Create 0000-license-check.md

Author: hzoo

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+- Start Date: 2016-10-12
+- RFC PR: 
+- Yarn Issue: 
+
+# Summary
+
+Original issue: https://github.com/yarnpkg/yarn/issues/904
+
+From tweet: https://twitter.com/rstacruz/status/786052262841896960
+
+Integrate a license checker as a yarn command.
+
+> I have published a standalone package that does this: https://github.com/behance/license-to-fail
+
+Yarn has `yarn licenses ls`. It would also be useful to know if certain packages
+don't satify your license (or other similar files) requirements rather than just a list of them.
+
+```bash
+$ yarn licesnes check
+yarn licenses v0.14.0
+Disallowed Licenses
+├─ [email protected]
+│  ├─ License: not-allowed-license
+│  └─ URL: git+https://github.com/pkg/here.git
+# error
+```
+
+# Motivation
+
+Most apps/projects have certain assumptions about the kinds of dependencies they bring in.
+Even if you check each new dependency, the dependencies of those dependencies may have issues.
+There's isn't an easy/manual way to do this outside of checking the license of all dependencies.
+
+It's most likely that there aren't issues but having an command to do so would allow running it on CI
+just like a linter. Issues can be caught automatically and with confidence.
+
+This solves the problem of checking the license of a new dependency brought in through a new PR
+or of an existing package updating it's license (whether it's a direct dependency or indirect dependency).
+
+The outcome is that users could run the command to notify which packages are disallowed.
+
+# Detailed design
+
+The basic idea is straightforward: Given an array of packages and their licenses, match that against an array of
+licenses that are disallowed. If any match, error and print them out.
+
+It would be useful to have a way to make a list of exceptions for when you want to whitelist a properitary pacakage.
+
+In reality you will probably need to make a lot of exceptions for packages since not all projects have a license
+or the program to check what license a project is doesn't always work.
+
+## Exceptions/What to do with packages that have an "unknown" license
+
+- the license checker isn't able to figure out the license
+  - license is in the readme or some other form (not in package.json)
+  - the license is correctly updated in master on git but not published (not maintained)
+  - future version of the package has a license but it's an indirect dependency
+
+The way license-to-fail does it is let you pass in a config file.
+
+```bash
+$ ./node_modules/.bin/license-to-fail ./path-to-config.js
+```
+
+The config file is just an object with a list of `allowedPackages` and a list of `allowedLicenses`.
+
+```js
+module.exports = {
+  allowedPackages: [
+    {
+      "name": "allowed-package-name-here",
+      "extraFieldsForDocumentation": "hello!", // optional
+      "date": "date added", // optional
+      "reason": "reason for allowing" // optional
+    }
+  ],
+  allowedLicenses: [
+    "MIT",
+    "Apache",
+    "ISC",
+    "WTF"
+  ],
+  warnOnUnknown: true
+};
+```
+
+# Alternatives
+
+Just use a separate package rather than making it built-in like https://github.com/behance/license-to-fail already is (and others).
+
+# Unresolved questions
+
+How do users specify the allowed licenses and exceptions (differences for apps/libraries)?
+
+- use package.json config
+- infer from the package's own license which licenses would be acceptable
+- use an yarnrc config
+- use cli arguments for options
+
+Should it warn or error with unknown licenses?