Re: Make's assumption that files it does not create do not change

On 2022-04-13 06:39, Renaud Pacalet wrote:
> On 13/04/2022 15:27, Paul Smith wrote:
>> On Tue, 2022-04-12 at 14:58 -0700, Kaz Kylheku wrote:
>>> I also somewhat regret implementing the feature because it is
>>> unnecessary.
>>> The feature is built on the the implementation of pattern rules,
>>> which already supported multiple targets many years before &:.
>> 
>> Just to mention that these options including &: and just using pattern
>> rules were discussed in the GitHub issue and the followup comments so
>> these were known to the folks implementing the change.
>> 
>> I don't really agree that using pattern rules is a sufficient
>> alternative.  Forcing targets to have a common stem is not always
>> great.  I for one am happy that this feature exists!
> 
> Agreed. I was really expecting this feature. Not only because of the
> common stem constraint but also because most pattern rules that I was
> writing just for this grouped target feature were extremely unnatural.
> It even happened that somebody else decided to "fix" them because I
> had forgotten to add a big "don't touch" comment.
> 
> I am very happy too that this feature exists.

I hear both of you. My alpha charlie example deliberately emphasizes
the weirdness of looking for an incidental common stem which bears no
semantics.

But there is a (seemingly) simple fix for that: allow empty stems.

That's it: allow the % pattern wildcard to have an empty match,
and then you can do the following, which is much better:

   alpha% bravo% : delta% charlie%

When a file called bravo needs to be updated, it will match bravo%,
binding the % variable to the empty string "". and off we go:
the grouped target rule we want is instantiated for us.

Any non-empty set of strings always has a common infix: the empty 
string,
and by convention you can use the leading or trailing one.

(Is there some deep reason why % cannot bind to an empty string?)

Oh, I just remembered something. I last thought about this issue
several years ago, and had this idea at that time.

Also, a way to give a common stem to some files is to move them
into a subdirectory. This will work even if the tool involved has
hard-coded names.

   %/alpha %/bravo : %/delta %/charlie
         cd $* ; tool-with-fixed-names

   whole: part subdir/alpha
         @echo build $@ from $^

   part: subdir/bravo
         @echo build $@ from $^

too-with-fixed-names accesses "alpha", "bravo", in its working
directory.

   $ make -f Makefile2
   cd subdir ; echo tool-with-fixed-names
   tool-with-fixed-names
   build part from subdir/bravo
   build whole from part subdir/alpha