Pattern Matching

Expr.scala

abstract class Expr
case class Var(name: String) extends Expr
case class Number(num: Double) extends Expr
case class UnOp(operator: String, arg: Expr) extends Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr
class Plus(left: Expr, right: Expr) extends BinOp("+", left, right)
def simplifyTop(expr: Expr): Expr = expr match {
  case UnOp("-", UnOp("-", e)) => e
  case BinOp("+", e, Number(0)) => e
  case BinOp("*", e, Number(1)) => e
  case _ => expr
}

def simplifyAll(expr: Expr): Expr = expr match {
  case UnOp("-", UnOp("-", e)) => simplifyAll(e)
  case BinOp("+", e, Number(0)) => simplifyAll(e) 
  case BinOp("*", e, Number(1)) => simplifyAll(e)
  case UnOp(op, e) => UnOp(op, simplifyAll(e))
  case BinOp(op, l, r) => BinOp(op, simplifyAll(l), simplifyAll(r))
  case _ => expr
}