Locked makes it possible to lock certain variables by a key

Abstract - Locked.md

Locked

var myLocked = Locked(value: Int(5), key: 5)
try myLocked.access(key: 5) { $0 = 10 }

Locked.swift

protocol Locked {
    associatedtype Value
    associatedtype Key : Equatable
    mutating func access<T>(key: Key, _ f: (inout Value) throws -> T) throws -> T
}

struct LockedStruct<Element> : Locked {

    typealias Key = Int
    typealias Value = Element
    
    private var value : Element
    private var key : Int
    mutating func access<T>(key: Key, _ f: (inout Value) throws -> T) throws -> T {
        guard key == self.key else { throw LockedError.wrongKey }
        return try f(&value)
    }
}
enum LockedError : Error {
    case wrongKey
}