bug#36772: feature request: checked variant of "substitute*"

[Robert Vollmert <rob@vllmrt.net>]

> On 23. Jul 2019, at 15:35, Ricardo Wurmus <rekado@elephly.net> wrote:
> 
> 
> Hi Robert,
> 
>> I think it would be great to have the following variant of substitute*:
>> 
>> (substitute*-once filename (pattern vars) body)
>> 
>> which acts like the usual substitute-*, except it also asserts that the
>> substitution applies to exactly one line in the file, causing a build
>> failure otherwise.
> 
> I agree that the effect of substitute* should be checked.  I think
> substitute* should fail when one of its clauses failed to match
> anything.
> 
> Each clause could also accept an optional argument to make them only
> match one location.  We wouldn’t have to duplicate the macro for that
> and it’s a simple extension to failing on zero matches.
> 
> What do you think?

That sounds like a good improvement, too. I think the important part is
ensuring the substitution matches at all, maybe we could leave out the
“at most once” part. (I doubt it would be used a lot if it’s not the default.)

Cheers
Robert