External Cartfile specifications for dependencies

[jspahrsummers]

It'd be nice to inject a custom Cartfile for use by a dependency of your project—in other words, specify external Cartfiles for dependencies listed in your top-level Cartfile.

I can foresee a few uses for this:

1. Updating a project's dependencies before the maintainers are able to do so themselves, without needing to fork the project
2. Relaxing a project's dependency requirements, if they've been specified too strictly
3. Using Carthage on projects that have no awareness of it, or no desire to use it

That last point might make this a candidate for our initial release, since few—if any—projects will have Cartfiles when this repository is made public.

@Carthage/owners Any thoughts on whether this should be a prerequisite to shipping?

[jspahrsummers]

Tentatively dropping this into the milestone, mostly just so I don't overlook it later. Let me know if you think it should be pulled out.

[alanjrogers]

@jspahrsummers Great idea, I think this is important for an initial ship so that people can start using it without having to wait for library maintainers to update.

[robrix]

Agreed.

[jspahrsummers]

I've kinda 180ºd on this. I can't think of a good syntax off of the top of my head, and this introduces some additional complexity around locating those external references.

Since there is a workaround available (forking), I'm interested in punting this in the interest of minimizing scope. Please speak up if you think I'm crazy. 😉

[jspahrsummers]

This hasn't really been a problem in practice—probably partly thanks to Carthage's interoperability with other dependency management solutions (CocoaPods, submodules, subtrees).

After all, this only applies to dependencies that themselves have dependencies, which is a fairly small group.