Is there a way to let arguments be var instead of val?

Question

When debugging command line argument handling in Java I'm used to doing

args = new String[] { "some", "new", "arguments" };

(especially useful if have a filename as argument which you frequently change, but don't want to go through some dialog windows in the IDE). This has the benefit that I can simply comment out the line when building a release.

So when I tried this in Scala I discovered that arguments are vals. (And I can't write var in front of the parameter).

• Q1: What's the rationale for this?

• Q2: So is there any obvious work-around except for doing

  val newArgs = if (...) args else Array("some", "new", "arguments")
  

  and stick to newArgs in the remaining main method?

Open

Answer 1

Q1: Mutating the input parameters is often seen as bad style and makes it harder to reason about code.

Q2: You could assign the args to a var before doing anything with it.

Crowell

Q1, good point. Q2, yes. That's better than the if-line, because then I can simply comment out the reassignment as in the case of Java. Thanks! – Open

Answer 2

Arrays are mutable, so if you insist:

Seq("some", "new", "arguments").copyToArray(args, 0, 3)

That does, of course, only work if there is enough space in the passed array.

Remember that you can use default parameters in Scala to solve your original probem in a much cleaner way.

Celestina

Could you give an example of such default parameter for the main method? – Open

Ah, sorry, got carried away there. Does not work since in case of no parameters, an empty array is passed. Maybe that might be worth a change. – Celestina

Answer 3

If you only want to modify the args inside of the function, then your approach in the description is enough.

However, if you need to treat it as a true "reference" type and keep the modifications valid outside the function, you can wrap the arguments in a case class, e.g.:

case class Ref[A](var value: A)

And use it like:

def modify(refInt: Ref[Int]) = refInt.value = 3

Then, when you use refInt.value outside the function, it would still be 3.

Rake