Importing compiled flow libraries

[Gozala]

I've being struggling to type check apps that use compiled flow libraries.

The problem is that in my program I import a package 'foo' who's package.json#main points to ./dist/index.js where dist file are generated via babeljs and have an UMD format and look roughly like this:

(function (global, factory) {
  if (typeof define === 'function' && define.amd) {
    define(['exports', 'react'], factory);
  } else if (typeof exports !== 'undefined') {
    factory(exports, require('react'));
  } else {
    var mod = {
      exports: {}
    };
    factory(mod.exports, global.React);
    global.index = mod.exports;
  }
})(this, function (exports, _react) {
  'use strict';
/*::
  import type {VirtualElement} from "reflex/src/core";
  import type {Address} from "reflex/src/signal";

  type RenderTarget = Node|Element|HTMLElement;
  type Configuration = {target: RenderTarget};
...

What this gives us is:

• If you ignore dist flow fails to import foo and fails to type check for that reason.
• If you do not ignore dist it will attempt to also type check foo/dist/index.js and still fail but in this case with "Unexpected token import", presumably because imports supposed to be at the top level not inside the function.

Either options seems bad though and even if flow was able to handle foo/dist/index.js and not complain about import inside a function it still would not be ideal as comments could be stripped out in dist files. It would make much more sense if it was passible to tell flow that source for this package is in this in this directory. I guess it has to be something like [include] directive but with a package name mapping so that imports would be resolved correctly.

Another (probably better) alternative could be to introduce "flow" field for the package.json so that package.json#flow.main could take precedence over package.json#main when flow resolves import path.

[Gozala]

Note that this issue arises only if your program A depends on flow checked library B that depends on flow checked library C.

[Gozala]

I think #838 is somewhat relevant here as it could have resolved this issue.

[Gozala]

Posting a quote from IRC that helps with this issue:

2:13 PM — •jeffmo looks for the docs on module.name_mapper ...
2:14 PM <•jeffmo> gozala: This *might* do what you’re looking for: http://flowtype.org/docs/advanced-configuration.html#options (see “module.name_mapper")
2:15 PM <•jeffmo> it lets you intercept Flow’s internal module name resolution steps
2:15 PM <•jeffmo> by specifying a regexp replacement pattern
2:15 PM <gozala> jeffmo: oh right I have not realized I could use that for this
2:15 PM <gozala> jeffmo: thanks
2:15 PM <•jeffmo> Maybe you could specify that “node_modules/your_package” maps to “node_modules/your_package/pre-build-version.js"

[Gozala]

I'm keeping this one open as I still think that mapping via package.json field is worth having specially because .flowconfig based configuration becomes more complicated with a number of libraries (see #238)

[Macil]

Possibly relevant: #593.

[samwgoldman]

I think there's room for improvement here, but we've implemented a scheme where you can include .js.flow files in your npm module, and Flow will use those instead of the corresponding .js files. Check out the "flow" build step in the fbjs project for an example of this in practice.