The explaination is wrong in typescript guide

[GuichiZhao]

I believe the section Basic usage from the typescript guide is completely wrong

The reason why "Why can't we simply infer the type from the initial state" is wrong per the typescript guide, it has nothing to do with invariant, the real reason is that we do not provide any useful information for TS engine does the pattern matching

If we provide proper information

declare const create: <T>(f: (get: () => T) => T) => T
const a = create(() => 'hello') //const a: string

We can see clearly that a is inferred as a number rather than unknown as the typescript guide said.

So, as long as the generic to be inferred is invariant (i.e. both covariant and contravariant), TypeScript will be unable to infer it.

Let me speak aloud about how TS engine think

When the typescript engine sees () => 'hello' it tries to pattern match (get: () => T) => T and it figures out that if it binds T with string, it will match both.
Note that because the ()=>'hello' provides no param, the TS engine will not bother to match the argument, it will just let it go
Remember that ()=>sting is perfectly assignable to (a:number)=>string (but no the other way)

However, if we put an argument as a get param,

declare const create: <T>(f: (get: () => T) => T) => T
const a = create((_) => 'hello') // const a: unknown

TS engine can not bind T properly, it is not because of a chicken and egg problem as the guide said

TypeScript wonders. It's like that chicken or egg problem. At the end TypeScript, gives up and infers T as unknown.

Let us think aloud again, the TS engine tries to match
(_) => 'hello' and (get: () => T) => T it sees the parameter is not empty as in the previous example, so it DOES try to match it, so the problem becomes:
match _ and () => T, where _ is implicitly of any type. there is no exra hint with _ at all, so <T> is inferred as unknown

If we annotate it to hint the typescript, it works again

declare const create: <T>(f: (get: () => T) => T) => T
const a = create((_: (s: string) => string) => 'hello') //const a: unknown

Similar behavior can be observed in Promise

const x = new Promise((resolve) => {
  resolve(11)
}) //const x: Promise<unknown>

The Promise can not be bound properly because when TS tries to pattern match the arguments, it finds no hint at all, AGAIN nothing to do with "invariant"

If we provide the TS engine with some hint, it will work again

const x = new Promise((resolve: (s: number) => void) => {
  resolve(11)
}) //const x: Promise<number>

To conclude, the real reason why TS engine infers <T> as unknown is there is no enough information provided, rather the the "chicken-egg" or "invariant" as the documentation put it.

Reproduction Link
https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEYD2A7AzgF0XKGQC54AeAFQD4AKAM0IoHMQNaBKeAXjPhNY6585IAoZOixR22ELhAUKAfVppCmGAEsUdfvBXrN7TgHIAFiAgQkB5oOsjM8AB4SUIAO7wACjCQBbVWhkUcGhIEABuBPAUSvAoAK7eAEYgMFqhSKrAWgDegvDwQSHhFACMxVYAvsxAA

If the idea is approved, I am happy to issue a PR to fix the documentation

[dai-shi]

Are you suggesting to just fix the docs, or do you have any idea to improve types in the code?

[dai-shi]

I think you want to talk with @devanshj in advance, as he's the one who created the TypeScript guide.

[GuichiZhao]

I think you want to talk with @devanshj in advance, as he's the one who created the TypeScript guide.

What is your opinion though?

[dai-shi]

Apart from the wording, I think it feels like you are missing some points of code and/or docs. I can't explain it very well. Maybe #725 will help. And, #710. I don't understand them 100% and that's why it's better for you guys to discuss.

[GuichiZhao]

Apart from the wording, I think it feels like you are missing some points of code and/or docs. I can't explain it very well. Maybe #725 will help. And, #710. I don't understand them 100% and that's why it's better for you guys to discuss.

If you can not fully understand, How can expect the user to understand the documentation? LOL

By the way, I am 90% sure the real reason you do not understand it is that the current documentation is not only wrong but also confusing and misleading. It points in the wrong direction. That is why no one can follow its logic

If you read my text above, you will find it easier to follow and make more sense

[dai-shi]

I mean about #710 and #725. If it's about the wording in the guide, I think I can understand. Let's see if @devanshj catches up, otherwise will see.

[devanshj]

Hi, I'm the author of the said documentation. Thanks for pointing out it's not because of invariance but because of the naive pattern matching TypeScript uses (as illustrated in the promise example too). Please go ahead and make a PR to improve the documentation.

[GuichiZhao]

I tried to contribute to the documentation and follow the contribution guide. , but I faced some trouble setting up the environment

1. There is no branch names docs in the website repo

2. There is no src folder as well

@devanshj

[dai-shi]

I'm VERY sorry about it. CONTRIBUTING.md isn't well maintained. ( @dbritto-dev Can you fix it quickly? )

@GuichiZhao It's here: https://github.com/pmndrs/zustand/blob/main/docs/guides/typescript.md
Just open a PR in this repo.

[GuichiZhao]

@dai-shi Here is the PR #2900

[dbritto-dev]

Hey peeps I'll update CONTRIBUTING.md