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| sealed trait Expr | |
| case class Num(n: Int) extends Expr | |
| case class Add(e1: Expr, e2: Expr) extends Expr | |
| case class Sub(e1: Expr, e2: Expr) extends Expr | |
| def value(e: Expr): Int = e match { | |
| case Num(n) => n | |
| case Add(e1, e2) => value(e1) + value(e2) | |
| case Sub(e1, e2) => value(e1) - value(e2) | |
| } |
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