Info Catalog m4: Arguments m4: Definitions m4: Undefine

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
 

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