Question

我是Scala的新手，并从以下代码陷入了编译器错误。我正在尝试实现代表+ ve整数的自然数类。我在定义应跟在def + (that: Nat): Nat之后的+运算符时遇到问题，但是我没有在抽象类中定义此运算符，因此出现错误。但是一旦在抽象类中定义它，它就会消失。

[error] D:\Scala\NatuarlNum\src\main\scala\NatuarlNum.scala:31:46: type mismatch;
[error]  found   : Nat
[error]  required: String
[error]   def + (that: Nat): Nat = new Succ(previous.+(that))

我不明白为什么它说必填项是“ String”而不是“ Nat”。即使在定义之前就存在函数声明。

以下是代码：

abstract class Nat {
  def isZero: Boolean
  def predecessor: Nat
  def successor: Nat
  //def + (that: Nat): Nat  // By uncommentng this line the error will go away
  //def - (that: Nat): Nat
  override def toString: String = {
    if (this.isZero) {
      "0"
    } else {
      "{" + this.predecessor.toString + "}"
    }
  }
}

object Zero extends Nat {
  def isZero = true
  def predecessor = throw new Error("0.predecessor")
  def successor: Nat = new Succ(this)
  def + (that: Nat): Nat = that
}

class Succ(previous: Nat) extends Nat {
  def isZero = false
  def predecessor = previous
  def successor: Nat = new Succ(this)
  def + (that: Nat): Nat = new Succ(previous.+(that))  // The error is from this line
}

object NatuarlNum {
  def main(args: Array[String]): Unit = {
    val zero = Zero
    val one = new Succ(zero)
    val two = new Succ(one)
    val three = new Succ(new Succ(new Succ(Zero)))

    println("0 = " + zero)
    println("1 = " + one)
    println("2 = " + two)
    println("3 = " + three)

    println("0 + 2 = " + (zero + two))
    println("2 + 2 = " + (two + two))   

  }
}

没有+运算符，我的代码将编译并给出如下结果：

0 = 0
1 = {0}
2 = {{0}}
3 = {{{0}}}
0 + 2 = {{0}}

Answer 1

好吧，您在代码中回答了问题。

  //def + (that: Nat): Nat  // By uncommentng this line the error will go away
  //def - (that: Nat): Nat

问题是

def + (that: Nat): Nat = new Succ(previous.+(that))

previous的类型为Nat，没有+运算符替代。

如果您取消注释Nat中的运算符定义，它将起作用。

+运算符的默认实现采用String作为参数，它解释了您收到的错误消息，有关更多信息，请参见any2stringadd，并且有一个ticket删除隐式+运算符定义。

来自编译器警告：

不建议使用Predef（从2.13.0开始）：不建议使用+的隐式注入。转换为字符串以调用+

未在抽象类中定义类型时发生类型不匹配错误

1 个答案: