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+- Start Date: 2016-10-31
+- RFC PR: (leave this empty)
+- Yarn Issue: (leave this empty)
+
+# Summary
+
+> The ability to dynamically determine packages based on variable substitution.
+
+While there could be many great use cases for variable substitution, I will
+address the two most important variables that I've encountered, `platform` and
+`cpu`. The syntax for substitution within dependencies is `.(variableName)`,
+which I'll explain later.
+
+
+### What variable substitution does:
+
+``` json
+"devDependencies": {
+   "rebel": "ourGitUrl#0.4.4.(anyOs).(anyCpu)"
+}
+```
+
+Would result in `yarn` interpreting the `package.json` as if it had been written as:
+
+``` json
+"devDependencies": {
+   "rebel": "ourGitUrl#0.4.4.darwin.64"
+}
+```
+
+if you are on Mac OS, 64 bit. 
+
+I'm seeking feedback on the *exact* parameters/variables names to use (it has
+been suggested that we simply copy what Brew does).
+
+
+# Motivation
+
+
+This enabled blazing fast installs for what would normally take as much as `30`
+minutes on `npm` - for `postinstall`-heavy packages.
+
+
+# Detailed design
+
+This convention is not only great for package installers (users), but great for
+package authors because they can configure travis builds to automatically
+upload successful builds to `ourGitUrl#0.4.4-.(anyOs).(anyCpu)` where
+`platform`/`cpu` matches the platform/architecture that travis built on.  So,
+that `package.json` snippet above is actually something that package authors
+could commit and publish, knowing that it will work everywhere when people
+begin to depend on their packages. There is no way to do this with `yarn` or
+`npm` today that I know of (not even with the `os`/`cpu` fields).
+
+You can imagine allowing this usage of `yarn`, in order to include prebuilts
+for the exact architecture you want:
+
+```
+yarn install --vars -platform=linux -cpu=64
+```
+
+But what about OSs/CPUs that we haven't built for? What about people who use
+the `npm` client to install a package with `yarn` variables in it? The proposed
+variable syntax is intentionally chosen to be `.(varName)`, because `.(foo)` is
+valid syntax for git tags, even if it's not substituted with anything.
+
+So the solution, is that in addition to having travis publish
+`#0.4.4-.darwin.64` that contains the prebuilt version of the package for
+`darwin`, we _also_ publish the _not_-prebuilt version of the package from
+travis with `#0.4.4-.(anyOs).(anyCpu)` - yes, the literal string
+`"#0.4.4-.(anyOs).(anyCpu)"` including parens. That means `npm` clients can
+_still_ depend on our packages that contain `yarn` specific substitution
+features, it's just that they end up rebuilding the package from source. It's a
+nice incentive to use `yarn`, without breaking compatibility with `npm`, and
+hopefully `npm` can adopt the convention if people find it useful.
+
+We could also allow installation of *some* packages to avoid variable
+substitution, and therefore force rebuilding of those packages.
+
+Feature 1: Ability to control how one *`package.json`*'s variables are substituted:
+
+In this example, every package will have `linux` and `64` substituted for
+`platform`/`cpu` respective, *except* `rebel`'s `package.json`.
+
+```
+yarn install --vars -platform=linux -cpu=64 --vars-for-package rebel --platform=anyOs -cpu=anyCpu
+```
+
+Feature 2: Ability to control how one *dependency* has its variables
+substituted, regardless of which `package.json` depends on it.
+
+```
+yarn install --vars -platform=linux -cpu=64 --vars-for-dependency rebel --platform=anyOs -cpu=anyCpu
+```
+
+
+# How We Teach This
+
+This is a pretty simple concept to explain - it's basically template substitution.
+
+# Drawbacks
+
+`yarn` git tag version constraints should also work here, although these would
+not be compatible with `npm` (sadly), but the `npm` incompatibility is only due
+to `npm` not supporting git tag ranges, and that has nothing to do with
+variable substitution.
+
+```
+"dependencies": {
+   "rebel":
+      "gitUrl#0.4.(os).(cpu) - gitUrl#0.5.(os).(cpu)"
+}
+```
+
+The main drawback is that while it empowers developers to depend on
+*potentially* pre-built packages and while `travis` enables publishing them in
+the proper form very easily, `package.json` files become strange looking, and
+`package.json` files start to leak some of the implementation details about how
+a package should be installed.
+
+# Alternatives
+
+The impact of not solving this problem *somehow* is that `yarn` and `npm` won't
+be capable of modeling languages outside of JS, or JS projects that integrate
+with other languages which have heavey `postinstall` steps.
+
+Instead of saying: "I depend on `rebel`, and here's where you can get the
+prebuilt binary", it might be better to say, "I depend on rebel", and then
+allow `rebel` itself to specify where to get the prebuilt binaries. Or perhaps,
+there should be a way for any of those entities to configure where that binary
+is and then it should be negotiated. But I've made the proposal as is,
+specifically because it may be incredibly easy to implement, and even works
+with `npm` in most cases!
+
+# Unresolved questions
+
+- This proposal has `npm` compatibility via full rebuilds and with `yarn` we
+  can substitute `.(os)`/`.(cpu)` across all packages, or no packages at all,
+  but how do we substitute variables for only the subset of packages that
+  _have_ a tag for that os/cpu? This could be solved much later, but I imagine
+  that the `yarn` client could _first_ try substituting `os/cpu` and then if no
+  git tag is found, it could fall back to the literal string
+  `#4.0.0.(anyOs).(anyCpu)`, and then finally `#4.0.0`.
+
+