Type instantiation for type level generic function

🔍 Search Terms

type instantiation
type level generic function
generic return type

Related: https://github.com/microsoft/TypeScript/issues/40542, https://github.com/microsoft/TypeScript/issues/61133 and maybe https://github.com/microsoft/TypeScript/issues/52035?

Edit: Just found https://github.com/microsoft/TypeScript/issues/55435 and https://github.com/microsoft/TypeScript/issues/40179

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

#47607 added support for instantiation expressions, but those do not apply at the type level. I understand it'd be ambiguous in some cases (https://github.com/microsoft/TypeScript/pull/47607#issuecomment-1058192563), but in some cases there really isn't a reasonable workaround.

📃 Motivating Example

I actually have a very similar case to the example provided in https://github.com/microsoft/TypeScript/issues/40542

const provideBox = () => {
    const box = <T>(value: T) => ({value});
    return box;
};

type BoxMaker = ReturnType<typeof provideBox>
// type BoxMaker = <T>(value: T) => { value: T; }

type Box1<T> = ReturnType<BoxMaker<T>>;
// Type 'BoxMaker' is not generic.(2315)

const box = undefined as unknown as BoxMaker;

type Box2<T> = ReturnType<typeof box<T>>;
//type Box2<T> = { value: T; }

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "target": "ES2017",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

Playground Link: Provided

💻 Use Cases

1. What do you want to use this for?

In our case we have a more complex provideBox that accept arguments. We also need to rely on type inference for the generic type of the box function and its return value;

3. What shortcomings exist with current approaches?

I cannot extract and export a generic type for a return value of a generic function if I don't have direct reference to the function.

5. What workarounds are you using in the meantime?

The only workaround I found is to use a dummy runtime value to apply the type, and use and instantiation expression on that dummy value (see Box2 above)

Bắt đầu bằng cách tái hiện ví dụ tạo động lực trong liên kết Playground được cung cấp, sau đó đọc các issue liên quan và PR #47607 để hiểu hành vi hiện có của các biểu thức khởi tạo và những điểm mơ hồ đã được nêu. Công việc được xem là hoàn tất khi một dạng ở cấp độ kiểu có thể trích xuất kiểu trả về generic mà không cần một giá trị runtime giả, tạo ra kết quả Box2 như mong đợi trong khi vẫn giữ nguyên hành vi hiện có.