def sideEffecting(): Map[String, List[String]] = { println("bazinga"); Map("a" -> List("a", "b")) }

//doesn't print anything here
val x1: String => List[String] = sideEffecting().getOrElse(_, Nil)

//but it'll print bazinga any time you call one of these:
x1("a") //here
x1("a") //also here
x1("b") //here too

//prints bazinga exactly once, here
val x2: String => List[String] = { val m = sideEffecting(); m.getOrElse(_, Nil) }

x2("a") //doesn't run side effect
x2("a") //neither does this