Scala vs Kotlin: Operator overloading

Last week, I started my comparison of Scala and Kotlin with the Pimp my library pattern. In the second part of this serie, I’d like to address operator overloading.

Overview

Before to dive into the nitty-gritty details, let’s try first to tell what it’s all about.

In every language where there are functions (or methods), a limited set of characters is allowed to define the name of said functions. Some languages are more lenient toward allowed characters: naming a function \O/ might be perfectly valid.

Some others are much more strict about it. It’s interesting to note that Java eschewed the ability to use symbols in function names besides $ - probably in response to previous abuses in older languages. It definitely stands on the less lenient part of the spectrum and the Java compiler won’t compile the previous \O/ function.

The name operator overloading is thus slightly misleading, even if widespread. IMHO, it’s semantically more correct to talk about operator characters in function names.

Scala

Scala stands on the far side of leniency spectrum, and allows characters such as + and £ to be used to name functions, alone or in combinations. Note I couldn’t find any official documentation regarding accepted characters (but some helpful discussion is available here).

This enables libraries to offer operator-like functions to be part of their API. One example is the foldLeft function belonging to the TraversableOnce type, which is also made available as the /: function.

This allows great flexibility, especially in defining DSLs. For example, mathematics: functions can be named π, ∑ or √. On the flip side, this flexibility might be subject to abuse, as \O/, ^_^ or even |-O are perfectly valid function names. Anyone for an emoticon-based API?

def ∑(i: Int*) = i.sum

val s = ∑(1, 2, 3, 5) // = 11

Kotlin

Kotlin stands on the middle of the leniency scale, as it’s possible to define only a limited set of operators.

Each such operator has a corresponding standard function signature. To define a specific operator on a type, the associated function should be implemented and prepended with the operator keyword. For example, the + operator is associated with the plus() method. The following shows how to define this operator for an arbitrary new type and how to use it:

class Complex(val i: Int, val j: Int) {
    operator fun plus(c: Complex) = Complex(this.i + c.i, this.j + c.j)
}

val c = Complex(1, 0) + Complex(0, 1) // = Complex(1, 1)

Conclusion

Scala’s flexibility allows for an almost unlimited set of operator-looking functions. This makes it suited to design DSL with a near one-to-one mapping between domains names and function names. But it also relies on implicitness: every operator has to be known to every member of the team, present and future.

Kotlin takes a much more secure path, as it allows to define only a limited set of operators. However, those operators are so ubiquitous that even beginning software developer know them and their meaning (and even more so experienced ones).