Add new rfc for yarn tasks

accepted/0000-yarn-tasks.md

@@ -0,0 +1,103 @@
+- Start Date: 2018-11-21
+- RFC PR:
+- Yarn Issue: https://github.com/yarnpkg/yarn/issues/5773
+
+# Summary
+
+Yarn CLI command `yarn run` will be able to orchestrate various npm scripts, allowing them to run in parallel by default and matching any glob patterns.
+
+# Motivation
+
+Many build processes involve Grunt, Gulp, or other task runners that have great use cases but also require separate plugins, configuration files, and understanding of the API. `yarn run` would remove the need for installing these often global dpendencies and lower maintenance costs by providing a simpler interface defined in package.json. This would also cut down on the script run times and would be useful with workspaces as mentioned in https://github.com/yarnpkg/yarn/issues/6689.
+
+Feature Requirements:
+
+- Be able to run multiple commands in parallel across platform
+- Show and differentiate console output for each command
+- Compatible with existing behavior of `npm run` command
+
+Prior Art:
+
+- [npm-run-all](https://www.npmjs.com/package/npm-run-all)
+- [gulp](https://gulpjs.com/)
+
+# Detailed design
+
+## Simple use case
+
+// In `package.json`
+
+```js
+{
+    "scripts": {
+        "lint": "eslint .",
+        "test": "jest"
+    }
+    "tasks": {
+        "verify": ["lint", "test"]
+    }
+}
+```
+
+`yarn run verify` would run lint and test commands in parallel
+
+## Advanced use case - pattern matching
+
+```js
+{
+    "scripts": {
+        "lint": "eslint .",
+        "test:unit": "jest __tests__",
+        "test:integration": "jest __integration__"
+    }
+    "tasks": {
+        "verify": ["lint", "test**"]
+    }
+}
+```
+
+`yarn run verify` would run lint, test:unit, and test:integration commands in parallel
+
+## Advanced use case - nested tasks
+
+```js
+{
+    "scripts": {
+        "build": "webpack",
+        "lint": "eslint .",
+        "test": "jest",
+    }
+    "tasks": {
+        "verify": ["lint", "test"],
+        "dev": ["verify", "build"]
+    }
+}
+```
+
+`yarn run dev` would run lint, test, and build in parallel
+
+_NOTE_ Nested arrays or serial runs will not be supported in tasks because they can be done via scripts using `&&` operator
+
+## Additional flags
+
+`--hide-name`: By default, tasks will run by appending the name of the command to each line. Enabling this flag would allow it to be hidden from the output.
+
+# How We Teach This
+
+This will be an opt-in feature that would be compatible with existing behavior of `yarn/npm run`. However we should add documentation, as well as a simple migration guide.
+
+# Drawbacks
+
+- Increased complexity of yarn
+- Feature not yet supported by npm
+- Functionality has been partially implemented as a separate package by other open source projects
+
+# Alternatives
+
+`&` operator, which creates a sub-shell and would not allow modifying the output for readability
+Third-party libraries, which are additional dependencies and can require complex command line args
+Third-party task runners, which have issues mentioned above, mostly due to dependence on ecosystem/maintainers of plugins
+
+# Unresolved questions
+
+- Order of operations for pre/post hooks for installs if they are defined in tasks. Should they happen in parallel or serial?