foo
foo)
=>#foobar
=>bar'
A '$' sign in the expansion text, that is not followed by anything
'm4' understands, is simply copied to the macro expansion, as any other
text is.
define(`foo', `$$$ hello $$$')
=>
foo
=>$$$ hello $$$
If you want a macro to expand to something like '$12', the judicious
use of nested quoting can put a safe character between the '$' and the
next character, relying on the rescanning to remove the nested quote.
This will prevent 'm4' from interpreting the '$' sign as a reference to
an argument.
define(`foo', `no nested quote: $1')
=>
foo(`arg')
=>no nested quote: arg
define(`foo', `nested quote around $: `$'1')
=>
foo(`arg')
=>nested quote around $: $1
define(`foo', `nested empty quote after $: $`'1')
=>
foo(`arg')
=>nested empty quote after $: $1
define(`foo', `nested quote around next character: $`1'')
=>
foo(`arg')
=>nested quote around next character: $1
define(`foo', `nested quote around both: `$1'')
=>
foo(`arg')
=>nested quote around both: arg