Re: [PATCH] add m4_stack_foreach and m4_stack_foreach_lifo

[Eric Blake]

Paolo Bonzini <bonzini <at> gnu.org> writes:

> 
> This patch extracts the pushdef-stack manipulation code of m4_copy into
> new general-purpose macros.  The resulting m4_copy implementation is quite
> elegant, using m4_curry to push new definitions.

In documenting this trick for M4, I noticed that m4_curry and builtins don't 
mix unless you use M4 1.6 or newer (1.4.x has the limitation that builtins are 
silently converted to the empty string if passed through a $n reference of a 
text macro; it took a lot of infrastructure in m4.git branch-1.6 to overcome 
this limitation, and will be another couple years before we can blindly ask 
people to use newer m4 than 1.4.x).

m4_curry([m4_define], [mylen])(m4_defn([m4_len]))

results in mylen defined as [], not a synonym for <m4_len>.

I also noticed that Paolo's original suggestion for m4_copy as m4_stack_foreach 
and m4_curry suffers a similar fate, but fortunately, my rewrite to use the 
faster m4_stack_foreach_sep has the serendipitous side-effect of avoiding this 
problem.  I'm checking this in to make it clear on the limitations, and ensure 
we don't regress.

From: Eric Blake <address@hidden>
Date: Thu, 18 Dec 2008 15:55:58 -0700
Subject: [PATCH] Mention limitation of M4 1.4.x on builtin tokens.

* doc/autoconf.texi (Redefined M4 Macros) <m4_dumpdef>: Document
ramification of M4 1.4.x's inability to pass builtin tokens
through text macros.
(Evaluation Macros) <m4_curry>: Likewise.
* tests/m4sugar.at (m4@&address@hidden): Enhance test.
* NEWS: Mention subtle change in m4_dumpdef semantics.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog         |    8 ++++++++
 NEWS              |    4 +++-
 doc/autoconf.texi |   11 +++++++++++
 tests/m4sugar.at  |    8 ++++++--
 4 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 86c6c4b..c1d367a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,13 @@
 2008-12-18  Eric Blake  <address@hidden>
 
+       Mention limitation of M4 1.4.x on builtin tokens.
+       * doc/autoconf.texi (Redefined M4 Macros) <m4_dumpdef>: Document
+       ramification of M4 1.4.x's inability to pass builtin tokens
+       through text macros.
+       (Evaluation Macros) <m4_curry>: Likewise.
+       * tests/m4sugar.at (m4@&address@hidden): Enhance test.
+       * NEWS: Mention subtle change in m4_dumpdef semantics.
+
        Document m4_version_prereq.
        * doc/autoconf.texi (Number processing Macros)
        <m4_version_prereq>: Add documentation.
diff --git a/NEWS b/NEWS
index 2e7fa1e..30ac715 100644
--- a/NEWS
+++ b/NEWS
@@ -29,7 +29,9 @@ GNU Autoconf NEWS - User visible changes.
    m4_chomp  m4_curry  m4_default_quoted  m4_esyscmd_s  m4_map_args
    m4_map_args_pair  m4_set_map
 
-** The following m4sugar macros are documented now:
+** The following m4sugar macros are documented now, but in some cases
+   with slightly different semantics than what the previous
+   undocumented version had:
    m4_copy  m4_dumpdefs  m4_rename  m4_version_prereq
 
 ** The m4sugar macro m4_expand has been taught to handle unterminated
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index aeeb388..1e211a1 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -10563,6 +10563,12 @@ Redefined M4 Macros
 calls @code{m4_dumpdef} for all of the
 @code{m4_pushdef} stack of definitions, starting with the current, and
 silently does nothing if @var{name} is undefined.
+
+Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a
+builtin is output as the empty string.  This behavior is rectified by
+using M4 1.6 or newer.  However, this behavior difference means that
address@hidden should only be used while developing m4sugar macros,
+and never in the final published form of a macro.
 @end defmac
 
 @defmac m4_esyscmd_s (@var{command})
@@ -11270,6 +11276,11 @@ Evaluation Macros
 m4_curry([m4_curry], [m4_reverse], [1])([2])([3])
 @result{}3, 2, 1
 @end example
+
+Unfortunately, due to a limitation in M4 1.4.x, it is not possible to
+pass the definition of a builtin macro as the argument to the output of
address@hidden; the empty string is used instead of the builtin token.
+This behavior is rectified by using M4 1.6 or newer.
 @end defmac
 
 @defmac m4_do (@var{arg}, @dots{})
diff --git a/tests/m4sugar.at b/tests/m4sugar.at
index 36787fc..8a538f0 100644
--- a/tests/m4sugar.at
+++ b/tests/m4sugar.at
@@ -138,6 +138,7 @@ AT_CHECK([grep 'm4@&address@hidden: undefined.*oops' 
stderr], 
[0], [ignore])
 # Check that pushdef stacks can be renamed.
 AT_CHECK_M4SUGAR_TEXT([[m4_pushdef([a], [1])dnl
 m4_pushdef([a], [2])dnl
+m4_pushdef([a], m4_defn([m4_divnum]))dnl
 a b c
 m4_rename([a], [b])dnl
 a b c
@@ -147,8 +148,11 @@ m4_popdef([b], [c])dnl
 a b c
 m4_popdef([b], [c])dnl
 a b c
-]], [[2 b c
-a 2 c
+m4_popdef([b], [c])dnl
+a b c
+]], [[0 b c
+a 0 c
+a 0 0
 a 2 2
 a 1 1
 a b c
-- 
1.6.0.4