typeHelper.swift

protocol StateType {}

func withTypes<SpecificState>(_ state genericState: StateType?, function: (_ state: SpecificState?) -> SpecificState) -> StateType {
    guard let genericState = genericState else {
        return function(nil) as! StateType
    }

    guard let specificState = genericState as? SpecificState else {
        return genericState
    }

    return function(specificState) as! StateType
}