Re: a syntax for multitarget rules

[Steven Simpson]

On 14/11/14 10:35, Tim Murphy wrote:
> To save typing here's an example of some of the possible hacks:
> http://www.cmcrossroads.com/article/rules-multiple-outputs-gnu-make
>
> You'll see the problem with "chaining" dependencies - there is a 
> situation when a file is deleted and running make again doesn't 
> recreate it.  Basically only one target in your chain has a "command" 
> or "recipe" so if that file exists then make won't run the recipe even 
> if some other file in the chain is missing.

What I was proposing seems most like the one on the second page, but 
there is a difference.  It suggests:

parser.h parser.c: generate_parser

generate_parser: parser.i
        create parser.h and parser.c
        touch $@

I'm suggesting that the first rule is removed.  If something now needs 
(say) parser.h, it should have rules like:

foo: generate_parser
foo: parser.h

The first foo rule should ensure that parser.h exists and is up-to-date, 
before the second tests for parser.h.

In a recent thread:

<http://lists.gnu.org/archive/html/help-make/2014-10/msg00014.html>

...I was doing something similar, but the first foo rule was 
order-only.  My issue was that this failed on a parallel build, which is 
probably a problem here too.  I was shown a way round it, which works, 
with caveats, and might apply here too.

On the issue of defining a new kind of rule, rather than it declaring 
that a group of files are created together, perhaps it could declare 
that several items are by-products of creating a target:

parser.h parser.c &: generate_parser

foo: generate_parser
foo: parser.h

A parallel Make could now infer that generate_parser must complete 
before testing parser.h.

Note that the new rule doesn't actually group parser.h and parser.c; it 
just adds two new independent relationships (between parser.h and 
generate_parser) and (between parser.c and generate_parser), so they 
could be expressed independently too:

parser.h &: generate_parser
parser.c &: generate_parser

Cheers!