Record definitions are sloppy

[Methuselah96]

From @gryn on Wed, 03 Jul 2019 17:20:54 GMT

Flow version: 0.97.0 / immutable 4.0.0-rc12

What happened

When defining records, you have to do all the typecasting because Record() has a incorrect type definition.

How to reproduce

type Product = { address: string };
type App = { name: string };
const AppDefaults: App = { name: '' };

const AppRecordImplicit = Record(AppDefaults);        // class RecordInstance<T: any = any>
const AppRecordExplicit = Record<App>({ name: '' });  // class RecordInstance<T: any = any>
const AppRecordBorked: RecordFactory<App> =           // RecordFactory<App>
  Record<Product>({ address: '' });

const appImplicit = AppRecordImplicit({ name: 'i' }); // RecordOf<empty>
const appExplicit = AppRecordExplicit({ name: 'e' }); // RecordOf<empty>
const appBorked = AppRecordBorked({ name: 'b' });     // RecordOf<App>

In the above code, none of the ways of creating records work. The only way that actually works is to say:

const AppRecordExtraExplicit: RecordFactory<App> = Record<App>(AppDefaults);

Ideally, I'd expect either of the first two to work (AppRecordImplicit or AppRecordExplicit). However, they create RecordOf<empty> since they are not typed themselves (they receive class RecordInstance<T: any>).
The third call illustrates how that is a bit crazy, since you can successfully cast a record factory of one type to another (which leads you to be able to make records of the wrong type entirely).

Only by using AppRecordExtraExplicit -- and using it correctly, since it is the same form as AppRecordBorked -- can you get a RecordFactory<App> and produce actual RecordOf<App>.

Copied from original issue: immutable-js#1722