:link dependencies

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+- Start Date: 2016-10-12
+- RFC PR:
+- Yarn Issue:
+
+# Summary
+
+Add symlink `link:` dependency type to enable complex cross-project development
+workflows.
+
+# Motivation
+
+This RFC is a spinoff of yarn's [issue #884](https://github.com/yarnpkg/yarn/issues/884).
+
+We've been using some kind of monorepo approach for our projects for quite some
+time, a bit like [lerna](https://github.com/lerna/lerna) but with a more private
+and nested approach (our packages aren't expected to be published for now) along
+some specific needs: we must to link some public dependencies (eg. mongoose)
+that must be the exact same instance accross our subpackages (otherwise you
+encounter a lot of exotic bugs, edge cases), we also link our devDeps (they are
+shared accross all our subpackages).
+
+At first we used [linklocal](https://github.com/timoxley/linklocal) with npm@2,
+leveraging a custom use of the `file:` prefix (basically just symlinking them),
+but npm@3 broke a lot of things related to their handling (eg.
+[#10343](https://github.com/npm/npm/issues/10343)). We ended up moving to
+[ied](https://github.com/alexanderGugel/ied) where we implemented the `file:`
+prefix handling using simple symlinks, that tackled our need.
+
+I would love to be able to switch theses project to yarn but I would need a way
+to create these links.
+
+# Detailed design
+
+I can see two (non exclusive) ways to achieve that:
+
+- Either add a new `link:` prefix that would just create symlinks and that's it
+  (regardless of destination's existence)
+
+- Add an option flag `--link-file-dependencies` that would override default
+  `file:` handling (copying) and use symlinks instead.
+
+I've already implemented the changes in yarn's [pr#1109](https://github.com/yarnpkg/yarn/pull/1109) and I'm
+currently maintaining a fork since my team already rely on this for several
+projects.
+
+# How We Teach This
+
+I think `link:` is pretty explicit, an update to the docs/cli-help should be
+enough.
+
+# Drawbacks
+
+Not sure how exactly cross-platform symlinks are today. However it looks like
+[Microsoft might be catching up](https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/) on this issue.
+
+# Alternatives
+
+Beside the two alternatives exposed above, I can't think of anything else for
+now.
+
+Drawback of not implementing this is that we restrict how creative developers
+can be with complex multi-packages workflows.
+
+# Unresolved questions
+
+Not sure how we should handle actually publishing packages with such
+dependencies, the existing behavior for `file:` types?