Disk+PropertyWrapper.swift

import Disk

@propertyWrapper
struct File<Value: Codable> {
    let path: String
    let directory: Disk.Directory
    let defaultValue: Value

    init(_ path: String, directory: Disk.Directory, defaultValue: Value) {
        self.path = path
        self.directory = directory
        self.defaultValue = defaultValue
    }

    var wrappedValue: Value {
        get { return (try? Disk.retrieve(path, from: directory, as: Value.self)) ?? defaultValue }
        set { try? Disk.save(newValue, to: directory, as: path) }
    }
}