scala syntax to match on multiple case class types without decomposing the case class

Question

I have a sealed trait with various case class implementations. I want to pattern match on multiple classes at once for the same match expression. I can't seem to do it without decomposing the case classes and "|" between them

currently looks like:

sealed trait MyTrait {
  val param1: String
  ...
  val param100: String
}

case class FirstCase(param1: String ...... param100: String) extends MyTrait
...
case class NthCase(param1: String ..... param100: String) extends MyTrait

another place in code:

def myFunction(something: MyTrait) = {
   ...
   val matchedThing = something match {
      // this doesn't work with "|" character
      case thing: FirstCase | SecondCase => thing.param1
      ...
      case thing: XthCase | JthCase => thing.param10
   }
} 

Answer 1

Let's go there step by step:

1. The | operator, in the context of pattern matching, allows you to define alternative patterns, in the following form:

   pattern1 | pattern2
   

2. If you want to define a pattern that matches a type, the pattern must be provided in the following form:

   binding: Type
   

3. Providing a choice between two different types should then be provided in the following form:

   binding1: Type1 | binding2: Type2
   

4. To bind a single name to the two alternative bindings you can discard the name of the individual bindings (using the _ wildcard) and bind the name of the overall pattern to another binding using the @ operator, as expressed in the following example:

   binding @ (_ : Type1 | _ : Type2)
   

The following is an example:

sealed trait Trait {
  def a: String
  def b: String
}

final case class C1(a: String, b: String) extends Trait
final case class C2(a: String, b: String) extends Trait
final case class C3(a: String, b: String) extends Trait

object Trait {
  def f(t: Trait): String =
   t match {
    case x @ (_ : C1 | _ : C2) => x.a // the line you are probably interested in
    case y: C3 => y.b
  }
}

Here is some sample output on invoking f:

scala> Trait.f(C1("hello", "world"))
res0: String = hello

scala> Trait.f(C2("hello", "world"))
res1: String = hello

scala> Trait.f(C3("hello", "world"))
res2: String = world

You can play around with the presented examples here on Scastie.