experimental.well_formed_exports is on by default in React Native 0.62, yet has unclear error messages and is undocumented

[jamesisaac]

This commit (cc @panagosg7 ) has added these config options to the default .flowconig template in new React Native projects starting from 0.62:

experimental.well_formed_exports=true
experimental.types_first=true

From what I can see, neither of these options are documented anywhere (would expect them to be here).

The issue I think people will have with experimental.well_formed_exports=true, is that it will introduce hundreds of errors into any mature codebase. The Flow docs for React components suggest to type functional components like so:

function MyComponent(props: Props) {

This will lead to an error under well_formed_exports, as (from what I understand) they need to be typed like so:

function MyComponent(props: Props): React.Node {

The error message doesn't even make it clear that this option is the reason. It makes it sounds like it's the lint rule's fault (signature-verification-failure=warn), but turning that to off makes no difference:

Cannot build a typed interface for this module. You should annotate the exports of this module with types. Missing type annotation at function return: (`signature-verification-failure`)

The only way I was able to remove these hundreds of new errors (as I didn't have time to update hundreds of export signatures), was process of elimination trying disabling various lines in the flowconfig until the errors disappeared.

[panagosg7]

Thanks for bringing this up @jamesisaac ! I'll remove these flags from the template, at least until we formally announce types-first mode, and update the necessary documentation.

[jamesisaac]

@panagosg7 Ok, great! Looking forward to hearing more about Types First when it gets closer to release.

Even with a commit on master removing them from the template, I think you'd need to also get the commit cherry picked into 0.62 (react-native-community/releases#157) otherwise the template will ship as is, as it's already in RC stage.

[panagosg7]

Sure! I'll request the pick once the fix is landed.

[jamesisaac]

Fixed in the latest RC 👍 react-native-community/releases#157 (comment)