How to extract the matching object with 1 operation in Scala?

Question

I have the following piece of code:

foo collectFirst {
  case x if check(expensive(x)) => do_something(expensive(x))
  case x if check2(expensive2(x)) => do_something_else(expensive2(x))
}  

To prevent expensive() being called twice, I can do this:

object expensive_helper {
  def unapply[A, B](x: A): Option[B] =
    ???
}

But, this is pretty verbose. Is there a better way?

Answer 1

I have this helper in my common utils:

case class matches[A, B](f: A => B)(check: B => Boolean) {
  def unapply(a: A): Option[B] = Option(f(a)).filter(check)
}

Here's how to use it:

val x      = "abcd"
val base64Url = matches((s: String) => Try(Base64.urlDecode(s))
(_.isSuccess)
val base64 = matches((s: String) => Try(Base64.decode(s))
(_.isSuccess)

x match {
  case base64Url(y) => println("Decoded URL =" + y)
  case base64(y)    => println("Decoded     =" + y)
  case _            => println("Could not decode " + x)
}