progfunsamples

src/mooc/progfunsamples/Expr.scala

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+package week4
+
+trait Expr {
+ def isNumber: Boolean
+ def isSum: Boolean
+ def numValue: Int
+ def leftOp: Expr
+ def rightOp: Expr
+}
+class Number(n: Int) extends Expr {
+ def isNumber: Boolean = true
+ def isSum: Boolean = false
+ def numValue: Int = n
+ def leftOp: Expr = throw new Error("Number.leftOp")
+ def rightOp: Expr = throw new Error("Number.rightOp")
+}
+
+class Sum(e1: Expr, e2: Expr) extends Expr {
+ def isNumber: Boolean = false
+ def isSum: Boolean = true
+ def numValue: Int = throw new Error("Sum.numValue")
+ def leftOp: Expr = e1
+ def rightOp: Expr = e2
+}
+