Custom loader integration

[guybedford]

Is it currently possible to specify a custom resolve hook? It would be really useful with this to be able to integrate a custom loader like SystemJS into Flow, which has its own resolution rules.

Basically, allowing a loader resolution hook to be provided as input. Generally the resolve hook takes the following form:

function resolve(url, parentUrl) {
  return resolvedUrl;
}

In the loader I believe it may also allow a promise to be returned for asynchronous resolution.

Does such a thing exist? Does it sound sensible?

[akxcv]

The issue is also described well in #2939 by @JohnLouderback.
At our company, we have the following lookup process:

import './myFile'
// => apps/[currentProjectName]/assets/javascripts/myFile
// => apps/common/assets/javascripts/myFile (if the former file does not exist)

This is currently accomplished by a custom Babel plugin.

AFAIK, this isn't currently possible to do with Flow. What would be the best way to accomplish such behavior? I don't think we can allow babel to process files before Flow because then we'd need source maps and it would really complicate things overall. But clearly, we need to do some kind of processing that includes checking if a file exists (accounting for many possible extensions).

I'm willing to submit a PR, but first, we need to decide what we want to do (if anything).

[jakesylvestre]

Looks like this got resolved in #6132/ 08b0c8f

[apsavin]

@goodmind should it be closed?

[goodmind]

I don't think it should, Flow team planned to remove custom loaders

[nicolo-ribaudo]

This is needed to be able to still use Flow with Yarn 2.

[jakesylvestre]

@goodmind where's that thread?

[alexandrukis]

Is there any (at this point I would accept any ugly, hack-ish solution) way of achieving this using the latest version of Flow?

[SamChou19815]

I don't think we are likely to allow this in the future. Module resolution gets more complicated than a simple function call, when you also need to worry about updating the dependency graph incrementally.