8.3.3 Diversion support

M4sugar makes heavy use of diversions, because it is often the case that text that must appear early in the output is not discovered until late in the input. Additionally, some of the topological sorting algorithms used in resolving macro dependencies use diversions. However, most macros should not need to change diversions directly, but rather rely on higher-level M4sugar macros to manage diversions transparently.

In the rare case that it is necessary to write a macro that explicitly outputs text to a different diversion, it is important to be aware of an M4 limitation regarding diversions: text only goes to a diversion if it is not part of argument collection. Therefore, any macro that changes the current diversion cannot be used as an unquoted argument to another macro, but must be expanded at the top level. The macro m4_expand will diagnose any attempt to change diversions, since it is generally useful only as an argument to another macro. The following example shows what happens when diversion manipulation is attempted within macro arguments:

     m4_do([normal text]
     m4_divert_push([KILL])unwanted[]m4_divert_pop([KILL])
     [m4_divert_push([KILL])discarded[]m4_divert_pop([KILL])])dnl
     ⇒normal text
     ⇒unwanted

Notice that the unquoted text unwanted is output, even though it was processed while the current diversion was KILL, because it was collected as part of the argument to m4_do. However, the text discarded disappeared as desired, because the diversion changes were single-quoted, and were not expanded until the top-level rescan of the output of m4_do.

To make diversion management easier, M4sugar uses the concept of named diversions. Rather than using diversion numbers directly, it is nicer to associate a name with each diversion; the diversion number associated with a particular diversion name is an implementation detail, so you should only use diversion names. In general, you should not output text to a named diversion until after calling the appropriate initialization routine for your language (m4_init, AS_INIT, AT_INIT, ...), although there are some exceptions documented below.

M4sugar defines two named diversions.

KILL

Text written to this diversion is discarded. This is the default diversion once M4sugar is initialized.

GROW

This diversion is used behind the scenes by topological sorting macros, such as AC_REQUIRE.

M4sh adds several more named diversions.

BINSH

This diversion is reserved for the ‘#!’ interpreter line.

HEADER-REVISION

This diversion holds text from AC_REVISION.

HEADER-COMMENT

This diversion holds comments about the purpose of a file.

HEADER-COPYRIGHT

This diversion is managed by AC_COPYRIGHT.

M4SH-SANITIZE

This diversion contains M4sh sanitization code, used to ensure M4sh is executing in a reasonable shell environment.

M4SH-INIT

This diversion contains M4sh initialization code, initializing variables that are required by other M4sh macros.

BODY

This diversion contains the body of the shell code, and is the default diversion once M4sh is initialized.

Autotest inherits diversions from M4sh, and changes the default diversion from BODY back to KILL. It also adds several more named diversions, with the following subset designed for developer use.

PREPARE_TESTS

This diversion contains initialization sequences which are executed after atconfig and atlocal, and after all command line arguments have been parsed, but prior to running any tests. It can be used to set up state that is required across all tests. This diversion will work even before AT_INIT.

For now, the named diversions of Autoconf and Autoheader, and the remaining diversions of Autotest, are not documented. In other words, intentionally outputting text into an undocumented diversion is subject to breakage in a future release of Autoconf.