How to clone a case class instance and change just one field in Scala?

Question

Let's say I have a case class that represents personas, people on different social networks. Instances of that class are fully immutable, and are held in immutable collections, to be eventually modified by an Akka actor.

Now, I have a case class with many fields, and I receive a message that says I must update one of the fields, something like this:

case class Persona(serviceName  : String,
                   serviceId    : String,
                   sentMessages : Set[String])

// Somewhere deep in an actor
val newPersona = Persona(existingPersona.serviceName,
                         existingPersona.serviceId,
                         existingPersona.sentMessages + newMessage)

Notice I have to specify all fields, even though only one changes. Is there a way to clone existingPersona and replace only one field, without specifying all the fields that don't change? Can I write that as a trait and use it for all my case classes?

If Persona was a Map-like instance, it would be easy to do.

Answer 1

case classcomes with a copy method that is dedicated exactly to this usage:

val newPersona = existingPersona.copy(sentMessages = 
                   existingPersona.sentMessages + newMessage)

Answer 2

Since 2.8, Scala case classes have a copy method that takes advantage of named/default params to work its magic:

val newPersona =
  existingPersona.copy(sentMessages = existing.sentMessages + newMessage)

You can also create a method on Persona to simplify usage:

case class Persona(
  svcName  : String,
  svcId    : String,
  sentMsgs : Set[String]
) {
  def plusMsg(msg: String) = this.copy(sentMsgs = this.sentMsgs + msg)
}

then

val newPersona = existingPersona plusMsg newMsg

Answer 3

existingPersona.copy(sentMessages = existingPersona.sentMessages + newMessage)

Answer 4

Consider using lens in Shapeless library:

import shapeless.lens

case class Persona(serviceName  : String,
                   serviceId    : String,
                   sentMessages : Set[String])
// define the lens
val messageLens = lens[Persona] >> 'sentMessages 

val existingPersona = Persona("store", "apple", Set("iPhone"))

// When you need the new copy, by setting the value,
val newPersona1 = messageLens.set(existingPersona)(Set.empty)
// or by other operation based on current value.
val newPersona2 = messageLens.modify(existingPersona)(_ + "iPad")

// Results:
// newPersona1: Persona(store,apple,Set())
// newPersona2: Persona(store,apple,Set(iPhone, iPad))

Moreover, in case you have nested case classes, the getter and setter methods can be a bit tedious to compose. It will be a good chance to simplify by using lens library.

Please also refer to:

• Shapeless Github / Boilerplate-free lenses for arbitrary case classes
• Quicklens Github
• Lens in scala

Answer 5

I didn't want to include a big library to do complex lenses that let you set values deep in nested case classes. It turns out it is just a few lines of code in the scalaz library:

  /** http://stackoverflow.com/a/5597750/329496 */
  case class Lens[A, B](get: A => B, set: (A, B) => A) extends ((A) => B) with Immutable {
    def apply(whole: A): B = get(whole)

    def mod(a: A, f: B => B) = set(a, f(this (a)))

    def compose[C](that: Lens[C, A]) = Lens[C, B](
      c => this(that(c)),
      (c, b) => that.mod(c, set(_, b))
    )

    def andThen[C](that: Lens[B, C]) = that compose this
  }

You can then create lenses that set deeply nested values far easier than using the built in copy feature. Here is a link to a big set if complex lenses that that my library uses to set heavily nested values.