Metro: Support for resolving Yarn Plug 'n Play modules

[empyrical]

Do you want to request a feature or report a bug?
Request a feature

What is the current behavior?
Metro does not (yet) have PnP support

If the current behavior is a bug, please provide the steps to reproduce and a minimal repository on GitHub that we can yarn install and yarn test.

What is the expected behavior?
I think it would be very handy if Metro had support for resolving Yarn PnP modules.

Not only would there be the benefits you'd get for using it in other project, I believe that if this Yarn feature request is able to be resolved, this could be a reasonable alternative to using symlinks (since they are not able to be supported for technical reasons)

[arcanis]

It actually already does! We just need to open-source the resolver in a separate package (or make it native, possible) 😃 ping @rafeca

[empyrical]

Curious how this is going. If the internal resolver is inappropriate for open source, perhaps I could make an attempt at implementing one.

I think this could probably be a first-class metro feature. Only thing I can't really figure out is a good way to unit test it. Non-PnP mode has really nice virtual filesystem for testing, and this wouldn't be able to make use of that without heavy modification (probably)

[vjpr]

I found it implemented here: https://github.com/facebook/metro/commit/60bad942f6f32d2ac1af7dc58e3e768a9aee1752

[dalinarkholin]

@arcanis @empyrical was Pnp support removed? Im guessing that is why this is still open?

[arcanis]

Yep, I think FB removed it about a year ago since they don't use it internally. You should be able to add it back by overriding resolveRequest in your configuration, though.

[arlyon]

Now that yarn uses pnp by default, can we consider adding it again?

[dalinarkholin]

@arlyon I just added an example repo with pnp and react native working. Checkout the metro.config.js here: https://github.com/dalinarkholin/react-native-pnp-enabled

Also my comment here: https://github.com/react-native-community/cli/issues/27#issuecomment-625912523

But yes, it would be good to get it back as a option as well :)

[Dirklectisch]

Would it help if I provide a pull request to get this back in? If there is no interest in adding this back in we could create a separate package for it so that at least React Native has a way forward for adopting Yarn 2.

[dvicory]

Would it help if I provide a pull request to get this back in? If there is no interest in adding this back in we could create a separate package for it so that at least React Native has a way forward for adopting Yarn 2.

I'd love to see this either way, but would use a separate package if metro isn't interested in adding it.