6. Conditionals, loops, and recursion

Macros, expanding to plain text, perhaps with arguments, are not quite enough. We would like to have macros expand to different things, based on decisions taken at run-time. For that, we need some kind of conditionals. Also, we would like to have some kind of loop construct, so we could do something a number of times, or while some condition is true.

6.1 Testing if a macro is defined

There are two different builtin conditionals in m4. The first is ifdef:

Builtin: ifdef (name, string-1, [string-2])

If name is defined as a macro, ifdef expands to string-1, otherwise to string-2. If string-2 is omitted, it is taken to be the empty string (according to the normal rules).

The macro ifdef is recognized only with parameters.

ifdef(`foo', ``foo' is defined', ``foo' is not defined')
⇒foo is not defined
define(`foo', `')
⇒
ifdef(`foo', ``foo' is defined', ``foo' is not defined')
⇒foo is defined
ifdef(`no_such_macro', `yes', `no', `extra argument')
error-->m4:stdin:4: Warning: excess arguments to builtin `ifdef' ignored
⇒no

6.2 If-else construct, or multibranch

The other conditional, ifelse, is much more powerful. It can be used as a way to introduce a long comment, as an if-else construct, or as a multibranch, depending on the number of arguments supplied:

Builtin: ifelse (comment)

Builtin: ifelse (string-1, string-2, equal,   [not-equal])

Builtin: ifelse (string-1, string-2, equal-1,   string-3, string-4, equal-2, …, [not-equal])

Used with only one argument, the ifelse simply discards it and produces no output.

If called with three or four arguments, ifelse expands into equal, if string-1 and string-2 are equal (character for character), otherwise it expands to not-equal. A final fifth argument is ignored, after triggering a warning.

If called with six or more arguments, and string-1 and string-2 are equal, ifelse expands into equal-1, otherwise the first three arguments are discarded and the processing starts again.

The macro ifelse is recognized only with parameters.

Using only one argument is a common m4 idiom for introducing a block comment, as an alternative to repeatedly using dnl. This special usage is recognized by GNU m4, so that in this case, the warning about missing arguments is never triggered.

ifelse(`some comments')
⇒
ifelse(`foo', `bar')
error-->m4:stdin:2: Warning: too few arguments to builtin `ifelse'
⇒

Using three or four arguments provides decision points.

ifelse(`foo', `bar', `true')
⇒
ifelse(`foo', `foo', `true')
⇒true
define(`foo', `bar')
⇒
ifelse(foo, `bar', `true', `false')
⇒true
ifelse(foo, `foo', `true', `false')
⇒false

Notice how the first argument was used unquoted; it is common to compare the expansion of a macro with a string. With this macro, you can now reproduce the behavior of blind builtins, where the macro is recognized only with arguments.

define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')')
⇒
foo
⇒foo
foo()
⇒arguments:1
foo(`a', `b', `c')
⇒arguments:3

However, ifelse can take more than four arguments. If given more than four arguments, ifelse works like a case or switch statement in traditional programming languages. If string-1 and string-2 are equal, ifelse expands into equal-1, otherwise the procedure is repeated with the first three arguments discarded. This calls for an example:

ifelse(`foo', `bar', `third', `gnu', `gnats')
error-->m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored
⇒gnu
ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth')
⇒
ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh')
⇒seventh
ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8')
error-->m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored
⇒7

Naturally, the normal case will be slightly more advanced than these examples. A common use of ifelse is in macros implementing loops of various kinds.

6.3 Recursion in m4

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:

Builtin: shift (arg1, …)

Takes any number of arguments, and expands to all its arguments except arg1, separated by commas, with each argument quoted.

The macro shift is recognized only with parameters.

shift
⇒shift
shift(`bar')
⇒
shift(`foo', `bar', `baz')
⇒bar,baz

An example of the use of shift is this macro:

Composite: reverse (…)

Takes any number of arguments, and reverses their order.

It is implemented as:

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. It also shows that shift is usually used with ‘$@’. Another example of this is an implementation of a short-circuiting conditional operator.

Composite: cond (test-1, string-1, equal-1,   [test-2], [string-2], [equal-2], …, [not-equal])

Similar to ifelse, where an equal comparison between the first two strings results in the third, otherwise the first three arguments are discarded and the process repeats. The difference is that each test-<n> is expanded only when it is encountered. This means that every third argument to cond is normally given one more level of quoting than the corresponding argument to ifelse.

Here is the implementation of cond, along with a demonstration of how it can short-circuit the side effects in side. Notice how all the unquoted side effects happen regardless of how many comparisons are made with ifelse, compared with only the relevant effects with cond.

define(`cond',
`ifelse(`$#', `1', `$1',
        `ifelse($1, `$2', `$3',
                `$0(shift(shift(shift($@))))')')')dnl
define(`side', `define(`counter', incr(counter))$1')dnl
define(`example1',
`define(`counter', `0')dnl
ifelse(side(`$1'), `yes', `one comparison: ',
       side(`$1'), `no', `two comparisons: ',
       side(`$1'), `maybe', `three comparisons: ',
       `side(`default answer: ')')counter')dnl
define(`example2',
`define(`counter', `0')dnl
cond(`side(`$1')', `yes', `one comparison: ',
     `side(`$1')', `no', `two comparisons: ',
     `side(`$1')', `maybe', `three comparisons: ',
     `side(`default answer: ')')counter')dnl
example1(`yes')
⇒one comparison: 3
example1(`no')
⇒two comparisons: 3
example1(`maybe')
⇒three comparisons: 3
example1(`feeling rather indecisive today')
⇒default answer: 4
example2(`yes')
⇒one comparison: 1
example2(`no')
⇒two comparisons: 2
example2(`maybe')
⇒three comparisons: 3
example2(`feeling rather indecisive today')
⇒default answer: 4

Another common task that requires iteration is joining a list of arguments into a single string.

Composite: join ([separator], [args…])

Composite: joinall ([separator], [args…])

Generate a single-quoted string, consisting of each arg separated by separator. While joinall always outputs a separator between arguments, join avoids the separator for an empty arg.

Here are some examples of its usage, based on the implementation ‘m4-1.4.11/examples/join.m4’ distributed in this package:

$ m4 -I examples
include(`join.m4')
⇒
join,join(`-'),join(`-', `'),join(`-', `', `')
⇒,,,
joinall,joinall(`-'),joinall(`-', `'),joinall(`-', `', `')
⇒,,,-
join(`-', `1')
⇒1
join(`-', `1', `2', `3')
⇒1-2-3
join(`', `1', `2', `3')
⇒123
join(`-', `', `1', `', `', `2', `')
⇒1-2
joinall(`-', `', `1', `', `', `2', `')
⇒-1---2-
join(`,', `1', `2', `3')
⇒1,2,3
define(`nargs', `$#')dnl
nargs(join(`,', `1', `2', `3'))
⇒1

Examining the implementation shows some interesting points about several m4 programming idioms.

$ m4 -I examples
undivert(`join.m4')dnl
⇒divert(`-1')
⇒# join(sep, args) - join each non-empty ARG into a single
⇒# string, with each element separated by SEP
⇒define(`join',
⇒`ifelse(`$#', `2', ``$2'',
⇒  `ifelse(`$2', `', `', ``$2'_')$0(`$1', shift(shift($@)))')')
⇒define(`_join',
⇒`ifelse(`$#$2', `2', `',
⇒  `ifelse(`$2', `', `', ``$1$2'')$0(`$1', shift(shift($@)))')')
⇒# joinall(sep, args) - join each ARG, including empty ones,
⇒# into a single string, with each element separated by SEP
⇒define(`joinall', ``$2'_$0(`$1', shift($@))')
⇒define(`_joinall',
⇒`ifelse(`$#', `2', `', ``$1$3'$0(`$1', shift(shift($@)))')')
⇒divert`'dnl

First, notice that this implementation creates helper macros _join and _joinall. This division of labor makes it easier to output the correct number of separator instances: join and joinall are responsible for the first argument, without a separator, while _join and _joinall are responsible for all remaining arguments, always outputting a separator when outputting an argument.

Next, observe how join decides to iterate to itself, because the first arg was empty, or to output the argument and swap over to _join. If the argument is non-empty, then the nested ifelse results in an unquoted ‘_’, which is concatenated with the ‘$0’ to form the next macro name to invoke. The joinall implementation is simpler since it does not have to suppress empty arg; it always executes once then defers to _joinall.

Another important idiom is the idea that separator is reused for each iteration. Each iteration has one less argument, but rather than discarding ‘$1’ by iterating with $0(shift($@)), the macro discards ‘$2’ by using $0(`$1', shift(shift($@))).

Next, notice that it is possible to compare more than one condition in a single ifelse test. The test of ‘$#$2’ against ‘2’ allows _join to iterate for two separate reasons—either there are still more than two arguments, or there are exactly two arguments but the last argument is not empty.

Finally, notice that these macros require exactly two arguments to terminate recursion, but that they still correctly result in empty output when given no args (i.e., zero or one macro argument). On the first pass when there are too few arguments, the shift results in no output, but leaves an empty string to serve as the required second argument for the second pass. Put another way, ‘`$1', shift($@)’ is not the same as ‘$@’, since only the former guarantees at least two arguments.

Sometimes, a recursive algorithm requires adding quotes to each element, or treating multiple arguments as a single element:

Composite: quote (…)

Composite: dquote (…)

Composite: dquote_elt (…)

Takes any number of arguments, and adds quoting. With quote, only one level of quoting is added, effectively removing whitespace after commas and turning multiple arguments into a single string. With dquote, two levels of quoting are added, one around each element, and one around the list. And with dquote_elt, two levels of quoting are added around each element.

An actual implementation of these three macros is distributed as ‘m4-1.4.11/examples/quote.m4’ in this package. First, let's examine their usage:

$ m4 -I examples
include(`quote.m4')
⇒
-quote-dquote-dquote_elt-
⇒----
-quote()-dquote()-dquote_elt()-
⇒--`'-`'-
-quote(`1')-dquote(`1')-dquote_elt(`1')-
⇒-1-`1'-`1'-
-quote(`1', `2')-dquote(`1', `2')-dquote_elt(`1', `2')-
⇒-1,2-`1',`2'-`1',`2'-
define(`n', `$#')dnl
-n(quote(`1', `2'))-n(dquote(`1', `2'))-n(dquote_elt(`1', `2'))-
⇒-1-1-2-
dquote(dquote_elt(`1', `2'))
⇒``1'',``2''
dquote_elt(dquote(`1', `2'))
⇒``1',`2''

The last two lines show that when given two arguments, dquote results in one string, while dquote_elt results in two. Now, examine the implementation. Note that quote and dquote_elt make decisions based on their number of arguments, so that when called without arguments, they result in nothing instead of a quoted empty string; this is so that it is possible to distinguish between no arguments and an empty first argument. dquote, on the other hand, results in a string no matter what, since it is still possible to tell whether it was invoked without arguments based on the resulting string.

$ m4 -I examples
undivert(`quote.m4')dnl
⇒divert(`-1')
⇒# quote(args) - convert args to single-quoted string
⇒define(`quote', `ifelse(`$#', `0', `', ``$*'')')
⇒# dquote(args) - convert args to quoted list of quoted strings
⇒define(`dquote', ``$@'')
⇒# dquote_elt(args) - convert args to list of double-quoted strings
⇒define(`dquote_elt', `ifelse(`$#', `0', `', `$#', `1', ```$1''',
⇒                             ```$1'',$0(shift($@))')')
⇒divert`'dnl

It is worth pointing out that ‘quote(args)’ is more efficient than ‘joinall(`,', args)’ for producing the same output.

One more useful macro based on shift allows portably selecting an arbitrary argument (usually greater than the ninth argument), without relying on the GNU extension of multi-digit arguments (see section Arguments to macros).

Composite: argn (n, …)

Expands to argument n out of the remaining arguments. n must be a positive number. Usually invoked as ‘argn(`n',$@)’.

It is implemented as:

define(`argn', `ifelse(`$1', 1, ``$2'',
  `argn(decr(`$1'), shift(shift($@)))')')
⇒
argn(`1', `a')
⇒a
define(`foo', `argn(`11', $@)')
⇒
foo(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k', `l')
⇒k

6.4 Iteration by counting

Here is an example of a loop macro that implements a simple for loop.

Composite: forloop (iterator, start, end, text)

Takes the name in iterator, which must be a valid macro name, and successively assign it each integer value from start to end, inclusive. For each assignment to iterator, append text to the expansion of the forloop. text may refer to iterator. Any definition of iterator prior to this invocation is restored.

It can, for example, be used for simple counting:

$ m4 -I examples
include(`forloop.m4')
⇒
forloop(`i', `1', `8', `i ')
⇒1 2 3 4 5 6 7 8 

For-loops can be nested, like:

$ m4 -I examples
include(`forloop.m4')
⇒
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 the iterator has reached the final value. If it has not finished, it increments the iterator (using the predefined macro incr, see section Decrement and increment operators), and recurses.

Here is an actual implementation of forloop, distributed as ‘m4-1.4.11/examples/forloop.m4’ in this package:

$ m4 -I examples
undivert(`forloop.m4')dnl
⇒divert(`-1')
⇒# forloop(var, from, to, stmt) - simple version
⇒define(`forloop', `pushdef(`$1', `$2')_forloop($@)popdef(`$1')')
⇒define(`_forloop',
⇒       `$4`'ifelse($1, `$3', `', `define(`$1', incr($1))$0($@)')')
⇒divert`'dnl

Notice the careful use of quotes. Certain macro arguments are left unquoted, each for its own reason. Try to find out why these arguments are left unquoted, and see what happens if they are quoted. (As presented, these two macros are useful but not very robust for general use. They lack even basic error handling for cases like start less than end, end not numeric, or iterator not being a macro name. See if you can improve these macros; or see section Answers).

6.5 Iteration by list contents

Here is an example of a loop macro that implements list iteration.

Composite: foreach (iterator, paren-list, text)

Composite: foreachq (iterator, quote-list, text)

Takes the name in iterator, which must be a valid macro name, and successively assign it each value from paren-list or quote-list. In foreach, paren-list is a comma-separated list of elements contained in parentheses. In foreachq, quote-list is a comma-separated list of elements contained in a quoted string. For each assignment to iterator, append text to the overall expansion. text may refer to iterator. Any definition of iterator prior to this invocation is restored.

As an example, this displays each word in a list inside of a sentence, using an implementation of foreach distributed as ‘m4-1.4.11/examples/foreach.m4’, and foreachq in ‘m4-1.4.11/examples/foreachq.m4’.

$ m4 -I examples
include(`foreach.m4')
⇒
foreach(`x', (foo, bar, foobar), `Word was: x
')dnl
⇒Word was: foo
⇒Word was: bar
⇒Word was: foobar
include(`foreachq.m4')
⇒
foreachq(`x', `foo, bar, foobar', `Word was: x
')dnl
⇒Word was: foo
⇒Word was: bar
⇒Word was: foobar

It is possible to be more complex; each element of the paren-list or quote-list can itself be a list, to pass as further arguments to a helper macro. This example generates a shell case statement:

$ m4 -I examples
include(`foreach.m4')
⇒
define(`_case', `  $1)
    $2=" $1";;
')dnl
define(`_cat', `$1$2')dnl
case $`'1 in
⇒case $1 in
foreach(`x', `(`(`a', `vara')', `(`b', `varb')', `(`c', `varc')')',
        `_cat(`_case', x)')dnl
⇒  a)
⇒    vara=" a";;
⇒  b)
⇒    varb=" b";;
⇒  c)
⇒    varc=" c";;
esac
⇒esac

The implementation of the foreach macro is a bit more involved; it is a wrapper around two helper macros. First, _arg1 is needed to grab the first element of a list. Second, _foreach implements the recursion, successively walking through the original list. Here is a simple implementation of foreach:

$ m4 -I examples
undivert(`foreach.m4')dnl
⇒divert(`-1')
⇒# foreach(x, (item_1, item_2, ..., item_n), stmt)
⇒#   parenthesized list, simple version
⇒define(`foreach', `pushdef(`$1')_foreach($@)popdef(`$1')')
⇒define(`_arg1', `$1')
⇒define(`_foreach', `ifelse(`$2', `()', `',
⇒  `define(`$1', _arg1$2)$3`'$0(`$1', (shift$2), `$3')')')
⇒divert`'dnl

Unfortunately, that implementation is not robust to macro names as list elements. Each iteration of _foreach is stripping another layer of quotes, leading to erratic results if list elements are not already fully expanded. The first cut at implementing foreachq takes this into account. Also, when using quoted elements in a paren-list, the overall list must be quoted. A quote-list has the nice property of requiring fewer characters to create a list containing the same quoted elements. To see the difference between the two macros, we attempt to pass double-quoted macro names in a list, expecting the macro name on output after one layer of quotes is removed during list iteration and the final layer removed during the final rescan:

$ m4 -I examples
define(`a', `1')define(`b', `2')define(`c', `3')
⇒
include(`foreach.m4')
⇒
include(`foreachq.m4')
⇒
foreach(`x', `(``a'', ``(b'', ``c)'')', `x
')
⇒1
⇒(2)1
⇒
⇒, x
⇒)
foreachq(`x', ```a'', ``(b'', ``c)''', `x
')dnl
⇒a
⇒(b
⇒c)

Obviously, foreachq did a better job; here is its implementation:

$ m4 -I examples
undivert(`foreachq.m4')dnl
⇒include(`quote.m4')dnl
⇒divert(`-1')
⇒# foreachq(x, `item_1, item_2, ..., item_n', stmt)
⇒#   quoted list, simple version
⇒define(`foreachq', `pushdef(`$1')_foreachq($@)popdef(`$1')')
⇒define(`_arg1', `$1')
⇒define(`_foreachq', `ifelse(quote($2), `', `',
⇒  `define(`$1', `_arg1($2)')$3`'$0(`$1', `shift($2)', `$3')')')
⇒divert`'dnl

Notice that _foreachq had to use the helper macro quote defined earlier (see section Recursion in m4), to ensure that the embedded ifelse call does not go haywire if a list element contains a comma. Unfortunately, this implementation of foreachq has its own severe flaw. Whereas the foreach implementation was linear, this macro is quadratic in the number of list elements, and is much more likely to trip up the limit set by the command line option ‘--nesting-limit’ (or ‘-L’, see section Invoking m4). Additionally, this implementation does not expand ‘defn(`iterator')’ very well, when compared with foreach.

$ m4 -I examples
include(`foreach.m4')include(`foreachq.m4')
⇒
foreach(`name', `(`a', `b')', ` defn(`name')')
⇒ a b
foreachq(`name', ``a', `b'', ` defn(`name')')
⇒ _arg1(`a', `b') _arg1(shift(`a', `b'))

It is possible to have robust iteration with linear behavior and sane iterator contents for either list style. See if you can learn from the best elements of both of these implementations to create robust macros (or see section Answers).

This document was generated by root on April, 3 2008 using texi2html 1.78.