(m4.info.gz) Loops

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 Loops and recursion
 ===================
 
    There is no direct support for loops in `m4', but macros can be
 recursive.  There is no limit on the number of recursion levels, other
 than those enforced by your hardware and operating system.
 
    Loops can be programmed using recursion and the conditionals
 described previously.
 
    There is a builtin macro, `shift', which can, among other things, be
 used for iterating through the actual arguments to a macro:
 
      shift(...)
 
 It takes any number of arguments, and expands to all but the first
 argument, separated by commas, with each argument quoted.
 
      shift(bar)
      =>
      shift(foo, bar, baz)
      =>bar,baz
 
    An example of the use of `shift' is this macro, which reverses the
 order of its arguments:
 
      define(`reverse', `ifelse($#, 0, , $#, 1, ``$1'',
      			  `reverse(shift($@)), `$1'')')
      =>
      reverse
      =>
      reverse(foo)
      =>foo
      reverse(foo, bar, gnats, and gnus)
      =>and gnus, gnats, bar, foo
 
    While not a very interesting macro, it does show how simple loops
 can be made with `shift', `ifelse' and recursion.
 
    Here is an example of a loop macro that implements a simple forloop.
 It can, for example, be used for simple counting:
 
      forloop(`i', 1, 8, `i ')
      =>1 2 3 4 5 6 7 8
 
    The arguments are a name for the iteration variable, the starting
 value, the final value, and the text to be expanded for each iteration.
 With this macro, the macro `i' is defined only within the loop.  After
 the loop, it retains whatever value it might have had before.
 
    For-loops can be nested, like
 
      forloop(`i', 1, 4, `forloop(`j', 1, 8, `(i, j) ')
      ')
      =>(1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6) (1, 7) (1, 8)
      =>(2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) (2, 7) (2, 8)
      =>(3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6) (3, 7) (3, 8)
      =>(4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6) (4, 7) (4, 8)
      =>
 
    The implementation of the `forloop' macro is fairly straightforward.
 The `forloop' macro itself is simply a wrapper, which saves the
 previous definition of the first argument, calls the internal macro
 `_forloop', and re-establishes the saved definition of the first
 argument.
 
    The macro `_forloop' expands the fourth argument once, and tests to
 see if it is finished.  If it has not finished, it increments the
 iteration variable (using the predefined macro `incr',  Incr.),
 and recurses.
 
    Here is the actual implementation of `forloop':
 
      define(`forloop',
             `pushdef(`$1', `$2')_forloop(`$1', `$2', `$3', `$4')popdef(`$1')')
      define(`_forloop',
             `$4`'ifelse($1, `$3', ,
      		   `define(`$1', incr($1))_forloop(`$1', `$2', `$3', `$4')')')
 
    Notice the careful use of quotes.  Only three macro arguments are
 unquoted, each for its own reason.  Try to find out *why* these three
 arguments are left unquoted, and see what happens if they are quoted.
 
    Now, even though these two macros are useful, they are still not
 robust enough for general use. They lack even basic error handling of
 cases like start value less than final value, and the first argument
 not being a name.  Correcting these errors are left as an exercise to
 the reader.
 

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