Thursday, 10 November 2011

"Operator Overloading" in Scala

So, I've been teaching myself Scala recently, and it's a very interesting language.

One of the nice things I like about it, is it's support for creating DSLs, domain specific languages. A domain specific language - or at least my understanding of it - is a language that is written specifically for one problem domain. One example would be SQL, great for querying relational databases, useless for creating first person shooters.

Of course Scala itself is not a DSL, it's a general purpose language. However it does offer several features that allow you to simulate a DSL, in particular operator overloading, and implicit conversions. In this post I'm going to focus on the first of these...

Operator Overloading

So what's operator overloading?

Well operators are typically things such as +, -, and !. You know those things you use to do arithmetic on numbers, or occasionally for manipulating Strings. Well, operator overloading - just like method overloading - allows you to redefine their behaviour for a particular type, and give them meaning for your own custom classes.

Hang on a minute! I'm sure someone once told me operator overloading was evil?

Indeed, this is quite a controversial topic. It's considered far too open for abuse by some, and was so maligned in C++ that the creators of Java deliberately disallowed it (excepting "+" for String concatenation).

I'm of a slightly different opinion, used responsibly it can be very useful. For example lots of different objects support a concept of addition, so why not just use an addition operator?

Lets say you were developing a complex number class, and you want to support addition. Wouldn't it be nicer to write...

Complex result = complex1 + complex2;

...rather than...

Complex result = complex1.add(complex2);

The first example is much more natural don't you think?

So Scala allows you to overload operators then?

Well, not really. In fact, technically not at all.

So all this is just a tease? This is the most stupid blog post I've ever read. Scala's rubbish. I'm going back to Algol 68.

Wait a second, I've not finished. You see Scala doesn't support operator overloading, because it doesn't have operators!

Scala doesn't have operators? You've gone mad, I write stuff like "sum = 2 + 3" all the time, and what about all those funny list operations? "::", and ":/". They look like operators to me!

Well they're not. The thing is, Scala has a rather relaxed attitude to what you can name a method.

When you write...

sum = 2 + 3,'re actually calling a method called + on a RichInt type with a value of 2. You could even rewrite it as...

sum = 2.+(3)

...if you really really wanted to.

Aha, I got it. So how do you go about overloading an operator then?

Simple, it's exactly the same as writing a normal method. Here's an example.

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
Ok that's nice, what if I wanted a "not" operator though, ie something like a "!"

That's a unary prefix operator, and yes scala can support these, although in a more limited fashion than an infix operator like "+"

Only four operators can be supported in this fashion, +, -, !, and ~. You simply need to call your methods unary_! or unary_~, etc. Here's how you might add a "~" to calculate the magnitude of a Complex number to our complex number class

class Complex(val real : Double, val imag : Double) {
    // ...
    def unary_~ = Math.sqrt(real * real + imag * imag)

object Complex {
  def main(args : Array[String]) : Unit = {
     var b = new Complex(2.0,3.0)
     prinln(~b) //  3.60555

So that's all pretty simple, but please use responsibly. Don't create methods called "+" unless your class really does something that could be interpreted as addition. And never ever redefine the binary shift left operator "<<" as some sort of substitute for println. It's not clever and you'll make the Scala gods angry.

Hope you found that useful. Next up I'll cover implicit conversions. Another nice feature of Scala that really allows you to write your code in a more natural way


Jan Kammerath said...

Wow! That's quite tough stuff. Only worked with this in the past:

Anonymous said...

Operator overloading in C++

ADmin said...

Case in point there are chronicled heroes who are to a great degree -they're gay -however they're greatly manly.

Arsalan Yousuf said...

Thanks for sharing your good thoughts and idea here.are you struggling with your essays Just continue on sharing your talent and creation to us readers. There are valuable information. Thanks for sharing.

Amelie Bowen said...

academic paper writing

Essay writing services said...

I had been from suffering this problem for a long time but at last this code cleared all my problems..Thanks a lot for sharing this code and making it so simple..

Essay writing service

Aaliya Smith said...

Ranthambore, as one of the best national parks in the world to see Bengal tigers, wild, in their natural home. Here can see Bengal tigers as well as Sundari, Machali, Star male, Jumru and so on. Each of Ranthambore denizens is well recognized and The Ranthambhore Bagh, with its trained team, is perhaps your best bet to know who is who.
Ranthambore Tiger Reserve
Ranthambore Safari Booking

Stevens Stone said...

That’s really very useful. Alright, thanks for all this information! I learned a couple of things.
order assignment

aliya seen said...

If you are going to write cover letter for a job application so keep in mind judt one thing that your written statement will be direct approach if it is written well.

Curtis Albert said...

I also do like Scala as it provides a better and more natural support for functional and monadic constructs. Law School Admission Essay Proofreaders. I see we have a lot in common in preferences. Thanks for sharing. Purchase Essays Writer Service

Miriam Steve said...

Its quite a nice blog, it has very interesting and engaging content that would be great to read again. Such a post is well written, SMO professionals is another page to refer to for reliable information.

Annitah Agasa said...
This comment has been removed by the author.
Albert Smith said...

Tom has a good programming blog and I have learned a lot about operator overloading in scala. I have found scala programming language to be more interesting than I imagined. I have been using c++, c, Java, and, R programming languages. I am developing a computer program that will Help Write a Physics Lab Report and from what you have explained in this blog, scala will the perfect programming language to use.

Neil Jakson said...

Very nice posting ,i hope every one who has wish to read this blog will like this posting and will share it with his friends and families.i am happy to know that you are as hard working as Dissertation Writers UK
Coursework writing services

Nandhini said...

Thanks for splitting your comprehension with us. It’s really useful to me & I hope it helps the people who in need of this vital information. Python Training in Chennai

Keiron Tod said...