Question

Using Scala’s command line REPL:

def foo(x: Int): Unit = {} def foo(x: String): Unit = {println(foo(2))} 

gives

error: type mismatch; found: Int(2) required: String 

It seems that you can’t define overloaded recursive methods in the REPL. I thought this was a bug in the Scala REPL and filed it, but it was almost instantly closed with ‘wontfix: I don’t see any way this could be supported given the semantics of the interpreter, because these two methods must to be compiled together.’ He recommended putting the methods in an enclosing object.

Is there a JVM language implementation or Scala expert who could explain why? I can see it would be a problem if the methods called each other for instance, but in this case?

Or if this is too large a question and you think I need more prerequisite knowledge, does someone have any good links to books or sites about language implementations, especially on the JVM? (I know about John Rose’s blog, and the book Programming Language Pragmatics… but that’s about it. 🙂

Answer 1

The issue is due to the fact that the interpreter most often has to replace existing elements with a given name, rather than overload them. For example, I will often be running through experimenting with something, often creating a method called test:

def test(x: Int) = x + x 

A little later on, let’s say that I’m running a different experiment and I create another method named test, unrelated to the first:

def test(ls: List[Int]) = (0 /: ls) { _ + _ } 

This isn’t an entirely unrealistic scenario. In fact, it’s precisely how most people use the interpreter, often without even realizing it. If the interpreter arbitrarily decided to keep both versions of test in scope, that could lead to confusing semantic differences in using test. For example, we might make a call to test, accidentally passing an Int rather than List[Int] (not the most unlikely accident in the world):

test(1 :: Nil)  // => 1 test(2)         // => 4  (expecting 2) 

Over time, the root scope of the interpreter would get incredibly cluttered with various versions of methods, fields, etc. I tend to leave my interpreter open for days at a time, but if overloading like this were allowed, we would be forced to ‘flush’ the interpreter every so often as things got to be too confusing.

It’s not a limitation of the JVM or the Scala compiler, it’s a deliberate design decision. As mentioned in the bug, you can still overload if you’re within something other than the root scope. Enclosing your test methods within a class seems like the best solution to me.