Generic VariadicOperation type helper (Higher-Kind types)

[vladmw5]

🔍 Search Terms

type helpers as type parameters, recursive types, variadic type

✅ 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

Consider the following code sample:

type Intersection<L, R> = L & R;

I was not satisfied with the limitation of being able to pass only 2 operands, so I created a recursive type MultipleIntersection:

type MultipleIntersection<A extends any[]> = A extends [infer F, ...infer R extends any[]]
  ? Intersection<F, R extends [infer L] ? L : MultipleIntersection<R>>
  : never;

type x = {
  a: string;
}

type y = {
  b: number;
}

type z = {
  c: boolean;
}

type w = {
  d: string[];
}

type testCortege = [x, y, z, w];

type test = MultipleIntersection<testCortege> // x & y & z & w

// Basically my MultipleIntersection type does the following:
type validTest = Intersection<testCortege[0], Intersection<testCortege[1], Intersection<testCortege[2], testCortege[3]>>>

Generally speaking, in the implementation of the MultipleIntersection type, if I replace the Intersection type helper with any other binary type helper, I would get the "Multiple" (or n-ary, aka variadic) version of that type.

My question is: is it possible (is there any TS syntax for that) to create a generic VariadicOperation<A extends any[], O extends ???what> type that would allow to pass a binary type-helper?

full code example

📃 Motivating Example

Typescript is finally Turing complete with this feature!!!

💻 Use Cases

1. To be able to create generic and reusable type helpers
2. No TS syntax supports that behavior
3. I don't know any workarounds at the moment

[xiBread]

#1213

[RyanCavanaugh]

Typescript is finally Turing complete with this feature!!!

This has been true for a long time, for better or worse 🥵