Thursday, 10 November 2011

Implicit Conversions in Scala

Following on from the previous post on operator overloading I'm going to be looking at Implicit Conversions, and how we can combine them to with operator overloading to do some really neat things, including one way of creating a multi-parameter conversion.

So what's an "Implicit Conversion" when it's at home?

So lets start with some basic Scala syntax, if you've spent any time with Scala you've probably noticed it allows you to do things like:

   (1 to 4).foreach(println) // print out 1 2 3 4 

Ever wondered how it does this? Lets make things more explicit, you could rewrite the above code as:

    val a : Int = 1
    val b : Int = 4
    val myRange : Range = a to b
    myRange.foreach(println)

Scala is creating a Range object directly from two Ints, and a method called to.

So what's going on here? Is this just a sprinkling of syntactic sugar to make writing loops easier? Is to just a keyword in like def or val?

The answers to all this is no, there's nothing special going on here. to is simply a method defined in the RichInt class, which takes a parameter and returns a Range object (specifically a subclass of Range called Inclusive). You could rewrite it as the following if you really wanted to:

   val myRange : Range = a.to(b)

Hang on though, RichInt may have a "to" method but Int certainly doesn't, in your example you're even explicitly casting your numbers to Ints

Which brings me nicely on to the subject of this post, Implicit Conversions. This is how Scala does this. Implicit Conversions are a set of methods that Scala tries to apply when it encounters an object of the wrong type being used. In the case of the to example there's a method defined and included by default that will convert Ints into RichInts.

So when Scala sees 1 to 4 it first runs the implicit conversion on the 1 converting it from an Int primitive into a RichInt. It can then call the to method on the new RichInt object, passing in the second Int (4) as the parameter.

Hmm, think I understand, how's about another example?

Certainly. Lets try to improve our Complex number class we created in the previous post.

Using operator overloading we were able to support adding two complex numbers together using the + operator. eg.

class Complex(val real : Double, val imag : Double) {
  
  def +(that: Complex) = 
            new Complex(this.real + that.real, this.imag + that.imag)
  
  def -(that: Complex) = 
            new Complex(this.real - that.real, this.imag - that.imag)

  override def toString = real + " + " + imag + "i"
  
}

object Complex {
  def main(args : Array[String]) : Unit = {
       var a = new Complex(4.0,5.0)
       var b = new Complex(2.0,3.0)
       println(a)  // 4.0 + 5.0i
       println(a + b)  // 6.0 + 8.0i
       println(a - b)  // 2.0 + 2.0i
  }
}

But what if we want to support adding a normal number to a complex number, how would we do that? We could certainly overload our "+" method to take a Double argument, ie something like...

    def +(n: Double) = new Complex(this.real + n, this.imag)

Which would allow us to do...

    val sum = myComplexNumber + 8.5

...but it'll break if we try...

    val sum = 8.5 + myComplexNumber

To get around this we could use an Implicit Conversion. Here's how we create one.

  object ComplexImplicits {
    implicit def Double2Complex(value : Double) = 
                                     new Complex(value,0.0) 
 }

Simple! Although you do need to be careful to import the ComplexImplicits methods before they can be used. You need to make sure you add the following to the top of your file (even if your Implicits object is in the same file)...

   import ComplexImplicits._

And that's the problem solved, you can now write val sum = 8.5 + myComplexNumber and it'll do what you expect!

Nice. Is there anything else I can do with them?

One other thing I've found them good for is creating easy ways of instantiating objects. Wouldn't it be nice if there were a simpler way of creating one of our complex numbers other than with new Complex(3.0,5.0). Sure you could get rid of the new by making it a case class, or implementing an apply method. But we can do better, how's about just (3.0,5.0)

Awesome, but I'd need some sort of multi parameter implicit conversion, and I don't really see how that's possible!?

The thing is, ordinarily (3.0,5.0) would create a Tuple. So we can just use that tuple as the parameter for our implicit conversion and convert it into a Complex. how we might go about doing this...

  implicit def Tuple2Complex(value : Tuple2[Double,Double]) = 
                               new Complex(value._1,value._2);

And there we have it, a simple way to instantiate our Complex objects, for reference here's what the entire Complex code looks like now.

import ComplexImplicits._

object ComplexImplicits {
  implicit def Double2Complex(value : Double) = new Complex(value,0.0) 

  implicit def Tuple2Complex(value : Tuple2[Double,Double]) = new Complex(value._1,value._2);

}

class Complex(val real : Double, val imag : Double) {
  
  def +(that: Complex) : Complex = (this.real + that.real, this.imag + that.imag)
  
  def -(that: Complex) : Complex = (this.real - that.real, this.imag + that.imag)
      
  def unary_~ = Math.sqrt(real * real + imag * imag)
         
  override def toString = real + " + " + imag + "i"
  
}

object Complex {
  
  val i = new Complex(0,1);
  
  def main(args : Array[String]) : Unit = {
       var a : Complex = (4.0,5.0)
       var b : Complex = (2.0,3.0)
       println(a)  // 4.0 + 5.0i
       println(a + b)  // 6.0 + 8.0i
       println(a - b)  // 2.0 + 8.0i
       println(~b)  // 3.60555
      
       var c = 4 + b
       println(c)  // 6.0 + 3.0i
       var d = (1.0,1.0) + c  
       println(d)  // 7.0 + 4.0i
       
  }

}

6 comments:

Ajith Joseph said...

Very nice example, clearly explained. Thanks

winnetou said...

Very helpful, thanks!

Jack Chi said...

First - great demo, I was able to follow it and learn implicit conversions.

Could you also do a section on implicit parameters?

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Tzach Zohar said...

Thanks, perfectly written and very helpful.

Laurent Blain said...

Excellent, explained very simply, thanks!