Re: [Module::Build] Some updates, and catching up...

[David G. <da...@hy...>]

Adam Kennedy wrote:
> A META.yml file is packed up with the dist for the convenience of larger 
> systems, and META.yml in this case is known to only be an approximation.

I think it's for the convenience of telling indexers about:

(a) metadata (e.g. version numbers, dependencies) that would otherwise 
require them to run (potentially unsafe) code on all distributions submitted

(b) metadata about the distribution structure, e.g. files that 
should/shouldn't be indexed

> When true, the CPAN client should be aware it needs to run the installer 
> script first, which will localise the META.yml content, to get a 
> definitive result for that environment.

That seems redundant -- I think the installers can notify clients of 
dependencies directly without a new META.yml being created.  Once an 
installer has committed to running Makefile|Build.PL, the META.yml is 
irrelevant.

> The problem with doing some sort of optional feature is that it 
> introduces branch logic into your data structures.
> 
> So you need a way to describe the branch... platform => 'Win32'... [ 
> platform => 'Win32', perl => '> 5.005 || < 5.008004' ], and so on and so 
> on.
> 
> But because you are trying to express logic (code) as data, you are 
> always going to cases where you need just that bit more syntax and 
> flexibility, until before you know it you have invented your own new 
> mini-language, and well, then we might as well use Perl.

As long as it's a mini-language that can be parsed safely, then having 
some expressiveness is fine.  Most of where this will crop up is in 
module dependencies anyway, and the number of variables there is 
relatively contained:

* platforms (in some set)
* perl versions (in some set of ranges of versions)
* module versions (in some set of ranges of versions)

The logic is pretty simple, too.  If a given set of those conditions 
above is true, then a given set of dependencies exist (which can be 
expressed again as a set of ranges of versions).

> But then of course we have Perl inside META.yml, and we're back to 
> square 1.

A mini language for that shouldn't be too challenging to write and parse.

> I don't know whether or not it was planned, but the current situation, 
> with a flagged-imperfect uploaded META.yml, which is localised at 
> Makefile|Build.PL-time quite comfortably covers 100% of all use cases 
> I'm aware of.

Use case: Create an index of all dependencies for all distributions on 
the Win32 platform without running Makefile|Build.PL for all 
distributions.  (E.g. for a potential "CP-PAN" i.e. "CamelPack-PAN")

Personally, I think that YAML is complete overkill for CPAN distribution 
metadata.  Its major advantage seems to be that it's easily read by humans.

Regards,
David Golden