我应该如何实施“添加"功能?斯卡拉译员吗? [英] How should I implement "add" interpreter in scala?

问题描述

我正在实现Scala解释器，但遇到了一些问题

I am implementing scala interpreter and I have some problem

我具有Value.scala中的值类:

case class IntV(value: Int) extends Value

我也遵循文件Expr.scala中的类:

case class IntE(value: Int) extends Expr
case class Add(left: Expr, right: Expr) extends Expr

当我按如下方式实现Int解释器时:

When I implemented Int interpreter as followed:

case IntE(value)=>IntV(value)

效果很好，但是当我实现添加解释器时(它执行3 + 4 = 7之类的加法运算)，如下所示:

It worked well but when I implemented add interpreter(which does addition like 3+4=7)as followed:

case Add(left, right)=>IntV(left)+IntV(right)

返回错误，该错误表示变量为left和right，需要Int，但找到了expr.
我做错什么了?我该怎么做才能获得正确的回报?

It returns error which says for variable left and right, it requires Int but it found expr.
What did I do wrong? What should I do to get correct return?

推荐答案

您应递归调用解释器.

例如

sealed trait Value
case class IntV(value: Int) extends Value 

sealed trait Expr
case class IntE(value: Int) extends Expr
case class Add(left: Expr, right: Expr) extends Expr

def interpret(expr: Expr): Value = expr match {
  case IntE(value)      => IntV(value)
  case Add(left, right) => (interpret(left), interpret(right)) match {
    case (IntV(v1), IntV(v2)) => IntV(v1 + v2)
  }
}

interpret(IntE(1)) // IntV(1)
interpret(Add(IntE(3), IntE(4))) // IntV(7)

或使用针对IntV的已定义方法+

Or with a defined method + for IntV

sealed trait Value
case class IntV(value: Int) extends Value {
  def +(other: IntV): IntV = IntV(value + other.value)
}

sealed trait Expr
case class IntE(value: Int) extends Expr
case class Add(left: Expr, right: Expr) extends Expr

def interpret(expr: Expr): Value = expr match {
  case IntE(value)      => IntV(value)
  case Add(left, right) => (interpret(left), interpret(right)) match {
    case (v1: IntV, v2: IntV) => v1 + v2
  }
}

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