Incorrect mixed type from value of Object.entries

[patrickpei]

Flow assumes that value as returned by [key, value] for Object.entries is mixed. Not really sure where to start looking but willing to check it out / try to help!

const obj: {
  	[string]: number
} = {
	a: 1,
  	b: 2,
  	c: 3
};
for (const [char, num] of Object.entries(obj)) {
	const copy: number = num;
}

Error:

11: 	const copy: number = num;
                          ^ Cannot assign `num` to `copy` because mixed [1] is incompatible with number [2].
References:
[LIB] static/v0.66.0/flowlib/core.js:35:     static entries(object: any): Array<[string, mixed]>;
                                                                                         ^ [1]
11: 	const copy: number = num;
                 ^ [2]

Try Flow

[TrySound]

#5663
https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVBjOA7AzgFzGwEMBbAUwBU4BZYgB1wC4wBvVMMAbRIpYIBOAS2wBzALot2nTlzj4AFuQEA5MuX75hYyWw4zOxFtgCupAEbKANPoPmWAfkEjRtsAF99n92AC8ezmwARik3HBCAgzAjMCCbKLB7MAByDGS3T05vAG50KDgBMAAKLDxCHnUrMFIGcTA4KDAAeXMAK3IMfAA6cmwtIXJcIt4qWgZcAEoJyKFGopr6MAAyJbB8AE96cgbqhj9ff2S4No78ZOnpGVKCMCF8clI-XfoKinFcg1niu4fl1Y2tjsfo8Dodju1OudIgZroQACZCAjEbAYchPYFcZIIpEo8jJd5uThfIoA7aNbH4ZGo-aHUwWZRQy4JWFgAAexjMlkK-gpVPIHyimRkQu86CAA

[spudly]

Problem is here: https://github.com/facebook/flow/blob/master/lib/core.js#L48

[houshuang]

See #2221. I'm currently using the values/entries code in the bottom of this file to great effect. https://github.com/chili-epfl/FROG/blob/develop/frog-utils/src/index.js#L285-L293

[mrkev]

Would replacing the current declaration linked to by @spudly with this work? Anyone care to test it out and submit a PR?

[jcready]

No @mrkev, the definitions for values and entries that @houshuang linked to are unsound. Here is a simple example:

const entries = <T>(obj: { [string]: T }): Array<[string, T]> => {
  const keys: string[] = Object.keys(obj);
  return keys.map(key => [key, obj[key]]);
};

const values = <T>(obj: { [string]: T }): Array<T> => {
  const keys: string[] = Object.keys(obj);
  return keys.map(key => obj[key]);
};

function test(val: { foo: string }) {
  values(val).forEach(v => {
    v.toLowerCase();
  });
  entries(val).forEach(([ k, v ]) => {
    v.toLowerCase();
  });
}

test({ foo: 'x', bar: 123 });

This going to pass typechecking, but fail at runtime.

[houshuang]

Indeed. In our case, we only use it for well typed dictionary objects (of the type [id: string]: WellTypedObject) so we always know what to expect.

…

On Mon, Jun 4, 2018 at 6:09 PM, James Wyatt Cready-Pyle < ***@***.***> wrote: No @mrkev <https://github.com/mrkev>, the definitions for values and entries that @houshuang <https://github.com/houshuang> linked to are unsound. Here is a simple example: const entries = <T>(obj: { [string]: T }): Array<[string, T]> => { const keys: string[] = Object.keys(obj); return keys.map(key => [key, obj[key]]); }; const values = <T>(obj: { [string]: T }): Array<T> => { const keys: string[] = Object.keys(obj); return keys.map(key => obj[key]); }; function test(val: { foo: string }) { values(val).forEach(v => { v.toLowerCase(); }); entries(val).forEach(([ k, v ]) => { v.toLowerCase(); }); } test({ foo: 'x', bar: 123 }); This going to pass typechecking, but fail at runtime. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#5838 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AADwh5qf6t2LDPS_exJufrRGjpSVnythks5t5VuegaJpZM4SJ6wA> .

-- http://reganmian.net/blog -- Random Stuff that Matters

[mrkev]

@jcready ah true, inexact objects back at it again

[gkz]

With above commit, object types that have an indexer and no explicit properties (e.g. {[string]: number}) have the type of their dictionary values returned by Object.values (e.g. Array<number>), and similarly with Object.entries, except the first element of the tuple works exactly like Object.keys (in this case, Array<[string, number]>)