Re: M4 syntax $11 vs. ${11}

[Gary V. Vaughan]

Apologies for joining the conversation so late:

On 30 Jan 2007, at 07:23, Eric Blake wrote:
> > How about if M4 uses a slightly-different syntax, so that the two  
> > uses
> > are never confused?  That way, the conversion problem won't ever have
> > to be addressed, and Autoconf won't have to disable the extension
> > during a transition period.
> >
> > So, for example, suppose M4 uses the syntax ${{11}} to refer to
> > argument 11?  Or (if you prefer something shorter) ${+11}?  An
> > advantage to this sort of approach is that it gives you more leeway
> > for future extensions.
>
> You have a reasonable point there.  My current plan was to add new  
> syntax
> categories that could be controlled through the changesyntax builtin
> (available in beta since at least 1.4o several years ago).  So  
> using $<1>
> instead of ${1} is easy - just use changesyntax([{<], [}>]) up  
> front in
> m4sugar.  On the other hand, supporting a multi-character delimiter  
> that
> is still selectable by the m4 user is a bit tougher; I would need to
> introduce a new macro similar to changequote and changecom (perhaps
> changeextarg({{, }}) for choosing ${{1}} to mean an m4-parsed extended
> argument, leaving ${1} for literal output).

The intention with m4-2.0 and beyond was to make the rescanning engine
become modular enough that it would be possible to select a  
``language''.
For example we already have 3 such languages (albeit implemented outside
the current module system): traditional, gnu, and posix.  Ultimately,
we should freeze core parsing issues to make an ``autoconf'' language,
which autoconf-3.0 could extend with modules it chose to ship.  Also,
I'd like to have an m5 language (Prof Dain Samples extended m4), and
to split gnu into gnu-m4 (1.4.x syntax) and gnu-m42 (2.0 extensions).

Considering how little time I've had for gnu recently, I think it is
fine to push these ideas out past the 2.0 release, though the default
configuration of m4-2.0 (and beyond) should be fully compatible with
m4-1.4 -- any syntax improvements or extensions are the remit of
gnu-m42.

Cheers,
        Gary
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