best scala idiom for find & return

Question

This is something I encounter frequently, but I don't know the elegant way of doing. I have a collection of Foo objects. Foo has a method bar() that may return null or a Bar object. I want to scan the collection, calling each object's bar() method and stop on the first one returning an actual reference and return that reference from the scan.

Obviously:

foos.find(_.bar != null).bar

does the trick, but calls #bar twice.

Answer 1

Working on the Stream[T] returned by Seq.projection is a nice trick

foos.projection map (_.bar) find (_.size > 0)

This will map the values needed to execute find.

In Scala 2.8 it's:

foos.view map (_.bar) find (_.size > 0)

Answer 2

You can do it with any Iterable by using iterator (which evaluates lazily--it's called elements in 2.7). Try this out:

case class Foo(i: Int) {
  def bar = {
    println("Calling bar from Foo("+i+")")
    (if ((i%4)==0) "bar says "+i else null)
  }
}
val foos = List(Foo(1),Foo(2),Foo(3),Foo(4),Foo(5),Foo(6))
foos.iterator.map(_.bar).find(_!=null)