code reducing by using traits and the with-keyword

Question

I have some classes with the same super-type. Therefore all of this classes have to override the same methods. Now I can call a method and commit it an object of the common super-type. But it is not always useful to react to each committed type therefore an exception is thrown. First i tried to solve this behaviour like this:

def operation(s: SuperType) = s match {
  case t: SubType1 => ...
  case t: SubType2 => ...
  case _ => ...
}

Due to a lot of sub-types this will result to a lot of code (in each method and in each class) and I tried to solve this problem with traits. Each trait should test only one type and then forward the object to the higher method on the stack. The code below describes how I imagine that. But this don't work because the compiler can't dissolve the types. An other problem is that I have to declare each attribute of the classes in each behaviour class.

object TraitWithTest {
  def main(args: Array[String]) {
    val e1 = Even(2, 4)
    val e2 = Even(1, 3)
    val o1 = Odd(1.25, 3.75)
    val o2 = Odd(7.25, 9.25)
    val a1 = All(5.5)
    val a2 = All(3.5)

    println("e1 + e2: " + (e1 + e2))
    println("o1 + o2: " + (o1 + o2))
    try { println("e1 + o2: " + (e1 + o2)) } catch { case e => println(e) }
    println("o1 + e2: " + (o1 + e2))
    println("a1 + e1: " + (a1 + e2))
  }
}

abstract class Num {
  def +(n: Num): Num
}

trait OddBehaviour extends Num {
  val e1, e2: Int // here I don't want to declare all attributes
  val a1: Double
  abstract override def +(n: Num) = n match {
    case o: Odd => throw new UnsupportedOperationException("Even#+(Odd)")
    case _ => super.+(n)
  }
}

trait EvenBehaviour extends Num {
  val o1, o2: Double
  val a1: Double
  abstract override def +(n: Num) = n match {
    case e: Even => Odd(o1 + e.e1, o2 + e.e2)
    case _ => super.+(n)
  }
}

trait AllBehaviour extends Num {
  val o1, o2: Double
  val e1, e2: Int
  abstract override def +(n: Num) = n match {
    case a: All => Odd(o1 + a.a1, o2 + a.a1)
    case _ => super.+(n)
  }
}

object Even {
  def apply(e1: Int, e2: Int) = new Even(e1, e2) with OddBehaviour with AllBehaviour
}

abstract case class Even(e1: Int, e2: Int) extends Num {
  override def +(n: Num) = n match {
    case c: Even => Even(e1 + c.e1, e2 + c.e2)
    case _ => throw new IllegalArgumentException
  }
}

object Odd {
  def apply(o1: Double, o2: Double) = new Odd(o1, o2) with EvenBehaviour with AllBehaviour
}

abstract case class Odd(o1: Double, o2: Double) extends Num {
  override def +(n: Num) = n match {
    case o: Odd => Odd(o1 + o.o1, o2 + o.o2)
    case _ => throw new IllegalArgumentException
  }
}

object All {
  def apply(a1: Double) = new All(a1) with EvenBehaviour with OddBehaviour
}

abstract case class All(a1: Double) extends Num {
  override def +(n: Num) = n match {
    case a: All => All(a1 + a.a1)
    case _ => throw new IllegalArgumentException
  }
}

Can someone give say me whether it is possible to reduce lines of code by using traits? Or is the match-all-solution I currently using the best?

EDIT:

With your help I found a half-working-solution. My main-problem was that I tried to reduce the lines of code by using Scala-features. So I overlooked the easiest way: outsourcing the code! I only have to create a new object which checks the object-combinations. The objects themselves handle only their own types.

This is the code:

final object TraitWithTest {
  def main(args: Array[String]) {
    import traitwith.operations._
    val e1 = Even(2, 4)
    val e2 = Even(1, 3)
    val o1 = Odd(1.25, 3.75)
    val o2 = Odd(7.25, 9.25)
    val a1 = All(5.5)
    val a2 = All(3.5)

    val n1 = NumHolder(o1)
    val n2 = NumHolder(a1)

    println("e1 + e2: " + add(e1, e2))
    println("o1 + o2: " + add(o1, o2))
    try { println("e1 + o2: " + add(e1, o2)) } catch { case e => println(e) }
    println("o1 + e2: " + add(o1, e2))
    try { println("a1 + e2: " + add(a1, e2)) } catch { case e => println(e) }
    println("n1 + n2: " + add(n1, n2))
  }
}

final object operations {
  def add(a: Num, b: Num) = a -> b match {
    case (a1: Odd, b1: Odd) => a1 + b1
    case (a1: Odd, b1: Even) => Odd(a1.x + b1.x, a1.y + b1.y)
    case (a1: Odd, b1: All) => Odd(a1.x + b1.x, a1.y + b1.x)
    case (a1: Even, b1: Even) => a1 + b1
    case (a1: All, b1: All) => a1 + b1
    case _ => error("can't add " + b + " to " + a)
  }
}

abstract class Num {
  type A <: Num
  def +(a: A): A
}

final case class Odd(x: Double, y: Double) extends Num {
  override type A = Odd
  override def +(a: Odd) = Odd(x + a.x, y + a.y)
}

final case class Even(x: Int, y: Int) extends Num {
  override type A = Even
  override def +(a: Even) = Even(x + a.x, y + a.y)
}

final case class All(x: Double) extends Num {
  override type A = All
  override def +(a: All) = All(x + a.x)
}

final case class NumHolder(x: Num) extends Num {
  override type A = NumHolder
  override def +(a: NumHolder) = NumHolder(x + a.x)
}

I extended the code a bit and inserted the object NumHolder. Now, there is only one little flaw: In NumHolder I can't commit the super-type without getting a compile error in the add-method. I tried to use Generics instead of the type-keyword but that is unhandy because I have always to set a type to Num (also in the object operations).

How can I solve this little compile error?

Answer 1

based on Kevin Wright's answer I solved the problem now:

package de.traitwith

import de.traitwith.Operations._
import de.traitwith._

object TraitWithTest {
  def main(args: Array[String]) {
    val e1 = Even(2, 4)
    val e2 = Even(1, 3)
    val o1 = Odd(1.25, 3.75)
    val o2 = Odd(7.25, 9.25)
    val a1 = All(5.5)
    val a2 = All(3.5)
    val n1 = NumHolder(o1)
    val n2 = NumHolder(a1)
    println("e1 + e2: " + add(e1, e2))
    println("o1 + o2: " + add(o1, o2))
    try { println("e1 + o2: " + add(e1, o2)) } catch { case e => println(e) }
    println("o1 + e2: " + add(o1, e2))
    try { println("a1 + e2: " + add(a1, e2)) } catch { case e => println(e) }
    println("n1 + n2: " + add(n1, n2))
    println("o1 + n2: " + add(o1, n2))
  }
}

object Operations {
  def add(a: Num, b: Num): Num = a -> b match {
    case (a1: Odd, b1: Odd) => a1 + b1
    case (a1: Odd, b1: Even) => Odd(a1.x + b1.x, a1.y + b1.y)
    case (a1: Odd, b1: All) => Odd(a1.x + b1.x, a1.y + b1.x)
    case (a1: Odd, b1: NumHolder) => add(a1, b1.x)
    case (a1: Even, b1: Even) => a1 + b1
    case (a1: Even, b1: NumHolder) => add(a1, b1.x)
    case (a1: All, b1: All) => a1 + b1
    case (a1: All, b1: NumHolder) => add(a1, b1.x)
    case (a1: NumHolder, b1: NumHolder) => a1 + b1
    case (a1: NumHolder, b1: Odd)=> add(a1.x, b1)
    case (a1: NumHolder, b1: Even) => add(a1.x, b1)
    case (a1: NumHolder, b1: All) => add(a1.x, b1)
    case _ => error("can't add " + b + " to " + a)
  }
}

abstract class Num

final case class Odd(x: Double, y: Double) extends Num {
  def +(a: Odd) = Odd(x + a.x, y + a.y)
}

final case class Even(x: Int, y: Int) extends Num {
  def +(a: Even) = Even(x + a.x, y + a.y)
}

final case class All(x: Double) extends Num {
  def +(a: All) = All(x + a.x)
}

final case class NumHolder(x: Num) extends Num {
  def +(a: NumHolder) = NumHolder(add(x, a.x))
}

I don't have any more some methods in the super-type - I hope, same day, this will not cause problems. It is possible to delete all the add-methods in the classes but in my real application I have larger classes so I need them there.

Answer 2

Your problem is that you are trying to use object oriented features, such as classes and inheritance, with a design that is not object oriented.

The whole point of OOP is that you do not introspect what a class is. Use, instead, polymorphism to achieve the results. I particularly like this paper in illustrating how OO is supposed to work, but there are no shortage of resources out there in this respect.

EDIT

For example, the code provided roughly translates into the following, minus the things that don't work (the code provided doesn't compile precisely because of them).

abstract class Num {
  def +(n: Num): Num
  def plus(n1: Int, n2: Int): Num
  def plus(n1: Double, n2: Double): Num
  def plus(n: Double): Num
}

case class Even(e1: Int, e2: Int) extends Num {
  override def +(n: Num) = n.plus(e1, e2)
  override def plus(n1: Int, n2: Int) = Even(e1 + n1, e2 + n2)
  override def plus(n1: Double, n2: Double) = Odd(n1 + e1, n2 + e2)
  // the code provided references o1 and o2, which are not defined anywhere for Even
  // so I'm providing an alternate version
  override def plus(n: Double) = Odd(n + e1, n + e2)
}

case class Odd(o1: Double, o2: Double) extends Num {
  override def +(n: Num) = n.plus(o1, o2)
  override def plus(n1: Int, n2: Int) = throw new UnsupportedOperationException("Even#+(Odd)")
  override def plus(n1: Double, n2: Double) = Odd(o1 + n1, o2 + n2)
  override def plus(n: Double) = throw new UnsupportedOperationException("Even#+(Odd)")
}

case class All(a1: Double) extends Num {
  override def +(n: Num) = n.plus(a1)
  // the code provided references o1 and o2, which are not defined anywhere for All
  // so I'm providing an alternate version
  override def plus(n1: Int, n2: Int) = Odd(a1 + n1, a1 + n2)
  override def plus(n1: Double, n2: Double) = Odd(n1 + a1, n2 + a1)
  override def plus(n: Double) = All(a1 + n)
}

It looks to me it can be further improved with the visitor pattern, which makes sense, given that it targets the same problems that type matching usually does.

Answer 3

To misquote a certain advert: There's a type class for that...

Scala already supports ad-hoc polymorphism over "numbers" via Numeric, which is probably what you really want: http://www.scala-lang.org/archives/downloads/distrib/files/nightly/docs/library/scala/math/Numeric.html

But if this Even/Odd/All scheme is what you're actually doing, and not just a contrived example, then you can always roll your own type class!

---

Let's call it Addable:

case class Even(x:Int, y:Int)
case class Odd(x:Double, y:Double)
case class All(x:Double)

abstract class Addable[A, B] {
  def add(a: A, b: B): A
}

implicit object EvenCanAddEven extends Addable[Even, Even] {
  def add(a:Even, b:Even) = Even(a.x+b.x, a.y+b.y)
}

implicit object OddCanAddOdd extends Addable[Odd, Odd] {
  def add(a:Odd, b:Odd) = Odd(a.x+b.x, a.y+b.y)
}

implicit object OddCanAddEven extends Addable[Odd, Even] {
  def add(a:Odd, b:Even) = Odd(a.x+b.x, a.y+b.y)
}

implicit object AllCanAddAll extends Addable[All, All] {
  def add(a:All, b:All) = All(a.x+b.x)
}

def add[A,B](a:A, b:B)(implicit tc: Addable[A,B]) =
  tc.add(a, b)


val e1 = Even(2, 4)
val e2 = Even(1, 3)
val o1 = Odd(1.25, 3.75)
val o2 = Odd(7.25, 9.25)
val a1 = All(5.5)
val a2 = All(3.5)

println("e1 + e2: " + add(e1, e2))
println("o1 + o2: " + add(o1, o2))
println("e1 + o2: " + add(e1, o2)) //compiler should fail this line
println("o1 + e2: " + add(o1, e2))
println("a1 + e1: " + add(a1, e2))

disclaimer: I haven't actually tested the code, this machine doesn't (yet) have Scala installed

Answer 4

An alternative solution, for when types aren't known until runtime:

sealed trait Num
case class Even(x:Int, y:Int) extends Num
case class Odd(x:Double, y:Double) extends Num
case class All(x:Double) extends Num

object operations {
  def add(a: Num, b: Num) : Num = (a,b) match {
    case (a1:Even, b1:Even) => Even(a1.x+b1.x, a1.y+b1.y)
    case (a1:Odd, b1:Odd) => Odd(a1.x+b1.x, a1.y+b1.y)
    case (a1:Odd, b1:Even) => Odd(a1.x+b1.x, a1.y+b1.y)
    case (a1:All, b1:All) => All(a1.x, b1.x)
    case _ => error("can't add " + a + " to " + b)
  }
}

The trick here is to first wrap both params into a tuple, so you then have a single object to pattern-match on.

UPDATE

Following your edit; you don't seem to need the abstract type A anywhere, why not just leave Num as a marker trait and define the + method separately in each subclass?

sealed abstract trait Num

case class Odd(x: Double, y: Double) extends Num {
  def +(a: Odd) = Odd(x + a.x, y + a.y)
}

final case class Even(x: Int, y: Int) extends Num {
  def +(a: Even) = Even(x + a.x, y + a.y)
}

final case class All(x: Double) extends Num {
  def +(a: All) = All(x + a.x)
}

final case class NumHolder(x: Num) extends Num {
  def +(a: NumHolder) = NumHolder(x + a.x)
}

Answer 5

I don't know if I can solve your problem, but while thinking about it I tried to at least get your example to compile and work, which is hopefully at least somewhat helpful.

I've added some noise in the form of toString methods to be able to see the results of instantiations and expressions.

abstract class Num(val a: Double, val b: Double) {
  def +(that: Num): Num
  override def toString = (<z>Num({a}, {b})</z> text)
}

The "last resort" class, All, should be a case class to make the matching smoother, but since it's going to be inherited as true case class won't work well. A companion object with apply and unapply methods solves that.

An All can handle similar terms, but doesn't try to handle Even and Odd terms since those terms are secretly Alls at the same time.

class All(override val a: Double, override val b: Double) extends Num(a, b) {
  def +(that: Num): Num = that match {
    case All(n) => All(this.a + n)
    case _      => error("I don't know this subtype")
  }
  override def toString = (<z>All({a})</z> text)
}
object All {
  def apply(num: Double) = new All(num, num)
  def unapply(num: All) = Some(num.a)
}

Now, the way Evens and Odds work could be distilled into traits, but it isn't necessary for this example. Not doing so simplifies inheritance, but possibly goes against the point of the example, I don't know.

An Even knows how to handle Even and Odd terms, but passes any others to its superclass. Again, it's a faux case class for matching purposes.

class Even(override val a: Double, override val b: Double) extends All(a, b) {
  override def +(that: Num): Num = that match {
    case Even(a, b) => Even(this.a + a, this.b + b)
    case Odd(a, b)  => Odd(this.a + a, this.b + b)
    case x => super.+(x)
  }
  override def toString = (<z>Even({a}, {b})</z> text)
}
object Even {
  def apply(a: Double, b: Double) = new Even(a, b)
  def unapply(num: Even) = Some((num.a, num.b))
}

An Odd knows how to handle Even terms, but refuses to deal with Odd ones (I changed this from your example to make a weak pun, you're welcome).

class Odd(override val a: Double, override val b: Double) extends All(a, b) {
  override def +(that: Num): Num = that match {
    case Even(a, b) => Odd(this.a + a, this.b + b)
    case Odd(a, b)  => error("Adding two Odds is an odd thing to do")
    case x => super.+(x)
  }
  override def toString = (<z>Odd({a}, {b})</z> text)
}
object Odd {
  def apply(a: Double, b: Double) = new Odd(a, b)
  def unapply(num: Odd) = Some((num.a, num.b))
}

OK, let's take it for a spin.

object Try {
  def main(args: Array[String]) {
    val e1 = Even(2, 4)
    val e2 = Even(1, 3)
    val o1 = Odd(1.25, 3.75)
    val o2 = Odd(7.25, 9.25)
    val a1 = All(5.5)
    val a2 = All(3.5)

    println("e1 + e2: " + (e1 + e2))
    println("e1 + o2: " + (e1 + o2))
    try { println("o1 + o2: " + (o1 + o2)) } catch { case e => println(e) }
    println("o1 + e2: " + (o1 + e2))
    println("a1 + e1: " + (a1 + e2))
  }
}

Answer 6

It looks like generics might help you. Try something like this:

class Supertype[A <: Supertype] {
    def operation(s: A) {

    }
}

class Subtype extends SuperType[Subtype] {
    override def operation(s: Subtype) {

    }
}

Your problem description isn't very clear, so this is a guess...