Branding Drafts on Type level

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typescript
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developer-experience

调研方向

从 TypeScript Playground proof of concept 开始，将其与 branded Draft、MaybeDraft、current、isDraft 和 produce 相关的行为与 Immer 现有的类型签名进行比较。该 issue 没有指定仓库文件或测试；要视为完成，需要就编译时 branding 是否值得采用达成一致决定，并定义实现范围。

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描述

question

🙋‍♂ Question

I was wondering if it would be possible to not just brand at runtime, but branded type at compile time as well?

Cause

I was using redux/toolkit, which uses Immer and had an issue where current(val) was called on a non draft, typechecker was fine, executed it in development, all good, came to production, tried to unwrap a non draft and crashed. Not a big deal, just use isDraft, but it was something technically avoidable if....

Implementation

I could make a PR, I just wanted to figure out if that is even desirable since I assume this question came up already some time ago and was decided against it for a reason to me currently unknown (except maybe that some types are already relatively complex)

Proof of concept

I made a small proof of concept on typescript playground relative quickly to just check out the branding, the idea would be that produce etc return Drafted<T> instead of T hiding a potential source for errors (naming still up for discussion)

I made a small code example on TS playground where I do not duplicate the objects, Proxy them etc for simplicity. If no link should be used, here the snipped:

const brand : unique symbol = Symbol("foobar")

type Draft<T> = T & {[brand]:never}
type MaybeDraft<T> = Draft<T> | T

const a = 32;
const b = {
    a: 32,
    b: "Hello world"
}
const c = [1, 2, 3];
const d = [a,b,c] as const

const draft = <T extends object>(val: T)=> {
    const clone = val
    Object.defineProperty(clone, brand, {configurable: false, enumerable: false, writable: false, value: true})
    return clone as Draft<T>
}


const isDraft = <T extends unknown>(val: T): val is Draft<T> => {
    //@ts-ignore
    if (val[brand]) {
        return true
    }

    return false
}


const B = draft(b)
const C = draft(c)
const D = draft(d)


console.log("A: ", isDraft(a))
console.log("B: ", isDraft(B))
console.log("C: ", isDraft(C))
console.log("D: ", isDraft(D))


/**
 * Accepts a Draft or a Value, but differentiates them on a type level to enforce handling of each of the 2 scenarios
 */
const doSomething = (value: MaybeDraft<number[]>) => {
    //const val = current(value) // Error because value could also not be a draft
    const val = isDraft(value) ? current(value) : value
    return val
}

/**
 * Accepts a Draft or a Value, it doesn't care if it might be a Draft and just uses it
 */
const doSomething2 = (value: number[]) => {
    return value
}

/**
 * Accepts only a Draft, non Drafts are refused on a Type Level at compile time
 */
const current = <T extends object>(value: Draft<T>):T => {
    return value
}

doSomething(C)
doSomething2(C)
//const neverHappend = current(c)
const val = current(C)
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