使用本身是泛型的类型专门化泛型 function

Question

In general, "specializing" generic functions in Typescript can be implemented by returning functions:

export function twice<T>() {
  return (obj: T): [T, T] => {
    return [obj, obj];
  };
}

interface Bar {
  bar: string;
}

const twiceBar = twice<Bar>();

Here, twiceBar basically is identical to (obj: Foo) => [obj, obj] .

Is it possible to specialize a function with a type that is itself a generic?

interface Baz<T> {
  baz: T;
}

const twiceBaz = twice<Baz>(); // <--- does not compile

Here, I'd like twiceBaz to be <T>(obj: Baz<T>) => [obj, obj] . How can that be achieved?

Answer 1

What you want to do is twice<Baz<T>> while letting T remain generic. The function twiceBaz is generic over this T , by specifying it, your problem will be solved.

function twiceBaz<T>() {
  return twice<Baz<T>>();
}

You need to specify twiceBaz as a function that calls twice because that's the only way to get a new T generic parameter.

The signature ends up being exactly the one you want and it works:

interface Baz<T> {
  baz: T;
}

const twiceBazNumber = twiceBaz<number>();
const numberBaz = {
  baz: 2;
};

console.log(twiceBazNumber(numberBaz)); // [ { baz: 2 }, { baz: 2 } ]