(autoconf.info)Evaluation Macros

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8.3.6 Evaluation Macros

The following macros give some control over the order of the evaluation
by adding or removing levels of quotes.

 -- Macro: m4_apply (MACRO, LIST)
     Apply the elements of the quoted, comma-separated LIST as the
     arguments to MACRO.  If LIST is empty, invoke MACRO without
     arguments.  Note the difference between `m4_indir', which expects
     its first argument to be a macro name but can use names that are
     otherwise invalid, and `m4_apply', where MACRO can contain other
     text, but must end in a valid macro name.
          m4_apply([m4_count], [])
          m4_apply([m4_count], [[]])
          m4_apply([m4_count], [[1], [2]])
          m4_apply([m4_join], [[|], [1], [2]])

 -- Macro: m4_count (ARG, ...)
     This macro returns the decimal count of the number of arguments it
     was passed.

 -- Macro: m4_do (ARG, ...)
     This macro loops over its arguments and expands each ARG in
     sequence.  Its main use is for readability; it allows the use of
     indentation and fewer `dnl' to result in the same expansion.

 -- Macro: m4_dquote (ARG, ...)
     Return the arguments as a quoted list of quoted arguments.
     Conveniently, if there is just one ARG, this effectively adds a
     level of quoting.

 -- Macro: m4_dquote_elt (ARG, ...)
     Return the arguments as a series of double-quoted arguments.
     Whereas `m4_dquote' returns a single argument, `m4_dquote_elt'
     returns as many arguments as it was passed.

 -- Macro: m4_echo (ARG, ...)
     Return the arguments, with the same level of quoting.  Other than
     discarding whitespace after unquoted commas, this macro is a no-op.

 -- Macro: m4_expand (ARG)
     Return the expansion of ARG as a quoted string.  Whereas
     `m4_quote' is designed to collect expanded text into a single
     argument, `m4_expand' is designed to perform one level of expansion
     on quoted text.  The distinction is in the treatment of whitespace
     following a comma in the original ARG.  Any time multiple
     arguments are collected into one with `m4_quote', the M4 argument
     collection rules discard the whitespace.  However, with
     `m4_expand', whitespace is preserved, even after the expansion of
     macros contained in ARG.

     Note that `m4_expand' cannot parse everything.  The expansion of
     ARG must not contain unbalanced quotes (although quadrigraphs can
     get around this), nor unbalanced parentheses (portable shell `case'
     statements are a major culprit here, but creative shell comments
     can get around this).

          m4_define([active], [ACT, IVE])dnl
          m4_define([active2], [[ACT, IVE]])dnl
          m4_quote(active, active)
          m4_expand([active, active])
          =>ACT, IVE, ACT, IVE
          m4_quote(active2, active2)
          =>ACT, IVE,ACT, IVE
          m4_expand([active2, active2])
          =>ACT, IVE, ACT, IVE

 -- Macro: m4_ignore (...)
     This macro was introduced in Autoconf 2.62.  Expands to nothing,
     ignoring all of its arguments.  By itself, this isn't very useful.
     However, it can be used to conditionally ignore an arbitrary
     number of arguments, by deciding which macro name to apply to a
     list of arguments.
          dnl foo outputs a message only if [debug] is defined.
          [m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])])

     Note that for earlier versions of Autoconf, the macro `__gnu__' can
     serve the same purpose, although it is less readable.

 -- Macro: m4_make_list (ARG, ...)
     This macro exists to aid debugging of M4sugar algorithms.  Its net
     effect is similar to `m4_dquote'--it produces a quoted list of
     quoted arguments, for each ARG.  The difference is that this
     version uses a comma-newline separator instead of just comma, to
     improve readability of the list; with the result that it is less
     efficient than `m4_dquote'.
          m4_dquote(zero, [one], [[two]])
          m4_make_list(zero, [one], [[two]])
          m4_foreach([number], m4_dquote(zero, [one], [[two]]), [ number])
          => 0 1 two
          m4_foreach([number], m4_make_list(zero, [one], [[two]]), [ number])
          => 0 1 two

 -- Macro: m4_quote (ARG, ...)
     Return the arguments as a single entity, i.e., wrap them into a
     pair of quotes.  This effectively collapses multiple arguments
     into one, although it loses whitespace after unquoted commas in
     the process.

 -- Macro: m4_unquote (ARG, ...)
     This macro was introduced in Autoconf 2.62.  Expand each argument,
     separated by commas.  For a single ARG, this effectively removes a
     layer of quoting, and `m4_unquote([ARG])' is more efficient than
     the equivalent `m4_do([ARG])'.  For multiple arguments, this
     results in an unquoted list of expansions.  This is commonly used
     with `m4_split', in order to convert a single quoted list into a
     series of quoted elements.

   The following example aims at emphasizing the difference between
several scenarios: not using these macros, using `m4_defn', using
`m4_quote', using `m4_dquote', and using `m4_expand'.

     $ cat example.m4
     dnl Overquote, so that quotes are visible.
     m4_define([show], [$[]1 = [$1], $[]@ = [$@]])
     m4_define([a], [A])
     m4_define([mkargs], [1, 2[,] 3])
     m4_define([arg1], [[$1]])
     show(a, b)
     show([a, b])
     show(m4_quote(a, b))
     show(m4_dquote(a, b))
     show(m4_expand([a, b]))

     $ autom4te -l m4sugar example.m4
     $1 = A, $@ = [A],[b]
     $1 = a, b, $@ = [a, b]
     $1 = A,b, $@ = [A,b]
     $1 = [A],[b], $@ = [[A],[b]]
     $1 = A, b, $@ = [A, b]

     1, 2[,] 3
     1,2, 3
     [1],[2, 3]
     1, 2, 3

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