generator.swift

import Foundation
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

/// define an operation success or error result
enum Result<T> {
  case error(Error)
  case success(T)
}

/// define a generator function result. 
/// a generator can yield multiple values before returning
enum IteratorResult<Yield, Return> {
  case yield(Yield)
  case done(Return)
}

/// a generator is a function that return an IteratorResult every time its called
typealias Generator<Yield, Return, Next> = (Next?) -> IteratorResult<Yield, Return>
typealias Thunk<T> = (@escaping (Result<T>) -> Void) -> Void

/// run a generator step bt step until its done
func runner<T, V>(_ generator: Generator<Thunk<T>, Thunk<V>, T>) -> Void {
  func run(value: T?, callback: @escaping (Result<V>) -> Void) {
    let it = generator(value)

    switch it {
    case .yield(let thunk):
      thunk() { (result: Result<T>) -> Void in
        switch result {
        case .error(let err):
          callback(.error(err))
        case .success(let value):
          print("yield", value)
          return run(value: value, callback: callback)
        }
      }
    case .done(let thunk):
      thunk(callback)
    }
  }

  run(value: nil) { print($0) }
}


/// example of generator that yield incremented number and returns 10
func incGenerator() -> Generator<Thunk<Int>, Thunk<Int>, Int> {
  return { (prev: Int?) in
    let next = (prev ?? 0) + 1
    let thunk: Thunk<Int> = { callback in
      DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
        callback(Result.success(next))
      }
    }
    return next < 10
      ? .yield(thunk)
      : .done(thunk)
  }
}

runner(incGenerator())

You're missing an @escaping decorator on line 23