Generics Aliasing and Partial Specialization

Suggestion

🔍 Search Terms

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generics specialization

✅ Viability Checklist

My suggestion meets these guidelines:

• 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 feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Add Generics aliasing and partial Specialization

📃 Aliasing Example

// In external library

export type Monoid<T> = {
  concat: (x: T, y: T) => T;
  empty: T;
};
export type MonoidFn<T> = (concat: (x: T, y: T) => T, empty: T) => Monoid<T>;

// in user code
const monoid: MonoidFn<~T> = (concat, empty) => ({ concat, empty }); // impossible today, T is undefined

Here we want the monoid function to still be generic and still using the original library generic definition. Using ~T tells the compiler we know that T is undefined, just make an alias.

📃 Partial Specialization Example

// In external library

export type AggregateFn<T, U> =  (map: (item: T) => U) => (array: T[]) => U;

// in user code
const sum: AggregateFn<~T, number> = mapReduceSum<T,number>(); // impossible today, T is undefined

Here we want to specialize AggregateFn and make sum an AggregateFn<T, number>, that is still generic but specialized for numbers. Using ~T tells the compiler we know that T is undefined, so make a partial specialization.

💻 Use Cases

This is almost madatory for decent functional programming without having to redeclare functions of an external library.
Examples should be sufficient to explain a basic use case

No source files, tests, or entry points are named. Start by reviewing the aliasing and partial-specialization examples and the TypeScript design goals; done would require an agreed design for generic aliases and partial specialization plus implementation and coverage for the requested cases.