Functional setters using Swift key paths

lib.swift

// ----------------------------------------------------
// These 3 helpers is all you need to unlock functional 
// setters for every field of every immutable struct.
// ----------------------------------------------------

// A generic way to a lift writable key path and a value to a setter function.
func set<A, B>(_ kp: WritableKeyPath<A, B>, _ b: B) -> (A) -> A {
  return { a in
    var a = a
    a[keyPath: kp] = b
    return a
  }
}

// A generic way to lift writable key paths and an update function to a setter function.
func over<A, B>(_ kp: WritableKeyPath<A, B>, _ update: @escaping (B) -> B) -> (A) -> A {
  return { a in
    var a = a
    a[keyPath: kp] = update(a[keyPath: kp])
    return a
  }
}

// A generic way to apply many updates to a value.
func update<A>(_ a: A, _ update1: @escaping (A) -> A, _ updates: ((A) -> A)...) -> A {
  var a = update1(a)
  updates.forEach { a = $0(a) }
  return a
}

usage.swift

// ----------------------------------------------------
// And here is how you can use them.
// ----------------------------------------------------

// Some types to play with.
struct Food { private(set) var name: String }
struct Location { private(set) var name: String }
struct User {
  private(set) var favoriteFood: Food
  private(set) var location: Location
  private(set) var name: String
}

// A value to play with.
let user = User(
  favoriteFood: Food(name: "Tacos"),
  location: Location(name: "Brooklyn"),
  name: "Blob"
)

// Create a new user with their location set to Los Angeles.
update(
  user,
  set(\.location.name, "Los Angeles")
)

// Create a new user by setting a bunch of properties at once.
update(
  user,
  set(\.location.name, "Los Angeles"),
  set(\.name, "Blobby"),
  over(\.favoriteFood.name) { "\($0) & Nachos" }
)