m4sugar.m4

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# same as the one in which macro are expanded.  (The bug above can
# indeed be described as: a macro has been AC_PROVIDE'd, but it is
# emitted after: the lack of correlation between emission and expansion
# order is guilty).
#
# How to do that?  You keeping the stack of diversions to elaborate the
# macros, but each time a macro is fully expanded, emit it immediately.
#
# In the example above, when TEST2a is expanded, but it's epilogue is
# not run yet, you have:
#
#	   GROW - 2:
#	   GROW - 1: TEST2a
#	   GROW:     Elaboration of TEST1
#	   BODY:
#
# The epilogue of TEST2a emits it immediately:
#
#	   GROW - 2:
#	   GROW - 1:
#	   GROW:     Elaboration of TEST1
#	   BODY:     TEST2a
#
# TEST2b then requires TEST3, so right before the epilogue of TEST3, you
# have:
#
#	   GROW - 2: TEST3
#	   GROW - 1: Elaboration of TEST2b
#	   GROW:     Elaboration of TEST1
#	   BODY:      TEST2a
#
# The epilogue of TEST3 emits it:
#
#	   GROW - 2:
#	   GROW - 1: Elaboration of TEST2b
#	   GROW:     Elaboration of TEST1
#	   BODY:     TEST2a; TEST3
#
# TEST2b is now completely expanded, and emitted:
#
#	   GROW - 2:
#	   GROW - 1:
#	   GROW:     Elaboration of TEST1
#	   BODY:     TEST2a; TEST3; TEST2b
#
# and finally, TEST1 is finished and emitted:
#
#	   GROW - 2:
#	   GROW - 1:
#	   GROW:
#	   BODY:     TEST2a; TEST3; TEST2b: TEST1
#
# The idea, is simple, but the implementation is a bit evolved.  If you
# are like me, you will want to see the actual functioning of this
# implementation to be convinced.  The next section gives the full
# details.
#
#
# The Axel Thimm implementation at work
# -------------------------------------
#
# We consider the macros above, and this configure.ac:
#
#	    AC_INIT
#	    TEST1
#
# You should keep the definitions of _m4_defun_pro, _m4_defun_epi, and
# m4_require at hand to follow the steps.
#
# This implements tries not to assume that of the current diversion is
# BODY, so as soon as a macro (m4_defun'd) is expanded, we first
# record the current diversion under the name _m4_divert_dump (denoted
# DUMP below for short).  This introduces an important difference with
# the previous versions of Autoconf: you cannot use m4_require if you
# were not inside an m4_defun'd macro, and especially, you cannot
# m4_require directly from the top level.
#
# We have not tried to simulate the old behavior (better yet, we
# diagnose it), because it is too dangerous: a macro m4_require'd from
# the top level is expanded before the body of `configure', i.e., before
# any other test was run.  I let you imagine the result of requiring
# AC_STDC_HEADERS for instance, before AC_PROG_CC was actually run....
#
# After AC_INIT was run, the current diversion is BODY.
# * AC_INIT was run
#   DUMP:                undefined
#   diversion stack:     BODY |-
#
# * TEST1 is expanded
# The prologue of TEST1 sets AC_DIVERSION_DUMP, which is the diversion
# where the current elaboration will be dumped, to the current
# diversion.  It also m4_divert_push to GROW, where the full
# expansion of TEST1 and its dependencies will be elaborated.
#   DUMP:       BODY
#   BODY:       empty
#   diversions: GROW, BODY |-
#
# * TEST1 requires TEST2a: prologue
# m4_require m4_divert_pushes another temporary diversion GROW - 1 (in
# fact, the diversion whose number is one less than the current
# diversion), and expands TEST2a in there.
#   DUMP:       BODY
#   BODY:       empty
#   diversions: GROW-1, GROW, BODY |-
#
# * TEST2a is expanded.
# Its prologue pushes the current diversion again.
#   DUMP:       BODY
#   BODY:       empty
#   diversions: GROW - 1, GROW - 1, GROW, BODY |-
# It is expanded in GROW - 1, and GROW - 1 is popped by the epilogue
# of TEST2a.
#   DUMP:        BODY
#   BODY:        nothing
#   GROW - 1:    TEST2a
#   diversions:  GROW - 1, GROW, BODY |-
#
# * TEST1 requires TEST2a: epilogue
# The content of the current diversion is appended to DUMP (and removed
# from the current diversion).  A diversion is popped.
#   DUMP:       BODY
#   BODY:       TEST2a
#   diversions: GROW, BODY |-
#
# * TEST1 requires TEST2b: prologue
# m4_require pushes GROW - 1 and expands TEST2b.
#   DUMP:       BODY
#   BODY:       TEST2a
#   diversions: GROW - 1, GROW, BODY |-
#
# * TEST2b is expanded.
# Its prologue pushes the current diversion again.
#   DUMP:       BODY
#   BODY:       TEST2a
#   diversions: GROW - 1, GROW - 1, GROW, BODY |-
# The body is expanded here.
#
# * TEST2b requires TEST3: prologue
# m4_require pushes GROW - 2 and expands TEST3.
#   DUMP:       BODY
#   BODY:       TEST2a
#   diversions: GROW - 2, GROW - 1, GROW - 1, GROW, BODY |-
#
# * TEST3 is expanded.
# Its prologue pushes the current diversion again.
#   DUMP:       BODY
#   BODY:       TEST2a
#   diversions: GROW-2, GROW-2, GROW-1, GROW-1, GROW, BODY |-
# TEST3 requires TEST2a, but TEST2a has already been AC_PROVIDE'd, so
# nothing happens.  It's body is expanded here, and its epilogue pops a
# diversion.
#   DUMP:       BODY
#   BODY:       TEST2a
#   GROW - 2:   TEST3
#   diversions: GROW - 2, GROW - 1, GROW - 1, GROW, BODY |-
#
# * TEST2b requires TEST3: epilogue
# The current diversion is appended to DUMP, and a diversion is popped.
#   DUMP:       BODY
#   BODY:       TEST2a; TEST3
#   diversions: GROW - 1, GROW - 1, GROW, BODY |-
# The content of TEST2b is expanded here.
#   DUMP:       BODY
#   BODY:       TEST2a; TEST3
#   GROW - 1:   TEST2b,
#   diversions: GROW - 1, GROW - 1, GROW, BODY |-
# The epilogue of TEST2b pops a diversion.
#   DUMP:       BODY
#   BODY:       TEST2a; TEST3
#   GROW - 1:   TEST2b,
#   diversions: GROW - 1, GROW, BODY |-
#
# * TEST1 requires TEST2b: epilogue
# The current diversion is appended to DUMP, and a diversion is popped.
#   DUMP:       BODY
#   BODY:       TEST2a; TEST3; TEST2b
#   diversions: GROW, BODY |-
#
# * TEST1 is expanded: epilogue
# TEST1's own content is in GROW, and it's epilogue pops a diversion.
#   DUMP:       BODY
#   BODY:       TEST2a; TEST3; TEST2b
#   GROW:       TEST1
#   diversions: BODY |-
# Here, the epilogue of TEST1 notices the elaboration is done because
# DUMP and the current diversion are the same, it then undiverts
# GROW by hand, and undefines DUMP.
#   DUMP:       undefined
#   BODY:       TEST2a; TEST3; TEST2b; TEST1
#   diversions: BODY |-
#
#
# 2. Keeping track of the expansion stack
# =======================================
#
# When M4 expansion goes wrong it is often extremely hard to find the
# path amongst macros that drove to the failure.  What is needed is
# the stack of macro `calls'. One could imagine that GNU M4 would
# maintain a stack of macro expansions, unfortunately it doesn't, so
# we do it by hand.  This is of course extremely costly, but the help
# this stack provides is worth it.  Nevertheless to limit the
# performance penalty this is implemented only for m4_defun'd macros,
# not for define'd macros.
#
# The scheme is simplistic: each time we enter an m4_defun'd macros,
# we prepend its name in m4_expansion_stack, and when we exit the
# macro, we remove it (thanks to pushdef/popdef).
#
# In addition, we want to use the expansion stack to detect circular
# m4_require dependencies.  This means we need to browse the stack to
# check whether a macro being expanded is m4_require'd.  For ease of
# implementation, and certainly for the benefit of performances, we
# don't browse the m4_expansion_stack, rather each time we expand a
# macro FOO we define _m4_expanding(FOO).  Then m4_require(BAR) simply
# needs to check whether _m4_expanding(BAR) is defined to diagnose a
# circular dependency.
#
# To improve the diagnostic, in addition to keeping track of the stack
# of macro calls, m4_expansion_stack also records the m4_require
# stack.  Note that therefore an m4_defun'd macro being required will
# appear twice in the stack: the first time because it is required,
# the second because it is expanded.  We can avoid this, but it has
# two small drawbacks: (i) the implementation is slightly more
# complex, and (ii) it hides the difference between define'd macros
# (which don't appear in m4_expansion_stack) and m4_defun'd macros
# (which do).  The more debugging information, the better.


# m4_expansion_stack_push(TEXT)
# -----------------------------
m4_define([m4_expansion_stack_push],
[m4_pushdef([m4_expansion_stack],
	    [$1]m4_ifdef([m4_expansion_stack], [
m4_defn([m4_expansion_stack])]))])


# m4_expansion_stack_pop
# ----------------------
# Dump the expansion stack.
m4_define([m4_expansion_stack_pop],
[m4_popdef([m4_expansion_stack])])


# m4_expansion_stack_dump
# -----------------------
# Dump the expansion stack.
m4_define([m4_expansion_stack_dump],
[m4_ifdef([m4_expansion_stack],
	  [m4_errprintn(m4_defn([m4_expansion_stack]))])dnl
m4_errprintn(m4_location[: the top level])])


# _m4_divert(GROW)
# ----------------
# This diversion is used by the m4_defun/m4_require machinery.  It is
# important to keep room before GROW because for each nested
# AC_REQUIRE we use an additional diversion (i.e., two m4_require's
# will use GROW - 2.  More than 3 levels has never seemed to be
# needed.)
#
# ...
# - GROW - 2
#   m4_require'd code, 2 level deep
# - GROW - 1
#   m4_require'd code, 1 level deep
# - GROW
#   m4_defun'd macros are elaborated here.

m4_define([_m4_divert(GROW)],       10000)


# _m4_defun_pro(MACRO-NAME)
# -------------------------
# The prologue for Autoconf macros.
m4_define([_m4_defun_pro],
[m4_expansion_stack_push(m4_defn([m4_location($1)])[: $1 is expanded from...])dnl
m4_pushdef([_m4_expanding($1)])dnl
m4_ifdef([_m4_divert_dump],
	 [m4_divert_push(m4_defn([_m4_divert_diversion]))],
	 [m4_copy([_m4_divert_diversion], [_m4_divert_dump])dnl
m4_divert_push([GROW])])dnl
])


# _m4_defun_epi(MACRO-NAME)
# -------------------------
# The Epilogue for Autoconf macros.  MACRO-NAME only helps tracing
# the PRO/EPI pairs.
m4_define([_m4_defun_epi],
[m4_divert_pop()dnl
m4_if(_m4_divert_dump, _m4_divert_diversion,
      [m4_undivert([GROW])dnl
m4_undefine([_m4_divert_dump])])dnl
m4_expansion_stack_pop()dnl
m4_popdef([_m4_expanding($1)])dnl
m4_provide([$1])dnl
])


# m4_defun(NAME, EXPANSION)
# -------------------------
# Define a macro which automatically provides itself.  Add machinery
# so the macro automatically switches expansion to the diversion
# stack if it is not already using it.  In this case, once finished,
# it will bring back all the code accumulated in the diversion stack.
# This, combined with m4_require, achieves the topological ordering of
# macros.  We don't use this macro to define some frequently called
# macros that are not involved in ordering constraints, to save m4
# processing.
m4_define([m4_defun],
[m4_define([m4_location($1)], m4_location)dnl
m4_define([$1],
	  [_m4_defun_pro([$1])$2[]_m4_defun_epi([$1])])])


# m4_defun_once(NAME, EXPANSION)
# ------------------------------
# As m4_defun, but issues the EXPANSION only once, and warns if used
# several times.
m4_define([m4_defun_once],
[m4_define([m4_location($1)], m4_location)dnl
m4_define([$1],
	  [m4_provide_if([$1],
			 [m4_warn([syntax], [$1 invoked multiple times])],
			 [_m4_defun_pro([$1])$2[]_m4_defun_epi([$1])])])])


# m4_pattern_forbid(ERE, [WHY])
# -----------------------------
# Declare that no token matching the extended regular expression ERE
# should be seen in the output but if...
m4_define([m4_pattern_forbid], [])


# m4_pattern_allow(ERE)
# ---------------------
# ... but if that token matches the extended regular expression ERE.
# Both used via traces.
m4_define([m4_pattern_allow], [])


## ----------------------------- ##
## Dependencies between macros.  ##
## ----------------------------- ##


# m4_before(THIS-MACRO-NAME, CALLED-MACRO-NAME)
# ---------------------------------------------
m4_define([m4_before],
[m4_provide_if([$2],
	       [m4_warn([syntax], [$2 was called before $1])])])


# m4_require(NAME-TO-CHECK, [BODY-TO-EXPAND = NAME-TO-CHECK])
# -----------------------------------------------------------
# If NAME-TO-CHECK has never been expanded (actually, if it is not
# m4_provide'd), expand BODY-TO-EXPAND *before* the current macro
# expansion.  Once expanded, emit it in _m4_divert_dump.  Keep track
# of the m4_require chain in m4_expansion_stack.
#
# The normal cases are:
#
# - NAME-TO-CHECK == BODY-TO-EXPAND
#   Which you can use for regular macros with or without arguments, e.g.,
#     m4_require([AC_PROG_CC], [AC_PROG_CC])
#     m4_require([AC_CHECK_HEADERS(limits.h)], [AC_CHECK_HEADERS(limits.h)])
#   which is just the same as
#     m4_require([AC_PROG_CC])
#     m4_require([AC_CHECK_HEADERS(limits.h)])
#
# - BODY-TO-EXPAND == m4_indir([NAME-TO-CHECK])
#   In the case of macros with irregular names.  For instance:
#     m4_require([AC_LANG_COMPILER(C)], [indir([AC_LANG_COMPILER(C)])])
#   which means `if the macro named `AC_LANG_COMPILER(C)' (the parens are
#   part of the name, it is not an argument) has not been run, then
#   call it.'
#   Had you used
#     m4_require([AC_LANG_COMPILER(C)], [AC_LANG_COMPILER(C)])
#   then m4_require would have tried to expand `AC_LANG_COMPILER(C)', i.e.,
#   call the macro `AC_LANG_COMPILER' with `C' as argument.
#
#   You could argue that `AC_LANG_COMPILER', when it receives an argument
#   such as `C' should dispatch the call to `AC_LANG_COMPILER(C)'.  But this
#   `extension' prevents `AC_LANG_COMPILER' from having actual arguments that
#   it passes to `AC_LANG_COMPILER(C)'.
m4_define([m4_require],
[m4_expansion_stack_push(m4_location[: $1 is required by...])dnl
m4_ifdef([_m4_expanding($1)],
	 [m4_fatal([$0: circular dependency of $1])])dnl
m4_ifndef([_m4_divert_dump],
	  [m4_fatal([$0: cannot be used outside of an m4_defun'd macro])])dnl
m4_provide_if([$1],
	      [],
	      [m4_divert_push(m4_eval(m4_divnum - 1))dnl
m4_default([$2], [$1])
m4_divert(m4_defn([_m4_divert_dump]))dnl
m4_undivert(m4_defn([_m4_divert_diversion]))dnl
m4_divert_pop(m4_defn([_m4_divert_dump]))])dnl
m4_provide_if([$1],
	      [],
	      [m4_warn([syntax],
		       [$1 is m4_require'd but is not m4_defun'd])])dnl
m4_expansion_stack_pop()dnl
])


# m4_expand_once(TEXT, [WITNESS = TEXT])
# --------------------------------------
# If TEXT has never been expanded, expand it *here*.  Use WITNESS as
# as a memory that TEXT has already been expanded.
m4_define([m4_expand_once],
[m4_provide_if(m4_ifval([$2], [[$2]], [[$1]]),
	       [],
	       [m4_provide(m4_ifval([$2], [[$2]], [[$1]]))[]$1])])


# m4_provide(MACRO-NAME)
# ----------------------
m4_define([m4_provide],
[m4_define([m4_provide($1)])])


# m4_provide_if(MACRO-NAME, IF-PROVIDED, IF-NOT-PROVIDED)
# -------------------------------------------------------
# If MACRO-NAME is provided do IF-PROVIDED, else IF-NOT-PROVIDED.
# The purpose of this macro is to provide the user with a means to
# check macros which are provided without letting her know how the
# information is coded.
m4_define([m4_provide_if],
[m4_ifdef([m4_provide($1)],
	  [$2], [$3])])


## -------------------- ##
## 9. Text processing.  ##
## -------------------- ##


# m4_cr_letters
# m4_cr_LETTERS
# m4_cr_Letters
# -------------