Why the following scala code is valid?

Question

My understanding is Unit = void, but why I can pass in multiple argument?

So can anyone explain why the following code is valid?

def foo(x: Unit) = println("foo")                 
foo("ss", 1)  

Answer 1

If you run your snippet with scala -print you'll roughly get the following output for the code:

/* Definition of foo */
private def foo(x: scala.runtime.BoxedUnit): Unit = {

/* Invocation of foo */
foo({
  new Tuple2("ss", scala.Int.box(1));
  scala.runtime.BoxedUnit.UNIT
});

As you can see, the arguments to foo are rewritten into a code block that creates a tuple but then returns UNIT.

I can't see a good reason for this behaviour and I'd rather get a compiler error thrown instead.

Answer 2

A related question which gives a decent answer to this is here:

Scala: Why can I convert Int to Unit?

From Section 6.26.1 of the Scala Language Specification v2.9, "Value Discarding":

If e has some value type and the expected type is Unit, e is converted to the expected type by embedding it in the term { e; () }.

So, in your case it seems ("ss", 1) is being converted to a tuple so that it can be treated as a single argument, then as that argument type is not Unit, it is converted to a block which computes that tuple value then returns unit to match with the required type of the parameter.