Cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method

user_9710309 0
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I have a quite simple Scala code where I wanted to apply pattern matching:

trait Expr
case class Number(n: Int) extends Expr
case class Sum(a: Expr, b: Expr) extends Expr
case class Prod(a: Expr, b: Expr) extends Expr

def eval(e: Expr): Int = e match
  case Number(n) => n
  case Sum(a, b) => eval(a) + eval(b)
  case Prod(a, b) => eval(a) * eval(b)

But when I run the code the error rises: "Number cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method"

What should I do to avoid this error? Thank you

pattern-matching scala
2 Answers

You probably have imported java.lang.Number in the scope of the eval definition.

Make sure to import your Number definition instead (either by removing the Java import or renaming it, whatever fits best your use case...) and it will work fine.

user_53891270

Apparently it's true what was said before, that java.lang.Number shadows case class Number when run in IntelliJ. If you don't want to abandon your IDE, you can also rename case class Number to case class Num. This way, the code runs fine at least on my machine.

user_60671580
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