Re: AM_SUBST_IGNORE = AC_SUBST without Makefile variable definition

[Ralf Wildenhues]

[ removed automake-patches, its readers are likely to read here, too,
  and this isn't about a patch ]

Hi Stepan,

* Stepan Kasal wrote on Wed, Oct 19, 2005 at 01:37:22PM CEST:
> On Wed, Oct 19, 2005 at 11:32:37AM +0200, Ralf Wildenhues wrote:
> > I believe what Stepan meant was:
*snip*
> 
> not exactly.  It is like this:
> 
> 1) each occurence of @VAR@ is replaced by the value of VAR, for each
>    AC_SUBSTed variable VAR.
> 2) if the original line matched /@address@hidden/, then
>       - this match is removed
>       - each embedded newline is esaped by a backslash
>       - if the resulting string ends with a backslash, space is appendeded

Ah, ok.  Thank you for clearing this up.

> In other words, after the usual sed substtitutions, the following
> sed code is executed:
> /@address@hidden/ {
>       s/\n/\\&/g
>       s/@address@hidden//
>       s/\\$/& /
> }

Oh, ahh.  Nifty!  Only thing the user will have to keep in mind is that
she should add another white space (besides the newline, which will be
removed by `make') to separate values.  I guess that could be called a
feature even!

> > [...] unless an actual implementation can be shown not to
> > show execution time regression inside config.status.
> 
> The above proposal is suited for this.

ACK.

> (Having "@FOO\n@" for "flattened value of FOO" might look nicer, but
> effective implementation would be more difficult, if possible.
> The facts that the magical tag means "the whole line is flattened" and
> that the tag is at the end of the line enable easy effective
> implementation.)

ACK.

> > If you accept that fact, you might as well flatten the variable before
> > substituting it.
*snip*
> if autoconf "flattens" the variable, it is
> possible that the line would get too long, which could cause problems.

ACK.  Darn, I knew I must've missed something.

> Actually, there is a report that LIBOBJ can get too long with latest
> autotools and heavy gnulib usage:

Ahh.

> > [...] it also means that any occurrence of
> >   @FOO@ 
> > in a Makefile.in will be substituted by $FOO_FLATTENED rather than $FOO,
> 
> That doesn't sound like a good idea.  I think that one of the motivations
> for multiline substitutions was that we can AC_SUBST multiline make rules.

Ah, ok.  So you also intend to have an AC_SUBST_NOFLATTEN or
AC_SUBST_RULE that will cause Automake
- not to add the @\n@ at the end (as will be done with AC_SUBST),
- output this one after all the variables (and also after some or all
  rules?)

Nice idea!  Do we have a real-world example where this would help, though?

Cheers,
Ralf