Returning internal state as a val to a promise

example.pony

// This works
actor Router
  let _env : Env
  var _notifiers : Map[String val, ChatSession tag]

  new create(env: Env) =>
    _env = env
    _notifiers = Map[String val, ChatSession tag].create(10)

  fun ref _user_list() : Array[String] val =>
    var result : Array[String] iso = recover Array[String] end
    for k in _notifiers.keys() do result.push(k) end
    recover val result end


// This fails:
  fun ref _user_list() : Array[String] val =>
    recover val Iter[String](_notifiers.keys()).collect(Array[String]) end
    
// Error:
// (..)/chat/chat.pony:169:30: can't access field of non-sendable object inside of a recover expression
//     recover val Iter[String](_notifiers.keys()).collect(Array[String]) end
//                              ^
//     Info:
//     (..)/chat/chat.pony:160:3: this would be possible if the field was sendable
//       var _notifiers : Map[String val, ChatSession tag]
//       ^


// This fails as well:
  fun ref _user_list() : Array[String] val =>
    var result : Array[String] ref = recover Array[String] end
    Iter[String](_notifiers.keys()).collect(result)
    recover val result end


///chat.pony:172:5: can't recover to this capability
//    recover val result end
//    ^
//   Info:
//    ...chat.pony:172:17: expression type is Array[String val] tag^
//  recover val result end