How can I do a setter in functional programming like so? I would like to respect immutability and other functional programming principles.
private int age;
public void setAge(int age){
this.age=age;
}
Is that possible? If not, how can I represent the state (and its changes) of a program in a functional programming language?
In general you should create a new state instead of modifying the old one.
You could use copy method like this:
case class Person(name: String, age: Int)
val youngBob = Person("Bob", 15)
val bob = youngBob.copy(age = youngBob.age + 1)
// Person(Bob,16)
You could also use Lens:
import scalaz._, Scalaz._
val ageLens = Lens.lensu[Person, Int]( (p, a) => p.copy(age = a), _.age )
val bob = ageLens.mod(_ + 1, youngBob)
// Person(Bob,16)
See Learning scalaz/Lens. There are also other implementations of Lens.
For instance you could use shapeless Lens that implemented using macros, so you don't have to create lens manually:
import shapeless._
val ageLens = Lens[Person] >> 1
val bob = ageLens.modify(youngBob)(_ + 1)
// Person(Bob,16)
See examples on github.
There are a lot of Lens implementations. See Boilerplate-free Functional Lenses for Scala and Macrocosm.
While it's practically an oxymoron to talk about an FP mutator, Scala (as a hybrid object-functional language) does have a syntax for creating mutators and mutator-like methods:
scala> class C1 { def x: Int = xx; def x_=(i: Int): Unit = xx = i; private var xx: Int = 0 }
defined class C1
scala> val c1 = new C1
c1: C1 = C1@1e818d28
scala> c1.x = 23
c1.x: Int = 23
scala> c1.x
res18: Int = 23
Naturally, as shown there's no point in this but whatever backs the named accessor and mutator need not be a simple field and the mutator may impose range constraints on the value or do some logging when the value is mutated, so it's a fairly general construct.
Note that you cannot have the mutator alone. It has to have an accessor counterpart.
Addendum
Immutable w/ mutator w/o case class
scala> class C2(xInit: Int) { def this = this(0)
def x: Int = xx
def x_=(i: Int): C2 = new C2(i)
private var xx: Int = xInit
}
scala> val c2a = new C2()
c2a: C2 = C2@2f85040b
scala> val c2b = c2a.x = 23
c2b: C2 = C2@74f1bfa9
scala> c2a.x
res19: Int = 0
scala> c2b.x
res20: Int = 23
