Why Everyone Hates Operator Overloading

… no, don’t tell me you like Perl. Because you don’t. You never did. It does horrible things. It makes your code look like…

Designing Perl 6
Designing Perl 6 as found on the awesome Perl Humour page

Perl made heavy use of operator overloading and used operators for a variety of things. A similar tendency can be seen in C++ and Scala. See also people comparing the two. So what’s wrong with operator overloading?

People never agreed whether Scala got operator overloading right or wrong:

Usually, people then cite the usual suspects, such as complex numbers (getting things right):

class Complex(val real:Int, 
              val imaginary:Int) {
    def +(operand:Complex):Complex = {
        new Complex(real + operand.real, 
                    imaginary + operand.imaginary)
    }
 
    def *(operand:Complex):Complex = {
        new Complex(real * operand.real - 
                    imaginary * operand.imaginary,
            real * operand.imaginary + 
            imaginary * operand.real)
    }
}

The above will now allow for adding and multiplying complex numbers, and there’s absolutely nothing wrong with that:

val c1 = new Complex(1, 2)
val c2 = new Complex(2, -3)
val c3 = c1 + c2
 
val res = c1 + c2 * c3

But then, there are these weirdo punctuation things that make average programmers simply go mad:

 ->
 ||=
 ++=
 <=
 _._
 ::
 :+=

Don’t believe it? Check out this graph library!

To the above, we say:

Operator Overloading? Meh

How operator overloading should be

Operator overloading can be good, but mostly isn’t. In Java, we’re all missing better ways to interact with BigDecimal and similar types:

// How it is:
bigdecimal1.add(bigdecimal2.multiply(bigdecimal3));

// How it should be:
bigdecimal1 + bigdecimal2 * bigdecimal3

Of course, operator precedence would take place as expected. Unlike C++ or Scala, ideal operator overloading would simply map common operators to common method names. Nothing more. No one really wants API developers to come up with fancy ##-%>> operators.

While Ceylon, Groovy, and Xtend implemented this in a somewhat predictable and useful way, Kotlin is probably the language that has implemented the best standard operator overloading mechanism into their language. Their documentation states:

Binary operations

Expression Translated to
a + b a.plus(b)
a – b a.minus(b)
a * b a.times(b)
a / b a.div(b)
a % b a.mod(b)
a..b a.rangeTo(b)

That looks pretty straightforward. Now check this out:

“Array” access

Symbol Translated to
a[i] a.get(i)
a[i, j] a.get(i, j)
a[i_1, …, i_n] a.get(i_1, …, i_n)
a[i] = b a.set(i, b)
a[i, j] = b a.set(i, j, b)
a[i_1, …, i_n] = b a.set(i_1, …, i_n, b)

Now, I really don’t see a single argument against the above. This goes on, and unfortunately, Java 8 has missed this train, as method references cannot be assigned to variables and invoked like JavaScript functions (although, that’s not too late for Java 9+):

Method calls

Symbol Translated to
a(i) a.invoke(i)
a(i, j) a.invoke(i, j)
a(i_1, …, i_n) a.invoke(i_1, …, i_n)

Simply beautiful!

Conclusion

We’ve recently blogged about Ceylon’s awesome language features. But the above Kotlin features are definitely a killer and would remove any other sorts of desires to introduce operator overloading in Java for good.

Let’s hope future Java versions take inspiration from Kotlin, a language that got operator overloading right.