Scala - mutable (var) method parameter reference

Question

EDIT: I keep getting upvotes here. Just for the record, I no longer think this is important. I haven\'t needed it since I posted it.

I would like to do following in

Answer 1

String objects are immutable in Scala (and Java). The alternatives I can think of are:

1. Return the result string as return value.
2. Instead of using a String parameter, use a StringBuffer or StringBuilder, which are not immutable.

In the second scenario you would have something like:

def save(srcPath: String, destPath: StringBuilder) {
    if (!destPath.toString().endsWith("/"))
       destPath.append("/")
    // do something
    //
}

EDIT

If I understand correctly, you want to use the argument as a local variable. You can't, because all method arguments are val's in Scala. The only thing to do is to copy it to a local variable first:

def save(srcPath: String, destPath: String) {
    var destP = destPath
    if (!destP.endsWith("/"))
       destP += "/"
    // do something
    //
}

Answer 2

Maybe you could get the type system to do the work for you, so you don't even need to worry about adding a slash each time:

class SlashString(s: String) {
  override val toString = if (s endsWith "/") s else s + "/"
}
implicit def toSlashString(s: String) = new SlashString(s)

Now you don't need any code at all to change the input String:

def save(srcPath: String, destPath: SlashString) {
  printf("saving from %s to %s", srcPath, destPath)
}

val src: String = "abc"
val dst: String = "xyz"

scala> save(src, dst)
saving from abc to xyz/

True, there's a bit of setup at the start, but this will be less-so with implicit classes in version 2.10, and it removes all clutter from the method, which was what you were worried about.

Answer 3

Here's a couple of suggestions:

1) Update your function a bit

def save(srcPath: String, destPath: String) {
  var dp = destPath
  if (!dp.endsWith('/'))
    dp+= '/'
  // do something, but with dp instead of destPath
}

2) Create a utility function to use before calling save

def savedPath(path: String) = 
  if(path.endsWith("/")) 
    path
  else
    path + "/"

//call your save method on some path
val myDestPath = ...
val srcPath = ...
save(srcPath, savedPath(myDestPath))

Answer 4

The JVM does not allow pass-by-reference of pointers to objects (which is how you'd do this in C++), so you can't do exactly what you want.

One option is to return the new value:

def save(srcPath: String, destPath: String): String = {
  val newPath = (if (!destPath.endsWith("/")) destPath+'/' else destPath)
  // do something
  newPath
}

Another is to create a wrapper:

case class Mut[A](var value: A) {}

def save(srcPath: String, destPath: Mut[String]) {
  if (!destPath.value.endsWith("/")) destPath.value += '/'
  // do something
}

which users will then have to use on the way in. (Of course, they'll be tempted to save("/here",Mut("/there")) which will throw away the alterations, but this is always the case with pass-by-reference function arguments.)

---

Edit: what you're proposing is one of the biggest sources of confusion among non-expert programmers. That is, when you modify the argument of a function, are you modifying a local copy (pass-by-value) or the original (pass-by-reference)? If you cannot even modify it it is pretty clear that anything you do is a local copy.

Just do it that way.

val destWithSlash = destPath + (if (!destPath.endsWith("/")) "/" else "")

It's worth the lack of confusion about what is actually going on.

Answer 5

You can't.

You'll have to declare an extra var (or use a more functional style :-)).

Simplistic example:

def save(srcPath: String, destPath: String) {
    val normalizedDestPath =
      if (destPath.endsWith('/')) destPath
      else destPath + '/'
    // do something with normalizedDestPath 
}

Answer 6

I know this is an old question, but if you just want to reuse the argument name perhaps:

def save(srcPath: String, destPath: String) {
  ((destPath: String) => {
    // do something
  })(if (!destPath.endsWith('/')) destPath + '/' else destPath)
}