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[Eric Blake]

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- Log -----------------------------------------------------------------
commit 550a9fa1e1f12b297730bf6db132fc4818686a33
Author: Eric Blake <address@hidden>
Date:   Thu Aug 21 22:17:33 2008 -0600

    Avoid extra side effects in m4sugar list expansion.
    
    * lib/m4sugar/m4sugar.m4 (m4_mapall_sep, m4_list_cmp): Wrap
    around...
    (_m4_mapall_sep, _m4_list_cmp_raw): ...new helpers, to avoid
    duplicate side effects.
    (m4_version_compare): Adjust caller.
    * lib/m4sugar/foreach.m4 (m4_list_cmp): Rename...
    (_m4_list_cmp_raw): ...to match m4sugar.
    * doc/autoconf.texi (Looping constructs): Document the behavior of
    side effects.
    * tests/m4sugar.at (M4 loops, m4@&address@hidden, m4@&address@hidden):
    Ensure only one side effect.
    (recursion): Fix test typo.
    Reported by Ralf Wildenhues.
    
    Signed-off-by: Eric Blake <address@hidden>

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog              |   17 +++++++++++++++++
 doc/autoconf.texi      |   16 ++++++++++++++++
 lib/m4sugar/foreach.m4 |    8 +++++---
 lib/m4sugar/m4sugar.m4 |   22 ++++++++++++++++------
 tests/m4sugar.at       |   46 +++++++++++++++++++++++++++++++++++++++++++---
 5 files changed, 97 insertions(+), 12 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 7db4f57..99a0e05 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,20 @@
+2008-08-21  Eric Blake  <address@hidden>
+
+       Avoid extra side effects in m4sugar list expansion.
+       * lib/m4sugar/m4sugar.m4 (m4_mapall_sep, m4_list_cmp): Wrap
+       around...
+       (_m4_mapall_sep, _m4_list_cmp_raw): ...new helpers, to avoid
+       duplicate side effects.
+       (m4_version_compare): Adjust caller.
+       * lib/m4sugar/foreach.m4 (m4_list_cmp): Rename...
+       (_m4_list_cmp_raw): ...to match m4sugar.
+       * doc/autoconf.texi (Looping constructs): Document the behavior of
+       side effects.
+       * tests/m4sugar.at (M4 loops, m4@&address@hidden, m4@&address@hidden):
+       Ensure only one side effect.
+       (recursion): Fix test typo.
+       Reported by Ralf Wildenhues.
+
 2008-08-21  Ralf Wildenhues  <address@hidden>
 
        * TODO: Add item for compiler default flags.
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 5742f7e..75c110e 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -10756,6 +10756,22 @@ It is common to see lists with unquoted elements when 
those elements are
 not likely to be macro names, as in @samp{[fputc_unlocked,
 fgetc_unlocked]}.
 
+Although not generally recommended, it is possible for quoted lists to
+have side effects; all side effects are expanded only once, and prior to
+visiting any list element.  On the other hand, the fact that unquoted
+macros are expanded exactly once means that macros without side effects
+can be used to generate lists.  For example,
+
address@hidden
+m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [i])
address@hidden
address@hidden
+m4_define([list], [[1], [2], [3]])
address@hidden
+m4_foreach([i], [list], [i])
address@hidden
address@hidden example
+
 @defmac m4_car (@var{list})
 @msindex{car}
 Expands to the quoted first element of the comma-separated quoted
diff --git a/lib/m4sugar/foreach.m4 b/lib/m4sugar/foreach.m4
index 91a9643..c35d5d3 100644
--- a/lib/m4sugar/foreach.m4
+++ b/lib/m4sugar/foreach.m4
@@ -338,6 +338,8 @@ m4_define([m4_joinall],
 # -----------------
 # Compare the two lists of integer expressions A and B.
 #
+# m4_list_cmp takes care of any side effects; we only override
+# _m4_list_cmp_raw, where we can safely expand lists multiple times.
 # First, insert padding so that both lists are the same length; the
 # trailing +0 is necessary to handle a missing list.  Next, create a
 # temporary macro to perform pairwise comparisons until an inequality
@@ -346,9 +348,9 @@ m4_define([m4_joinall],
 #         m4_eval([($2) != ($4)]), [1], [m4_cmp([$2], [$4])],
 #         [0]_m4_popdef([_m4_cmp], [_m4_size]))
 # then calls _m4_cmp([1+0], [0], [1], [2+0])
-m4_define([m4_list_cmp],
-[m4_if([$1], [$2], 0,
-  [m4_pushdef([_m4_size])_$0($1+0_m4_list_pad(m4_count($1), m4_count($2)),
+m4_define([_m4_list_cmp_raw],
+[m4_if([$1], [$2], 0, [m4_pushdef(
+     [_m4_size])_m4_list_cmp($1+0_m4_list_pad(m4_count($1), m4_count($2)),
                             $2+0_m4_list_pad(m4_count($2), m4_count($1)))])])
 
 m4_define([_m4_list_pad],
diff --git a/lib/m4sugar/m4sugar.m4 b/lib/m4sugar/m4sugar.m4
index b337370..f2fde7a 100644
--- a/lib/m4sugar/m4sugar.m4
+++ b/lib/m4sugar/m4sugar.m4
@@ -1043,7 +1043,8 @@ m4_define([m4_mapall],
 # arguments.
 #
 # For m4_mapall_sep, merely expand the first iteration without the
-# separator, then include separator as part of subsequent recursion.
+# separator, then include separator as part of subsequent recursion;
+# but avoid extra expansion of LIST's side-effects via a helper macro.
 # For m4_map_sep, things are trickier - we don't know if the first
 # list element is an empty sublist, so we must define a self-modifying
 # helper macro and use that as the separator instead.
@@ -1052,8 +1053,10 @@ m4_define([m4_map_sep],
 [_m4_map([_m4_apply([m4_Sep([$2])[]$1]], [], $3)m4_popdef([m4_Sep])])
 
 m4_define([m4_mapall_sep],
-[m4_if([$3], [], [],
-       [m4_apply([$1], m4_car($3))_m4_map([m4_apply([$2[]$1]], $3)])])
+[m4_if([$3], [], [], [_$0([$1], [$2], $3)])])
+
+m4_define([_m4_mapall_sep],
+[m4_apply([$1], [$3])_m4_map([m4_apply([$2[]$1]], m4_shift2($@))])
 
 # _m4_map(PREFIX, IGNORED, SUBLIST, ...)
 # --------------------------------------
@@ -2235,15 +2238,19 @@ m4_define([m4_cmp],
 # long lists, since less text is being expanded for deciding when to end
 # recursion.  The recursion is between a pair of macros that alternate
 # which list is trimmed by one element; this is more efficient than
-# calling m4_cdr on both lists from a single macro.
+# calling m4_cdr on both lists from a single macro.  Guarantee exactly
+# one expansion of both lists' side effects.
 m4_define([m4_list_cmp],
+[_$0_raw(m4_dquote($1), m4_dquote($2))])
+
+m4_define([_m4_list_cmp_raw],
 [m4_if([$1], [$2], [0], [_m4_list_cmp_1([$1], $2)])])
 
 m4_define([_m4_list_cmp],
 [m4_if([$1], [], [0m4_ignore], [$2], [0], [m4_unquote], [$2m4_ignore])])
 
 m4_define([_m4_list_cmp_1],
-[_m4_list_cmp_2([$2], m4_dquote(m4_shift2($@)), $1)])
+[_m4_list_cmp_2([$2], [m4_shift2($@)], $1)])
 
 m4_define([_m4_list_cmp_2],
 [_m4_list_cmp([$1$3], m4_cmp([$3+0], [$1+0]))(
@@ -2335,8 +2342,11 @@ m4_dquote(m4_dquote(m4_defn([m4_cr_Letters])))[[+],
 #  -1 if VERSION-1 < VERSION-2
 #   0 if           =
 #   1 if           >
+#
+# Since _m4_version_unletter does not output side effects, we can
+# safely bypass the overhead of m4_version_cmp.
 m4_define([m4_version_compare],
-[m4_list_cmp(_m4_version_unletter([$1]), _m4_version_unletter([$2]))])
+[_m4_list_cmp_raw(_m4_version_unletter([$1]), _m4_version_unletter([$2]))])
 
 
 # m4_PACKAGE_NAME
diff --git a/tests/m4sugar.at b/tests/m4sugar.at
index a8cecf9..75c5057 100644
--- a/tests/m4sugar.at
+++ b/tests/m4sugar.at
@@ -527,6 +527,11 @@ m4_version_compare([1.1a], [1.1A])
 m4_version_compare([1z], [1aa])
 m4_version_compare([2.61a], [2.61a-248-dc51])
 m4_version_compare([2.61b], [2.61a-248-dc51])
+dnl Test that side effects to m4_list_cmp occur exactly once
+m4_list_cmp([[0], [0], [0]m4_errprintn([hi])],
+            [[0], [0], [0]m4_errprintn([hi])])
+m4_list_cmp([[0], [0], [0]m4_errprintn([hi])],
+            [[0], [0], [0]m4_errprintn([bye])])
 ]],
 [[-1
 1
@@ -542,6 +547,12 @@ m4_version_compare([2.61b], [2.61a-248-dc51])
 -1
 -1
 1
+0
+0
+]], [[hi
+hi
+hi
+bye
 ]])
 
 AT_CLEANUP
@@ -640,6 +651,8 @@ AT_CLEANUP
 
 AT_SETUP([M4 loops])
 
+AT_KEYWORDS([m4@&address@hidden m4@&address@hidden m4@&address@hidden)
+
 AT_CHECK_M4SUGAR_TEXT([[dnl
 m4_define([myvar], [outer value])dnl
 m4_for([myvar], 1, 3, 1, [ myvar])
@@ -683,6 +696,8 @@ m4_foreach([myvar], [[a], [b, c], [d], [e
 m4_foreach_w([myvar], [a  b c, d,e f
 g], [ myvar|])
 myvar
+dnl only one side effect expansion, prior to visiting list elements
+m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [m4_errprintn(i)])dnl
 ]],
 [[ 1 2 3
  1 2 3
@@ -716,7 +731,11 @@ myvar
 | f|
  a| b| c,| d,e| f| g|
 outer value
-]], [])
+]], [[hi
+1
+2
+3
+]])
 
 AT_DATA_M4SUGAR([script.4s],
 [[m4_init
@@ -790,6 +809,11 @@ m4_define([a1], [oops])dnl
 m4_define([pass1], [oops])dnl
 m4_map([a], [[[a]]])1
 m4_map([m4_unquote([a])], [m4_dquote([a])])
+dnl only one side effect expansion, prior to visiting list elements
+m4_map([m4_errprintn], [[[1]], [[2]], [[3]]m4_errprintn([hi])])dnl
+m4_map_sep([m4_errprintn], [], [[[1]], [[2]], [[3]]m4_errprintn([hi])])dnl
+m4_mapall([m4_errprintn], [[[1]], [[2]], [[3]]m4_errprintn([hi])])dnl
+m4_mapall_sep([m4_errprintn], [], [[[1]], [[2]], [[3]]m4_errprintn([hi])])dnl
 ]],
 [[
  1 2
@@ -806,7 +830,23 @@ m4_map([m4_unquote([a])], [m4_dquote([a])])
 -
 pass1
 pass
-]], [])
+]], [[hi
+1
+2
+3
+hi
+1
+2
+3
+hi
+1
+2
+3
+hi
+1
+2
+3
+]])
 
 AT_CLEANUP
 
@@ -1010,7 +1050,7 @@ m4_undefine(1m4_for([i], [2], [10000], [], [,i]))dnl
 m4_bpatsubsts([a1]m4_for([i], [1], [10000], [], [,i]), [a2], [A])
 m4_bmatch([9997]m4_for([i], [1], [10000], [], [,^i$]))
 m4_define([up], [m4_define([$1], m4_incr($1))$1])m4_define([j], 0)dnl
-m4_cond(m4_for([i], [1], [10000], [], [[up([j])], [9990], i,]) [oops])
+m4_cond(m4_for([i], [1], [10000], [], [[up([j])], [9990], i,]) [oops]) j
 m4_count(m4_transform_pair([,m4_quote], []m4_transform([,m4_echo]m4_for([i],
   [1], [10000], [], [,i]))))
 m4_divert_pop(0)


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