Adds the rfc for optional peer dependencies

accepted/0000-optional-peer-dependencies.md

@@ -0,0 +1,52 @@
+- Start Date: 30 Oct 18
+- RFC PR: https://github.com/yarnpkg/rfcs/pull/105
+- Yarn Issue: n/a
+- Champion: Maël Nison (@arcanis - [Twitter](https://twitter.com/arcanis))
+
+# Optional Peer Dependencies
+
+## 1. Which Problem Does It Solve
+
+> I'm a library author. I want my package to provide an optional integration with some other library, but I don't want to have to ship it.
+
+The current solution to the problem described above is peer dependencies. A package will list the optional dependency as peer dependency, meaning that it's to the package consumer to also provide the library that should be used.
+
+While it currently already works, it causes semantical issues: are peer dependencies meant to be optional or not? Should a missing peer dependency cause a warning? An error? Nothing at all, and expect the runtime to check that they are available?
+
+## 2. Detailed Design
+
+Yarn will introduce a new protocol, `optional:`. If a peer dependency cannot be found but the requested range begins with `optional:` then Yarn will silence any warning and assume that the package will check at runtime whether it should use it or not.
+
+**Important:** The `optional:` protocol will not be valid in anything else than peer dependencies. We reserve the right later to potentially implement it for other fields, but for the time being we're only interested in the `peerDependencies` use case.
+
+## 3. How We Teach This
+
+- We can just call the optional protocol.
+
+- Being stricly backward compatible, it doesn't require us to push changes onto our users, so the teaching should be simple and spread on the long time.
+
+## 4. Drawbacks
+
+- It requires adoption from other package managers, otherwise there's a risk to fracture the ecosystem if they decide to go with (for example) naming the field `opt:`.
+
+## 5. Alternatives
+
+- We could introduce an `optionalPeerDependencies` key.
+
+  - It would be one more key to teach users.
+
+  - This wouldn't be backward compatible - most package managers would have no idea what to do with such a key and would ignore it entirely, breaking the tree because of the hoisting.
+
+  - Semantic isn't clear if a dependency is both in `peerDependencies` and `optionalPeerDependencies`.
+
+  - It would have a completely different meaning from the already existing `optionalDependencies` field.
+
+- We could overhaul how the dependencies are defined, and fix them once and for all.
+
+  - This sounds a huge undertaking for a problem relatively minor at the moment.
+
+  - It seems unlikely we can reach a consensus in a reasonable timeframe.
+
+## 6. Unresolved Questions
+
+Will all the package managers work properly when seeing a `peerDependencies` range prefixed by `optional:`? Logic would say that yes, since peer dependency ranges are typically an hint rather than a strict enforcement (except maybe for npm 1 & 2), but it might trigger a bug in some versions of some package managers.