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
⇒define_blind
define_blind(`foo', `arguments were $*')
⇒
foo
⇒foo
foo(`bar')
⇒arguments were bar
define(`blah', defn(`foo'))
⇒
blah
⇒blah
blah(`a', `b')
⇒arguments were a,b
defn(`blah')
⇒ifelse(`$#', `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 ‘m4-1.4.16/examples/curry.m4’ included in the distribution.

$ m4 -I examples
include(`curry.m4')include(`stack.m4')
⇒
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
undivert(`curry.m4')dnl
⇒divert(`-1')
⇒# 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')')
⇒divert`'dnl

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
include(`curry.m4')include(`stack.m4')
⇒
define(`rename', `copy($@)undefine(`$1')')dnl
define(`copy', `ifdef(`$2', `errprint(`$2 already defined
')m4exit(`1')',
   `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