Question

In Scala, if I define a method called apply in a class or a top-level object, that method will be called whenever I append a pair a parentheses to an instance of that class, and put the appropriate arguments for apply() in between them. For example:

class Foo(x: Int) {
    def apply(y: Int) = {
        x*x + y*y
    }
}

val f = new Foo(3)
f(4)   // returns 25

So basically, object(args) is just syntactic sugar for object.apply(args).

How does Scala do this conversion?

Is there a globally defined implicit conversion going on here, similar to the implicit type conversions in the Predef object (but different in kind)? Or is it some deeper magic? I ask because it seems like Scala strongly favors consistent application of a smaller set of rules, rather than many rules with many exceptions. This initially seems like an exception to me.

Answer 1

I don’t think there’s anything deeper going on than what you have originally said: it’s just syntactic sugar whereby the compiler converts f(a) into f.apply(a) as a special syntax case.

This might seem like a specific rule, but only a few of these (for example, with update) allows for DSL-like constructs and libraries.