m4: Stacks
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
'm4-1.4.18/examples/stack.m4'.
$ m4 -I examples
include(`stack.m4')
=>
pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3')
=>
define(`show', ``$1'
')
=>
stack_foreach(`a', `show')dnl
=>1
=>2
=>3
stack_foreach_lifo(`a', `show')dnl
=>3
=>2
=>1
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
undivert(`stack.m4')dnl
=>divert(`-1')
=># stack_foreach(macro, action)
=># Invoke ACTION with a single argument of each definition
=># from the definition stack of MACRO, starting with the oldest.
=>define(`stack_foreach',
=>`_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.
=>define(`stack_foreach_lifo',
=>`_stack_reverse(`$1', `tmp-$1', `$2(defn(`$1'))')'dnl
=>`_stack_reverse(`tmp-$1', `$1')')
=>define(`_stack_reverse',
=>`ifdef(`$1', `pushdef(`$2', defn(`$1'))$3`'popdef(`$1')$0($@)')')
=>divert`'dnl