How to pattern match large Scala case classes?

42
votes

Consider the following Scala case class:

case class WideLoad(a: String, b: Int, c: Float, d: ActorRef, e: Date)

Pattern matching allows me to extract one field and discard others, like so:

someVal match {
    case WideLoad(_, _, _, d, _) => d ! SomeMessage(...)
}

What I would like to do, and what's more relevant when a case class has ~20 odd fields, is to extract only a few values in a way that does not involve typing out WideLoad(_, _, _, _, _, some, _, _, _, thing, _, _, interesting).

I was hoping that named args could help here, although the following syntax doesn't work:

someVal match {
    case WideLoad(d = dActor) => dActor ! SomeMessage(...)
    //              ^---------- does not compile
}

Is there any hope here, or am I stuck typing out many, many _, _, _, _?

EDIT: I understand that I can do case wl @ WideLoad(...whatever...) => wl.d, yet I'm still wondering whether there's even terser syntax that does what I need without having to introduce an extra val.

4
scalapattern-matching
Even with 4 or 5 fields, all the underscores make it pretty hard to read. A named parameter syntax would improve readability a lot, but as far as I know nothing like this exists yet. – Aaron Novstrup
I was under the impression that long parameter lists are something to avoid in general. – user382157
you meant case WideLoad (d == dActor) – user unknown
There's a SIP about this very issue: issues.scala-lang.org/browse/SI-6524 – James Moore
@JamesMoore Regarding the issue, I wonder whether inspiration can be taken from import statement syntax. We have that import org.apache.{foo,bar=>bat} imports foo and bar, renaming the latter to bat. In the same way we could do case WideLoad{d=>actorRef} if actorRef == 42 => SomeMessage(...) – Max Murphy

4 Answers

37
votes

I don't know if this is appropriate, but you can also build an object just to match that field, or that set of fields (untested code):

object WideLoadActorRef {
  def unapply(wl: WideLoad): Option[ActorRef] = { Some(wl.d) }
}

someVal match {
  case WideLoadActorRef(d) => d ! someMessage
}

or even

object WideLoadBnD {
  def unapplySeq(wl: WideLoad): Option[(Int,ActorRef)] = { Some((wl.b,wl.d)) }
}

someVal match {
  case WideLoadBnD(b, d) => d ! SomeMessage(b)
}
15
votes

You can always fall back to guards. It's not really nice, but better than normal pattern matching for you monster case classes :-P

case class Foo(a:Int, b:Int, c:String, d:java.util.Date)

def f(foo:Foo) = foo match {
  case fo:Foo if fo.c == "X" => println("found")
  case _ => println("arrgh!")
}

f(Foo(1,2,"C",new java.util.Date())) //--> arrgh!
f(Foo(1,2,"X",new java.util.Date())) //--> found

That said I think you should rethink your design. Probably you can logically group some parameters together using case classes, tuples, lists, sets or maps. Scala does support nested pattern matching:

case class Bar(a: Int, b:String)
case class Baz(c:java.util.Date, d:String)
case class Foo(bar:Bar, baz:Baz)

def f(foo:Foo) = foo match {
   case Foo(Bar(1,_),Baz(_,"X")) => println("found")
   case _ => println("arrgh!")
}

f(Foo(Bar(1,"c"),Baz(new java.util.Date, "X"))) //--> found
f(Foo(Bar(1,"c"),Baz(new java.util.Date, "Y"))) //--> arrgh!
0
votes

You can just specify the type in the matched pattern:

case class WideLoad(a: String, b: Int, c: Float, d: ActorRef, e: Date)

val someVal = WideLoad(...)

someVal match {
    case w: WideLoad => w.d ! SomeMessage(...)
}
0
votes

You can create a new case class which is a summary of your larger case class

case class WideLoad(a: String, b: Int, c: Float, d: ActorRef, e: Date)
case class WideLoadSummary(d: ActorRef, e: Date)

And then pattern match as normal.

val someVal = WideLoadSummary(wideload.d, wideload.e)

someVal match {
    case WideLoadSummary(d, _) => d ! SomeMessage(...)
}