Pattern matching on big/long case classes

Question

Is there a more readable and and more robust (to refactoring) way to match on case classes like this ?

Example

Case Class

Very long case class with many "fields".

case class Data(name: String, time: Long, ..., userId: Option[UUID] ..., orders: Int, ... ) //more fields fields more

Pattern match: Variant a

Works. But error prone when fields position changes. One ends up counting _.

res match {
  case data@Data(_,_,_,_,_,_,Some(id),_,_,_,6,_,_) => (id, data.orders)
  case _ => ... 

}

Pattern match: Variant b

Works also. Is stable to changing orders. Gets really cumbersome with more checks in the guard. Also reading the value has to be repeated.

res match {
  case data: Data if data.userId.isDefined && data.orders == 6 => (data.userId.get,data.orders)
  case _ => ...
}

Question rephrased

Is there a way to combine Variant A and B to get the benefit of both approaches ?

I know it's not necessarily helpful - but looks like your real problem is having such a complex case class to begin with. If possible, consider breaking it up into a hierarchy of smaller case classes contained as fields in a the `Data` case class. None of that is to say this isn't an interesting question :) — Tzach Zohar, Aug 25 '16 at 08:09
Similar question here: [How to pattern match large Scala case classes?](http://stackoverflow.com/questions/3474125/how-to-pattern-match-large-scala-case-classes) — Samar, Aug 25 '16 at 08:15

Alexey Romanov · Accepted Answer · 2016-08-25T08:16:26.150

14

You can use a custom extractor:

res match {
  case ExtractUserIdAndOrders(Some(id), 6) => ...
  case _ => ...
}

where

object ExtractUserIdAndOrders {
  def unapply(data: Data) = Some((data.userId, data.orders))
}

You can define it inside the method if you need it just once, or in a wider scope for multiple similar matches.

edited Aug 25 '16 at 08:16

answered Aug 25 '16 at 08:14

Alexey Romanov

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If I was about to click *post* when I saw this answer :( @Alexey Romanov – sebszyller Aug 25 '16 at 08:15
@sebszyller I know this feeling :( – Alexey Romanov Aug 25 '16 at 08:17

score 2 · Answer 2 · edited Sep 23 '17 at 16:49

As long as there is an unapply method in scope, which takes a parameter from the type you're matching against, you can define a custom extractor for this, e.g.

case class Big(foo: String, bar: String, baz: Option[String], x: Int, y: Int)

object SmallerBig {
  def unapply(x: Big): Option[(Option[String], Int)] = Some(x.baz, x.y)
}

val x = Big("a", "b", Some("c"), 1, 2)

x match {
  case Big(_, _, _, _, _) =>
  case SmallerBig(Some(a), b) =>
}

The return type of the unapply method must not be Option, but the type you're return must provide the methods get and isDefined.