Multiple return points in scala closure/anonymous function

As far as I understand it, there is no way in Scala to have multiple return points in an anonymous function, i.e.

someList.map((i) => {
    if (i%2 == 0) return i // the early return allows me to avoid the else clause
    doMoreStuffAndReturnSomething(i) // thing of this being a few more ifs and returns
})

raises an error: return outside method definition. (And if it weren’t to raise that, the code would not work as I’d like it to work.)

One workaround I could thing of would be the following

someList.map({
    def f(i: Int):Int = {
        if (i%2 == 0) return i
        doMoreStuffAndReturnSomething(i)
    }
    f
})

however, I’d like to know if there is another ‘accepted’ way of doing this. Maybe a possibility to go without a name for the inner function?

(A use case would be to emulate some valued continue construct inside the loop.)

Edit

Please believe me, that there is a need for avoiding the else statement, because, the doMoreStuff part might actually look like:

val j = someCalculation(i)
if (j == 0) return 8
val k = needForRecalculation(i)
if (k == j) return 9
finalRecalc(i)
...

which, when you only have an if–else structure available gets easily messed up.

Of course, in the simple example I gave in the beginning, it is easier to just use else. Sorry, I thought this was clear.

Edit Report

1 Answer

The example you've given is easily solved by an if statement. There are no performance or other penalties for doing this.

But you might have some other situation, which looks roughly like

if (test) {
  if (anotherTest) {
    val a = someComputation()
    if (testOf(a)) return otherComputation()
  }
  else if (yetAnotherTest) return whatever()
}
bigComputation()

There are a few ways to deal with this sort of situation if you want to avoid the tangle of if-statements and/or code duplication needed to convert this to a form without returns.

There are various sneaky things you can do with Option or Either to keep state flowing along (with orElse and fold) so that you do only the computations you need to.

You're really better off creating a def as you suggest. But just to compare, consider an Option-wrapping style:

i => {
  ( if ((i%2)==0) Some(i) 
    else None
  ).getOrElse(doStuffAndReturn(i))
}

On the large example above, this style would give

( if (test) {
    if (anotherTest) {
      val a = someComputation()
      if (testOf(a)) Some(otherComputation()) else None
    }
    else if (yetAnotherTest) Some(whatever())
    else None
}).getOrElse(bigComputation())

Personally, I don't think it's clearer (and it's certainly not faster), but it is possible.

answered 2010-05-24T18:28:37.563

Multiple return points in scala closure/anonymous function

1 Answer

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