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Accepted
Eliya Cohen
Asked: 2024-08-15 16:21:12 +0800 CST

TypeScript 无法获取正确的函数重载

  • 772

给出以下函数实现：

function fmap<T, U>(value: T, f: (x: T) => U): U;
function fmap<T, U>(value: T | undefined, f: (x: T) => U): U | undefined;
function fmap<T, U>(value: T | null, f: (x: T) => U): U | null;
function fmap<T, U>(value: T | null | undefined, f: (x: T) => U): U | null | undefined {
    return value === undefined ? undefined : value === null ? null : f(value);
}

在下列情况下我会得到正确的类型：

const test1 = (v: string) => fmap(v, x => x); // ✅ expected: string
const test2 = (v: string | null) => fmap(v, x => x); // ✅ expected: string | null
const test3 = (v: string | undefined) => fmap(v, x => x); // ✅ expected: string | undefined
const test4 = (v: string | null | undefined) => fmap(v, x => x); // ✅ expected: string | null | undefined
const test5 = (v: string) => fmap(v, x => 1); // ✅ expected: number

但在以下情况下则不然：

// ❌ expected: number | null (actual: number)
const test6 = (v: string | null) => fmap(v, x => 1);

// ❌ expected: number | undefined (actual: number)
const test7 = (v: string | undefined) => fmap(v, x => 1);

// ❌ expected: number | null | undefined (actual: number)
const test8 = (v: string | null | undefined) => fmap(v, x => 1);

查看游乐场

typescript

2 个回答

  1. Best Answer

    您需要从T泛型类型中提取可空类型

    type ExtractedNullable<T> = Extract<T, null | undefined>

function fmap<T, U>(value: T, f: (x: T) => U): U | ExtractedNullable<T> {
    return value === undefined ? undefined as ExtractedNullable<T> : value === null ? null as ExtractedNullable<T> : f(value);
}

const test1 = (v: string) => fmap(v, x => x); // ✅ expected: string
const test2 = (v: string | null) => fmap(v, x => x); // ✅ expected: string | null
const test3 = (v: string | undefined) => fmap(v, x => x); // ✅ expected: string | undefined
const test4 = (v: string | null | undefined) => fmap(v, x => x); // ✅ expected: string | null | undefined
const test5 = (v: string) => fmap(v, x => 1); // ✅ expected: number

const test6 = (v: string | null) => fmap(v, x => 1); // number | null

const test7 = (v: string | undefined) => fmap(v, x => 1); // number | undefined

const test8 = (v: string | null | undefined) => fmap(v, x => 1); // number | null | undefined
    • 2 
  2. const test1 = (v: string) => fmap(v, x => x); // ✅ expected: string
const test2 = (v: string | null) => fmap(v, x => x); // ✅ expected: string | null
const test3 = (v: string | undefined) => fmap(v, x => x); // ✅ expected: string | undefined
const test4 = (v: string | null | undefined) => fmap(v, x => x); // ✅ expected: string | null | undefined
const test5 = (v: string) => fmap(v, x => 1); // ✅ expected: number

// Should now correctly infer
const test6 = (v: string | null) => fmap(v, x => 1); // ✅ expected: number | null
const test7 = (v: string | undefined) => fmap(v, x => 1); // ✅ expected: number | undefined
const test8 = (v: string | null | undefined) => fmap(v, x => 1); // ✅ expected: number | null | undefined
    • 0

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