theoretical typed co function

co.ts

type CoGen<R> = {
  next(): IteratorYieldResult<undefined>
  next<T>(value: T): IteratorResult<Promise<T>, R>
}

async function co<R>(genfn: () => CoGen<R>): Promise<R> {
  const it = genfn()
  let result: IteratorResult<unknown, R> = it.next()
  while (true) {
    if (result.done) {
      return result.value
    } else {
      result = it.next(await result.value)
    }
  }
}

example.ts

async function test() {
  const result = await co(function* test() {
    const value = yield Promise.resolve(123)
    return String(value)
  })
  
  assert(result === '123')
}