Encoding of existential types in TypeScript

existential.ts

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)