Scala apply function on list elements to return a value

Question

I have found this answer in the forum earlier to a question I have been looking for, How to find a matching element in a list and map it in as an Scala API method?

// Returns Some(66)
List(1, 2, 3) collectFirst { case i if (i * 33 % 2 == 0) => i * 33 }

Now if I am replacing the if clause with a function

// 
List(1, 2, 3) collectFirst { case i if ( test(i) > 0) => test(i) }

this works but test() will be evaluated twice. Is there a better solution to apply a function to a list and return a result when a condition is met (not having to go through all elements and not havong to call the function twice (for evaluation and for returning the value.

Answer 1

Something like this, perhaps? Breaking it into two separate operations lets you save/reuse intermediate results.

List(1,2,3).iterator.map(test).find(_ > 0)

Answer 2

You can wrap your function in a custom extractor:

def test(i: Int): Int = i - 1

object Test {
  def unapply(i: Int): Option[Int] = Some(test(i))
}

scala> List(1, 10, 20) collectFirst { case Test(i) if i > 0 => i }
res0: Option[Int] = Some(9)

You can generalize this solution and make a class for that kind of extractors:

case class Extract[T, U](f: T => U) {
  def unapply(t: T): Option[U] = Some(f(t))
}

scala> val Test2 = Extract(test)
Test2: Extract[Int,Int] = Extract($$Lambda$1326/1843609566@69c33ea2)

scala> List(1, 10, 20) collectFirst { case Test2(i) if i > 0 => i }
res1: Option[Int] = Some(9)

You can also wrap the guard into the extractor as well:

case class ExtractWithGuard[T, U](f: T => U)(pred: U => Boolean) {
  def unapply(t: T): Option[U] = {
    val u = f(t)
    if (pred(u)) Some(u)
    else None
  }
}

scala> val Test3 = ExtractWithGuard(test)(_ > 0)
Test3: ExtractWithGuard[Int,Int] = ExtractWithGuard($$Lambda$1327/391731126@591a4d25)

scala> List(1, 10, 20) collectFirst { case Test3(i) => i }
res2: Option[Int] = Some(9)