Don't forget that .map works on a function. - MutableThings().addStuff will instantiate 1 MutableThings and return the addStuff function for that specific object - t => MutableThings().addStuff(t) returns a function that creates a MutableThings and calls the addStuff function on this new object.

gistfile1.txt

package main.scala

case class MutableThings(var stuff: List[String] = List()) {
  println("Creating new MutableThings")
  def addStuff(myStuff: String) = {
    this.stuff = List(myStuff)
    this
  }
}

case class ImmutableThings(stuff: List[String] = List()) {
  def addStuff(myStuff: String) = copy(stuff = List(myStuff))
}

object ImmutableThingsTester {
  def main(args: Array[String]) {
    def convert1(stuffToChange: String*) = stuffToChange.map(MutableThings().addStuff)
    def convert2(stuffToChange: String*) = stuffToChange.map(t => MutableThings().addStuff(t))
    def convert3(stuffToChange: String*) = stuffToChange.map(ImmutableThings().addStuff)
    def convert4(stuffToChange: String*) = stuffToChange.map(t => ImmutableThings().addStuff(t))

    println("Mutable case class with function:")
    println(convert1("1", "2", "3"))

    println("\nMutable case class with anonymous function:")
    println(convert2("1", "2", "3"))

    println("\nImmutable case class with function:")
    println(convert3("1", "2", "3"))

    println("\nImmutable case class with anonymous function:")
    println(convert4("1", "2", "3"))
  }
}