A function takes an A and returns a B for all As. But what if we are not able to handle all As? What if we want to build up to a proper function but in smaller building blocks? This is where PartialFunction comes in, lets take a look.
val even: PartialFunction[Int, String] = { case e: Int if e % 2 == 0 => "even" }
Here we define a PartialFunction even that only works for ints that are even. How would you use this?
even(2)
// even
even(1)
// scala.MatchError: 1 (of class java.lang.Integer)
Thats not nice! even(1) throws an exception, thats so not scala style! This is because even isn't a proper function, it only works with evens. So lets check to see if our input is proper for this partial function.
even.isDefinedAt(2)
// true
even.isDefinedAt(1)
// false
When building apis that are given PartialFunctions, its your responsibility to make sure the given input is proper for the function.
def doWork[A, B](f: PartialFunction[A, B])(work: A): Option[B] =
if(f.isDefinedAt(work)) Option(f(work)) else None
doWork(even)(1)
// None
doWork(even)(2)
// Some(even)
It was implied earlier that PartialFunctions are composable, lets show that.
val odd: PartialFunction[Int, String] = { case e: Int if e % 2 == 1 => "odd" }
val evenOrOdd = even orElse odd
if you have two PartialFunctions A => B and B => C, you can create a new one from A => C.
val length: PartialFunction[String, Int] = { case e: String => e.length }
val evensLength = even andThen length
evensLength(2)
// 4
Where is this really used? PartialFunctions are very common in collections apis and in dealing with events.
Collections example:
// think conditional map
List(1, 2, 3, 4) collect even
// List(even, even)
Events example:
// HTTP api that only works with GET requests at /hello. Other cases will get a 404
val service: PartialFunction[Request, Response] = {
case Get -> Root / "hello" => Ok("Hello World!")
}