Support flow:main in manifest

[steelbrain]

(I searched for an existing issue but couldn't find one so sorry if this is a duplicate)

The problem

The problem is that we want flow to understand the types of flow-typed modules correctly when requiring them.

The workaround

We currently have to keep .flow files along with .js files after they are babel compiled and that adds an extra build step and a ton of headache.

The solution

If Flow has support for flow:main (like jsnext:main for rollup), we would not have to add anything new to our build step. We can just leave the src dir in our npm package and set flow:main to src/.

This is a follow up to #1459

[steelbrain]

@STRML About your comment on the PR, AgentMe/flow-copy-source doesn't integrate well with babel so I've forked the babel cli and created a dropin replacement babel-cli-flow, it works well even with the watcher and updates $.flow copies instantly

[TrySound]

You might don't need to look for module field. Use this way

{
  "main": "./lib/index.js",
  "module": "./esm/index.js",
  "files": ["lib", "esm", "src"],
  "scripts": {
    "build:flow": "echo \"// @flow\\n\\nexport * from '../src';\" > lib/index.js.flow"
  }
}

[wtgtybhertgeghgtwtg]

I think that's what he was addressing under "The workaround". It's certainly an option some libraries are taking, but it feels hacky and potentially increases bundle package size.

[TrySound]

@wtgtybhertgeghgtwtg How this increases bundle size? lib/index.js.flow file is used only by flow. Bundlers do not care about it. Also with this you don't need to duplicate types anywhere. Sust place one .js.flow reference near entry point.

[TrySound]

Also there is #5729 with similar feature, so this can be closed

/cc @vkurchatkin

[wtgtybhertgeghgtwtg]

Sorry, I meant package size.
I don't think #5729 solves this issue. Ideally, Flow types should work without an extra build step and without the consumer having to alter their configuration.

[TrySound]

Package size? With a few bytes? Who cares? My way is pretty good for this without any new fields in package.json.

[wtgtybhertgeghgtwtg]

Yeah, I completely misinterpreted what the script did. I still feel that this would be a better solution, but I'm going to start using that script.

[TrySound]

The problem is that own entry point does not solve necessary cases. For example:

import map from 'lodash/map'

requires map.js.flow file. Such cases are not solvable with simple configuration. But with .js.flow and main field we have universal way to define types. So I consider it as non-hacky and even more productive than any config and additional fields.