Flow generator hackery to make it make it infer types

task.js

/* @flow strict */


type Future<x, a> = () => Task<x, a>

type Task<x, a> = {
  @@iterator(): $Iterator<empty, a, empty>;
  next(empty): IteratorResult<empty, a>;
  return(a): IteratorResult<empty, a>;
  throw(x): IteratorResult<empty, a>;
}
           

class Effect<x, a> { 
  value:a
  isDone:boolean;
  @@iterator: () => $Iterator<empty,a,empty>;
  constructor(value:a) {
    this.value = value;
    this.isDone = false
  }
//  [Symbol.iterator]() {
//    return this
//  }
  get done() {
    if (this.isDone) {
      return true
    } else {
      this.isDone = true
      return false
    }
  }
  next() {
    return this
  }
}

type Result <x, a> =
  | {ok:true, value:a}
  | {ok:false, error:x}

declare function spawn <x, a>(Future<x, a>):Promise<Result<x, a>>
declare function read(string):Task<Error, ArrayBuffer>
declare function write(string, ArrayBuffer):Effect<Error, void>
    
var out = spawn(function*() {
  var buffer =  yield * read("foo")
  const slice = buffer.slice(10, 20)
  const done = yield * write("another/path", slice)  
  return buffer
})