fix: add immerable Flow typedef

[nobitagit]

I noticed the Flow annotation for immerable was missing, so this PR introduces it. Some things to note though:

Before this fix

Given the following code:

// @flow

import { immerable } from "immer";

class A {}
A[immerable] = true

Flow produces this error:

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ src/symbol.js:3:10

Cannot import immerable because there is no immerable export in immer.

     1│ // @flow
     2│
     3│ import { immerable } from "immer";
     4│
     5│ class A {}
     6│ A[immerable] = true;

Also, this is a screenshot of my IDE:

After this fix

My IDE seems happier about it (type is Symbol and no error is shown on the import):

Interestingly, running Flow after adding the type annotation still produces an error, although a different one.

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ src/symbol.js:6:3

Cannot assign computed property using Symbol [1].

     src/symbol.js
      3│ import { immerable } from "immer";
      4│
      5│ class A {}
      6│ A[immerable] = true;
      7│

     node_modules/immer/dist/immer.js.flow
 [1] 80│ export var immerable: Symbol

This appears to be because Flow doesn't like Symbols as object keys as of now.

Rationale

While this fix won't make that snippet of code compile just yet, I think it's fair to surface the error to the user as clearly related to the shortcomings of Flow, not because of a missing definition in the lib.

Happy to hear your thoughts, change this PR or close it entirely if you think that things are fine as they are. Thanks!

[mweststrate]

I agree this is an improvement! A hackish solution / temporarily follow up would maybe to declare it as string instead. Not sure whether that is a good idea :-P.

[aleclarson]

🎉 This PR is included in version 4.0.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[nobitagit]

A hackish solution / temporarily follow up would maybe to declare it as string instead. Not sure whether that is a good idea!

I think that for the moment this is fine as is. This is a valid use case for a symbol, and the error can just be suppressed in userland:

// $FlowFixMe see: https://github.com/facebook/flow/issues/3258
A[immerable] = true;

AFAICS, the user is not really expected to tamper with immerable, so I think it's an acceptable stopgap for now while Flow catches up.