[Feature] Generic Reference Syntax Proposal

[MunMunMiao]

🔍 Search Terms

• Generic
• Generic Reference
• Partial Type
• Private Generic

✅ 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

Background

The current version of TypeScript often requires explicitly passing multiple generic parameters, leading to code duplication and redundancy in certain scenarios. Inspired by TypeScript Issue #26242, we propose a new keyword ref, aimed at simplifying the reference and application of generics, and enhancing the readability and maintainability of the code.

Function Description

The keyword ref is used to declare a generic reference, which can be reused in functions, classes, or other generic definitions without the need for repeated declaration. Using ref, one can define default values for generics, and it supports exporting and cross-file referencing.

Syntax

// Declaring a reference
// If no default value is set, it is inferred from the context.
ref T;

// Declaring a reference and setting a default value
ref T = string;

// The declaration reference can be constrained to certain types.
ref T extends string;

// The declaration of a reference can be constrained to certain types and a default value can be set.
ref T extends string = 'Hello';

// When defining a reference, if a value is set,
// then the reference will be constrained to the type of value.
//
// T type is: T extends 'Hello'
ref T = 'Hello';

// Exporting a reference
export ref T;

// ❌ The variable cannot be reassigned after being defined.
T = number;

📃 Motivating Example

Solving the Optional Generic Input Problem

Use ref to simplify function definitions, automatically infer generic parameters, and achieve private generic reference functionality:

function define<T, Agrs extends O[]>(value: T, ...agrs: Agrs): Ref<Agrs> {}

// If you need to manually limit the value to be a string
// you not only need to set the first generic but also input the second generic
// which cannot be automatically inferred.
define<string>('Hello', 1, 'A', true) // 👎 Error: Expected 2 type arguments, but got 1.


// when you't want the user pass O or only want the to pass type of
// you can use the ref generic reference.
ref O
function define<T>(value: T, options: O): Ref<O> {}
define<string>('input any strings') // 👍 <string>(value: string, options: number) => Ref<number>

// When you need to allow users to fill in the type
function define<T, O>(value: T, options: O): Ref<O> {}
define<string, string>('input any strings', 'i am here') // 👍 <string, string>(value: string, options: string) => Ref<string>

Cross-Function, Cross-File Constraints

ref supports cross-file referencing, achieving unified type constraints:

// option.ts
export ref Tref;

export function define(value: Tref): Ref<Tref> {}

// example.ts
import { type Tref } from 'option.ts';

export function example(value: Tref): Ref<Tref> {}

Conclusion

This proposal aims to simplify the use of generics by introducing the ref keyword, thereby enhancing TypeScript's development efficiency and code quality.

For instance, Vue.js V2 v3-component-options.d.ts#L75 If there is a ref keyword, then won't be so many generic variables stored on the function, and the code will be much cleaner.

We look forward to community feedback and further discussion.

❗️Warning❗️

This proposal does not conflict with optional input generics, but it can solve problems such as private generics.

[jcalz]

I don't understand the scoping for these ref type parameters. What does it mean for both define and example to have access to the same Tref? It almost feels like #31894. How does this differ from that?

For #26242 I think it would be almost required for the type parameter to be scoped to the function and not some outer scope. What are the benefits of ref U; declare const f: <T>(u: U) => F<T, U> compared to something like declare const f: <T, U=infer>(u: U) => F<T, U>?

[MunMunMiao]

The purpose is to address the issue of privatized generics and optional generics You can refer to my example 1.

(aside, you've got the [] in the wrong place in your first example)

I don't understand the scoping for these ref type parameters. What does it mean for both define and example to have access to the same Tref? It almost feels like #31894. How does this differ from that?

For #26242 I think it would be almost required for the type parameter to be scoped to the function and not some outer scope. What are the benefits of ref U; declare const f: <T>(u: U) => F<T, U> compared to something like declare const f: <T, U=infer>(u: U) => F<T, U>?

When we need to infer the return value a function, we need to infer it the function input values. This process does not require user input. However, the existing design approach requires that the generic variable be defined on the function, the user to interfere with our inference.

function define<T, Options extends Option[]>(value: T, ...agrs: Options): Ref<Options> {}
// In this case
// we do not want the user to set a second generic because it will interfere with our inference of args, 
// but we need the user to fill in the first generic to constrain the value.
define<string, any>('')

// So I need to hide my `Options[]`
ref Options
function define<T>(value: T, ...args: Options): Ref<Options> {}

define<string>('') // This is great.

[jcalz]

I still don't understand what it means for two different functions to access the same ref.

I also don't quite see the use case for preventing a user from manually specifying the type argument. You say "interfering with our inference" but why would that be a problem? What is the use case for private generics? Could you show an example where a user "interfering" with inference leads to a bad outcome?

[MunMunMiao]

I still don't understand what it means for two different functions to access the same ref.

For example, the code of Vue. Link Can be used in overloaded or more functions of the same type (this requires more examples to be given by everybody, and the use of the same ref for multiple functions is just one possibility)

I also don't quite see the use case for preventing a user from manually specifying the type argument. You say "interfering with our inference" but why would that be a problem? What is the use case for private generics? Could you show an example where a user "interfering" with inference leads to a bad outcome?

you can see this Playground

this's use ref syntax Playground