CODEWITHSUNDEEP
scala
Kevin Meredith
Kevin Meredith

Reputation: 41919

`error: object B is not a case class` - but `B` is a case class

Given two Algebraic Data Types: A and Foo:

scala> sealed trait A
defined trait A

scala> case object B extends A
defined object B

scala> sealed trait Foo
defined trait Foo

scala> case class FooImpl(x: Int) extends Foo
defined class FooImpl

a simple function f:

scala> def f: Foo = FooImpl(5)
f: Foo

Finally, I have a few nested match statements/expressions:

scala> def hoobar(x: A): Int = x match {
     |            case B(_) => f match {
     |              case FooImpl(_) => ???
     |            }
     |           }
<console>:18: error: object B is not a case class, nor does it have an unapply/unapplySeq member
                  case B(_) => f match {
                       ^

Why does the above error show up? B is certainly a case class, no?

Upvotes: 0

Views: 580

Answers (1)

Michael Zajac
Michael Zajac

Reputation: 55569

B is a case object, not a case class. B(_) doesn't make sense. What would _ be a substitution for when there is only one object B?

If you want to match on B, a specific object, use (back-ticks required if lower-case):

case B => ...

Though you could get the other syntax to work by providing an unapply method for B, it just doesn't really make sense.

case object B extends A {
    def unapply(b: this.type): Option[this.type] = Some(b)
}

Upvotes: 3

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