(autoconf.info)Changequote is Evil

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8.1.5 `changequote' is Evil

The temptation is often high to bypass proper quotation, in particular
when it's late at night.  Then, many experienced Autoconf hackers
finally surrender to the dark side of the force and use the ultimate
weapon: `changequote'.

   The M4 builtin `changequote' belongs to a set of primitives that
allow one to adjust the syntax of the language to adjust it to one's
needs.  For instance, by default M4 uses ``' and `'' as quotes, but in
the context of shell programming (and actually of most programming
languages), that's about the worst choice one can make: because of
strings and back-quoted expressions in shell code (such as `'this'' and
``that`'), and because of literal characters in usual programming
languages (as in `'0''), there are many unbalanced ``' and `''.  Proper
M4 quotation then becomes a nightmare, if not impossible.  In order to
make M4 useful in such a context, its designers have equipped it with
`changequote', which makes it possible to choose another pair of
quotes.  M4sugar, M4sh, Autoconf, and Autotest all have chosen to use
`[' and `]'.  Not especially because they are unlikely characters, but
_because they are characters unlikely to be unbalanced_.

   There are other magic primitives, such as `changecom' to specify
what syntactic forms are comments (it is common to see `changecom(<!--,
-->)' when M4 is used to produce HTML pages), `changeword' and
`changesyntax' to change other syntactic details (such as the character
to denote the Nth argument, `$' by default, the parentheses around
arguments, etc.).

   These primitives are really meant to make M4 more useful for specific
domains: they should be considered like command line options:
`--quotes', `--comments', `--words', and `--syntax'.  Nevertheless,
they are implemented as M4 builtins, as it makes M4 libraries self
contained (no need for additional options).

   There lies the problem....

   The problem is that it is then tempting to use them in the middle of
an M4 script, as opposed to its initialization.  This, if not carefully
thought out, can lead to disastrous effects: _you are changing the
language in the middle of the execution_.  Changing and restoring the
syntax is often not enough: if you happened to invoke macros in between,
these macros are lost, as the current syntax is probably not the one
they were implemented with.

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