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>
    );

    // ...
}

[cwohlman]

Existential types would also be useful for arrays of generics.

Example:

type Job<T, R> = { isJob(param: unknown): param is T, run(param: T): R }
class JobsHandler {
  jobs: Job<unknown, unknown>[] = [] // This breaks because of the error listed below
  // jobs: Job<any, any>[] = [] // too loose
  // jobs: Job<T, R>[] = [] // with existential types
    runJob<T, R>(thing: T): R {
    const job = this.jobs.find(job => job.isJob(thing));
    if (!job) {
        throw new Error('No job')
    }
    return job.run(thing); // ts error here, even though thing is guaranteed to be T
  }
  addJob<T, R>(job: Job<T, R>) {
      this.jobs.push(job)
  }
}

[pelotom]

For anyone interested in using existential types right now via the negation of universal types, the type annotation burden of doing so has been greatly reduced thanks to recent type inference improvements.

type StackOps<S, A> = {
  init(): S
  push(s: S, x: A): void
  pop(s: S): A
  size(s: S): number
}
 
type Stack<A> = <R>(go: <S>(ops: StackOps<S, A>) => R) => R

const arrayStack = <A>(): Stack<A> =>
  go => go<A[]>({
    init: () => [],
    push: (s, x) => s.push(x),
    pop: s => {
      if (s.length) return s.pop()!
      else throw Error('empty stack!')
    },
    size: s => s.length,
  })

const doStackStuff = (stack: Stack<string>) =>
  stack(({ init, push, pop, size }) => {
    const s = init()
    push(s, 'hello')
    push(s, 'world')
    push(s, 'omg')
    pop(s)
    return size(s)
  })

expect(doStackStuff(arrayStack())).toBe(2)