Poor man's lenses in swift

Cloneable.swift

infix operator .=: AssignmentPrecedence

func .=<T, U>(_ kp: WritableKeyPath<T, U>, _ value: U) -> (inout T) -> () {
    { $0[keyPath: kp] = value }
}

protocol Cloneable { }

extension Cloneable {
    func clone(with modifiers: (inout Self) -> ()...) -> Self {
        return modifiers.reduce(into: self) { $1(&$0) }
    }
}

struct Foo: Cloneable {
    var bar: Bar
    var baz: Baz
}

struct Bar {
    var baz: Baz
}

struct Baz {
    var value: String
}

let foo = Foo(bar: Bar(baz: Baz(value: "1")), baz: Baz(value: "2"))

print(foo)

let foo2 = foo.clone(with:
    \.bar.baz.value.="3",
    \.baz.value.="4"
)

print(foo2)