async + recursion + multi-thread serious simplification test code in playground

.swift

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

var freeV = 0
func recur(input: Int, completion: @escaping (Int) -> Void) -> Int {
  DispatchQueue.global().async {
    print("FOR: is main thread? : ", Thread.isMainThread)
    for i in (input * 10)...(input * 100) {
      print("[input : \(input)] check for : ", i)
    }
    print("[input : \(input)] end of for")
  }

  print("input : ", input)
  if input == 2 {
    completion(input)
    return input
  } else {
    recur(input: input + 1) { v in
      freeV = v
      print("[0] freeV :  ", freeV)
    }
  }
  
  print("when to return? : ", input)
  return input
}

let result = recur(input: 0) { v in
  freeV = v
  print("[1] freeV :  ", freeV)
}

print("RETURN is main thread? : ", Thread.isMainThread)
print("end of shit,. result : ", result)
print("freeV : ", freeV)



PlaygroundPage.current.needsIndefiniteExecution = true


/**
input :  0
FOR: is main thread? :  false
input :  1
FOR: is main thread? :  false
input :  2
FOR: is main thread? :  false
[0] freeV :   2
when to return? :  1
when to return? :  0
[input : 0] check for :  0
[input : 1] check for :  10
[input : 2] check for :  20
RETURN is main thread? :  true
[input : 0] end of for
end of shit,. result :  0
[input : 1] check for :  11
[input : 2] check for :  21
freeV :  2
[input : 1] check for :  12
[input : 1] check for :  13
[input : 2] check for :  22
[input : 1] check for :  14
[input : 2] check for :  23
[input : 1] check for :  15
[input : 2] check for :  24
[input : 1] check for :  16
[input : 2] check for :  25
[input : 1] check for :  17
[input : 2] check for :  26
[input : 1] check for :  18
[input : 2] check for :  27
[input : 1] check for :  19
[input : 2] check for :  28
[input : 1] check for :  20
[input : 2] check for :  29
[input : 1] check for :  21
[input : 2] check for :  30
[input : 1] check for :  22
[input : 2] check for :  31
[input : 1] check for :  23
[input : 2] check for :  32
[input : 1] check for :  24
[input : 2] check for :  33
[input : 1] check for :  25
[input : 2] check for :  34
[input : 1] check for :  26
[input : 2] check for :  35
[input : 1] check for :  27
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[input : 1] check for :  28
[input : 2] check for :  37
[input : 1] check for :  29
[input : 2] check for :  38
[input : 1] check for :  30
[input : 2] check for :  39
[input : 1] check for :  31
[input : 2] check for :  40
[input : 1] check for :  32
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[input : 1] check for :  33
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[input : 1] end of for
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[input : 2] end of for

*/