Re: [BuildStream] Specifying default dependencies for element kinds

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

Hi Sander,

On Thu, 2019-06-13 at 11:32 +0200, Sander Striker wrote:
> Hi,
>
> TL;DR let's try to minimize the syntactic sugar required in .bst files for list composition.
>
> > On Thu, Jun 13, 2019 at 10:31 AM Tristan Van Berkom via buildstream-list <buildstream-list gnome org> 
> > wrote:
> [...]
> > On Thu, 2019-06-13 at 08:17 +0200, Jürg Billeter wrote:
> > [...]
> > > I agree that the default composition behavior for lists is not ideal in
> > > this case.
>
> [...] 
> > Indeed, it's not ideal in this specific case that the overlaying
> > dictionary need to be aware of whether or not the underlying exists or
> > not, if they want to append/prepend.
> >
> > Perhaps we could introduce an error-free prepend/append semantic:
> >
> >   foo:
> >     (->):
> >     - append or create list
> >     - if doesn't already exist
> >
> > But I admit this would look a bit like voodoo if it was used in every
> > dependency list declaration in build elements in the project.
>
> That would be something that we should avoid.  Just like trying to
> avoid includes in every build element, it should be possible for the
> build elements themselves to be clean/minimal.
>  
> > Any better ideas ?
>
> Make error free append the default operation (even when it is not
> specified).  Which means that in the common case you don't need any
> syntactic sugar.  And use overwrites in your elements for the cases
> where you need to be very explicit, ignoring project.conf.

+1, I like this a lot.

That said, it may be worth some thought experiments to figure out what
protections we might lose with this (off hand it seems dangerous to me
to change the "error to append to non-existing list" to be the
opposite, but I could be wrong).

It also seems that it would not present difficulties in porting
BuildStream 1 projects - as it would technically be a compatible
change.

Cheers,
    -Tristan