Re: [Boost-build] Question about try-one-generator-really

From: Vladimir Prus (ghost_at_[hidden])
Date: 2007-03-10 03:59:23

On Saturday 10 March 2007 09:10, David Abrahams wrote:
> 
>   # Returns usage requirements + list of created targets
>   local rule try-one-generator-really ( project name ? : generator : 
>       target-type : property-set : sources * )
>   {
>       local targets =
>         [ $(generator).run $(project) $(name)
>                          : $(property-set)
>                          : $(sources)
>         ] ;
> 
>       local usage-requirements ;
>       local success ;
> 
>       generators.dout [ indent ] returned $(targets) ;
> 
>       if $(targets)
>       {
>           success = true ;
> 
>           if  [ class.is-a $(targets[1]) : property-set ]
>           {
>               usage-requirements = $(targets[1]) ;
>               targets = $(targets[2-]) ;
>           }
>           else
>           {
>               usage-requirements = [ property-set.empty ] ;
>           }
>       }
> 
> Apparently this means it's optional for the first element returned
> from running a generator property-set of usage requirements.  Why is
> that a good idea?  Wouldn't it be better to use a single protocol (all
> generators return either nothing, or a sequence starting with a
> property-set of usage requirements and followed by generated targets)?

It would be better if the need for this property-set return was known from
the very start. As it is, it was introduced relatively later, so auditing all
generators did not seem like a good idea, and user-written generators
are outside our control.

> Now as I modify gcc-linking-generator, I have to worry that
> unix-linking-generator might not give me a property set of usage
> requirements.

Practically, you can just check unix-linking-generator.

- Volodya