Existential type?

Here's a case where I need a few existentials. It's for a definition file, where I need to have parameters for Binding and Scheduler, but it doesn't matter to me what they are. All I care about is that they're internally correct (and any doesn't cover this), and I'd rather not simulate it by making the constructor unnecessarily generic.

The alternative for me is to be able to declare additional constructor-specific type parameters that don't carry to other methods, but existentials would make it easier.

export interface Scheduler<Frame, Idle> {
    nextFrame(func: () => any): Frame;
    cancelFrame(frame: Frame): void;
    nextIdle(func: () => any): Idle;
    cancelIdle(frame: Idle): void;
    nextTick(func: () => any): void;
}

export interface Binding<E> extends Component {
    binding: E;

    patchEnd?(): void;

    patchAdd?(
        prev: string | E | void,
        next: string | E | void,
        pos: number,
    ): void;

    patchRemove?(
        prev: string | E | void,
        next: string | E | void,
        pos: number
    ): void;

    patchChange?(
        oldPrev: string | E | void,
        newPrev: string | E | void,
        oldNext: string | E | void,
        newNext: string | E | void,
        oldPos: number,
        newPos: number
    ): void;
}

export class Subtree {
    constructor(
        onError?: (err: Error) => any,
        scheduler?: type<F, I> Scheduler<F, I>
    );

    // ...
}

export class Root extends Subtree {
    constructor(
        component: type<E> Binding<E>,
        onError?: (err: Error) => any,
        scheduler?: type<F, I> Scheduler<F, I>
    );

    // ...
}

从 issue 中的存在类型示例开始。没有指定源文件、测试或入口点，因此在评估设计之前，先确定涉及的 TypeScript 类型系统和声明处理领域。完成的标准是支持所请求的存在参数，同时保留 Binding 类型与 Scheduler 类型之间的内部关系。