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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.

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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]@c)

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

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

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')
define(`foo', `bar')
ifelse(foo, `bar', `true', `false')
ifelse(foo, `foo', `true', `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(`a', `b', `c')

For an example of a way to make defining blind macros easier, see Building macros with macros.

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

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.

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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(`foo', `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(`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]@c, [string-2]@c, [equal-2]@c, …, [not-equal]@c)

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.

`ifelse(`$#', `1', `$1',
        `ifelse($1, `$2', `$3',
define(`side', `define(`counter', incr(counter))$1')dnl
`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(`counter', `0')dnl
cond(`side(`$1')', `yes', `one comparison: ',
     `side(`$1')', `no', `two comparisons: ',
     `side(`$1')', `maybe', `three comparisons: ',
     `side(`default answer: ')')counter')dnl
⇒one comparison: 3
⇒two comparisons: 3
⇒three comparisons: 3
example1(`feeling rather indecisive today')
⇒default answer: 4
⇒one comparison: 1
⇒two comparisons: 2
⇒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]@c, [args…]@c)
Composite: joinall ([separator]@c, [args…]@c)

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 `m4-1.4.13/examples/join.m4' distributed in this package:

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

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

$ m4 -I examples
⇒# join(sep, args) - join each non-empty ARG into a single
⇒# string, with each element separated by SEP
⇒`ifelse(`$#', `2', ``$2'',
⇒  `ifelse(`$2', `', `', ``$2'_')$0(`$1', shift(shift($@)))')')
⇒`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(`$1', shift($@))')
⇒`ifelse(`$#', `2', `', ``$1$3'$0(`$1', shift(shift($@)))')')

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 `m4-1.4.13/examples/quote.m4' in this package. First, let's examine their usage:

$ m4 -I examples
-quote(`1', `2')-dquote(`1', `2')-dquote_elt(`1', `2')-
define(`n', `$#')dnl
-n(quote(`1', `2'))-n(dquote(`1', `2'))-n(dquote_elt(`1', `2'))-
dquote(dquote_elt(`1', `2'))
dquote_elt(dquote(`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
⇒# 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'',$0(shift($@))')')

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')
define(`foo', `argn(`11', $@)')
foo(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k', `l')

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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
forloop(`i', `1', `8', `i ')
⇒1 2 3 4 5 6 7 8 

For-loops can be nested, like:

$ m4 -I examples
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 `m4-1.4.13/examples/forloop.m4' in this package:

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

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).

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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 `m4-1.4.13/examples/foreach.m4', and foreachq in `m4-1.4.13/examples/foreachq.m4'.

$ m4 -I examples
foreach(`x', (foo, bar, foobar), `Word was: x
⇒Word was: foo
⇒Word was: bar
⇒Word was: foobar
foreachq(`x', `foo, bar, foobar', `Word was: x
⇒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
define(`_case', `  $1)
    $2=" $1";;
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";;

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
⇒# 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')')')

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')
foreach(`x', `(``a'', ``(b'', ``c)'')', `x
⇒, x
foreachq(`x', ```a'', ``(b'', ``c)''', `x

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

$ m4 -I examples
⇒# 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')')')

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
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).

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6.6 Working with definition stacks

Thanks to pushdef, manipulation of a stack is an intrinsic operation in m4. Normally, only the topmost definition in a stack is important, but sometimes, it is desirable to manipulate the entire definition stack.

Composite: stack_foreach (macro, action)
Composite: stack_foreach_lifo (macro, action)

For each of the pushdef definitions associated with macro, invoke the macro action with a single argument of that definition. stack_foreach visits the oldest definition first, while stack_foreach_lifo visits the current definition first. action should not modify or dereference macro. There are a few special macros, such as defn, which cannot be used as the macro parameter.

A sample implementation of these macros is distributed in the file `m4-1.4.13/examples/stack.m4'.

$ m4 -I examples
pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3')
define(`show', ``$1'
stack_foreach(`a', `show')dnl
stack_foreach_lifo(`a', `show')dnl

Now for the implementation. Note the definition of a helper macro, _stack_reverse, which destructively swaps the contents of one stack of definitions into the reverse order in the temporary macro `tmp-$1'. By calling the helper twice, the original order is restored back into the macro `$1'; since the operation is destructive, this explains why `$1' must not be modified or dereferenced during the traversal. The caller can then inject additional code to pass the definition currently being visited to `$2'. The choice of helper names is intentional; since `-' is not valid as part of a macro name, there is no risk of conflict with a valid macro name, and the code is guaranteed to use defn where necessary. Finally, note that any macro used in the traversal of a pushdef stack, such as pushdef or defn, cannot be handled by stack_foreach, since the macro would temporarily be undefined during the algorithm.

$ m4 -I examples
⇒# stack_foreach(macro, action)
⇒# Invoke ACTION with a single argument of each definition
⇒# from the definition stack of MACRO, starting with the oldest.
⇒`_stack_reverse(`$1', `tmp-$1')'dnl
⇒`_stack_reverse(`tmp-$1', `$1', `$2(defn(`$1'))')')
⇒# stack_foreach_lifo(macro, action)
⇒# Invoke ACTION with a single argument of each definition
⇒# from the definition stack of MACRO, starting with the newest.
⇒`_stack_reverse(`$1', `tmp-$1', `$2(defn(`$1'))')'dnl
⇒`_stack_reverse(`tmp-$1', `$1')')
⇒`ifdef(`$1', `pushdef(`$2', defn(`$1'))$3`'popdef(`$1')$0($@)')')

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6.7 Building macros with macros

Since m4 is a macro language, it is possible to write macros that can build other macros. First on the list is a way to automate the creation of blind macros.

Composite: define_blind (name, [value]@c)

Defines name as a blind macro, such that name will expand to value only when given explicit arguments. value should not be the result of defn (see section Renaming macros). This macro is only recognized with parameters, and results in an empty string.

Defining a macro to define another macro can be a bit tricky. We want to use a literal `$#' in the argument to the nested define. However, if `$' and `#' are adjacent in the definition of define_blind, then it would be expanded as the number of arguments to define_blind rather than the intended number of arguments to name. The solution is to pass the difficult characters through extra arguments to a helper macro _define_blind. When composing macros, it is a common idiom to need a helper macro to concatenate text that forms parameters in the composed macro, rather than interpreting the text as a parameter of the composing macro.

As for the limitation against using defn, there are two reasons. If a macro was previously defined with define_blind, then it can safely be renamed to a new blind macro using plain define; using define_blind to rename it just adds another layer of ifelse, occupying memory and slowing down execution. And if a macro is a builtin, then it would result in an attempt to define a macro consisting of both text and a builtin token; this is not supported, and the builtin token is flattened to an empty string.

With that explanation, here's the definition, and some sample usage. Notice that define_blind is itself a blind macro.

$ m4 -d
define(`define_blind', `ifelse(`$#', `0', ``$0'',
`_$0(`$1', `$2', `$'`#', `$'`0')')')
define(`_define_blind', `define(`$1',
`ifelse(`$3', `0', ``$4'', `$2')')')
define_blind(`foo', `arguments were $*')
⇒arguments were bar
define(`blah', defn(`foo'))
blah(`a', `b')
⇒arguments were a,b
⇒ifelse(`$#', `0', ``$0'', `arguments were $*')

Another interesting composition tactic is argument currying, or factoring a macro that takes multiple arguments for use in a context that provides exactly one argument.

Composite: curry (macro, …)

Expand to a macro call that takes exactly one argument, then appends that argument to the original arguments and invokes macro with the resulting list of arguments.

A demonstration of currying makes the intent of this macro a little more obvious. The macro stack_foreach mentioned earlier is an example of a context that provides exactly one argument to a macro name. But coupled with currying, we can invoke reverse with two arguments for each definition of a macro stack. This example uses the file `m4-1.4.13/examples/curry.m4' included in the distribution.

$ m4 -I examples
define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'',
                          `reverse(shift($@)), `$1'')')
pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3')
stack_foreach(`a', `:curry(`reverse', `4')')
⇒:1, 4:2, 4:3, 4
curry(`curry', `reverse', `1')(`2')(`3')
⇒3, 2, 1

Now for the implementation. Notice how curry leaves off with a macro name but no open parenthesis, while still in the middle of collecting arguments for `$1'. The macro _curry is the helper macro that takes one argument, then adds it to the list and finally supplies the closing parenthesis. The use of a comma inside the shift call allows currying to also work for a macro that takes one argument, although it often makes more sense to invoke that macro directly rather than going through curry.

$ m4 -I examples
⇒# curry(macro, args)
⇒# Expand to a macro call that takes one argument, then invoke
⇒# macro(args, extra).
⇒define(`curry', `$1(shift($@,)_$0')
⇒define(`_curry', ``$1')')

Unfortunately, with M4 1.4.x, curry is unable to handle builtin tokens, which are silently flattened to the empty string when passed through another text macro. This limitation will be lifted in a future release of M4.

Putting the last few concepts together, it is possible to copy or rename an entire stack of macro definitions.

Composite: copy (source, dest)
Composite: rename (source, dest)

Ensure that dest is undefined, then define it to the same stack of definitions currently in source. copy leaves source unchanged, while rename undefines source. There are only a few macros, such as copy or defn, which cannot be copied via this macro.

The implementation is relatively straightforward (although since it uses curry, it is unable to copy builtin macros, such as the second definition of a as a synonym for divnum. See if you can design a version that works around this limitation, or see section Answers).

$ m4 -I examples
define(`rename', `copy($@)undefine(`$1')')dnl
define(`copy', `ifdef(`$2', `errprint(`$2 already defined
   `stack_foreach(`$1', `curry(`pushdef', `$2')')')')dnl
pushdef(`a', `1')pushdef(`a', defn(`divnum'))pushdef(`a', `2')
copy(`a', `b')
rename(`b', `c')
a b c
⇒2 b 2
popdef(`a', `c')c a
⇒ 0
popdef(`a', `c')a c
⇒1 1

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