InstanceBuilder.swift

protocol Buildable {
    init(dict: [AnyKeyPath: Any]) throws
}

struct BuilderError : Error {
    let reason: String
}

struct SomeBuildable : Buildable {
    let field1: String
    let field2: Int?
    let field3: [SomeEnum]
    
    enum SomeEnum {
        case first
    }
    
    init(dict: [AnyKeyPath: Any]) throws {
        guard let field1 = dict[\SomeBuildable.field1] as? String else {
            throw BuilderError(reason: "Missing field1")
        }
        self.field1 = field1
        guard let field2 = dict[\SomeBuildable.field2] as? Int else {
            throw BuilderError(reason: "Missing field2")
        }
        self.field2 = field2
        guard let field3 = dict[\SomeBuildable.field3] as? [SomeEnum] else {
            throw BuilderError(reason: "Missing field3")
        }
        self.field3 = field3
    }
}


struct Builder<E : Buildable> {
    var dict: [AnyKeyPath: Any] = [:]
    
    mutating func setValue<T>(_ val: T, for keyPath: KeyPath<E, T>) {
        dict[keyPath] = val
    }
    
    func build() throws -> E {
        return try E.init(dict: dict)
    }
}

var builder = Builder<SomeBuildable>.init()
builder.setValue("Value", for: \SomeBuildable.field1)
builder.setValue(nil, for: \SomeBuildable.field2)
builder.setValue([.first], for: \SomeBuildable.field3)
let product = try? builder.build()