scala Ywarn-all 用法

Question

我正在尝试使用 scala 编译器 Y 警告，但我认为我做得不对。在下面的示例中，nums 未使用，因此我希望 -Ywarn-value-discard 打印一个警告。有两个 if 条件，一个嵌套在另一个内。 child if 的条件与 parent 的完全相反，因此其中的任何内容都是死代码。但 -Ywarn-dead-code 不会对此发出警告。谁能建议我可能做错了什么？任何指示将不胜感激。

object DeadCodeWarn{
  def main( args: Array[ String ] ) {
    val nums = 1 to 100
    //There should be a warning for squares
    //as its a non-unit expression thats not
    //used in any computation. Y-warn-value-discard
    val squares = nums.map( x => x * x ) 

    if( nums.length == 100 ){
      println( "should be printed" )
      if( nums.length !=100 )
      {
        //-Ywarn-dead-code
        println( "Dead code, so should be warned against" )
      }
    }
  }
}

$scalac -Xlint -Ywarn-all DeadCodeWarn.scala 默默地成功。

Answer 1

-Ywarn-value-discard

该值不会被丢弃，而是被分配给val!问题是 val 未使用

带有代码:

package foo

case class Foo(x: Int) {
  def foo: Int = {
    val y = 2;
    x
  }
}

我使用 scalac 2.11.5 和 -Ywarn-unused:

[warn] /.../foo.scala:5: local val in method foo is never used
[warn]     val y = 2;

---

-Ywarn-dead-code

很容易编写通用逻辑不起作用的反例:

// Evil mutable class
class Arrayish {
  private var x: Int = 100
  def length(): Int = {
    val res = x
    x += 1
    res
  }
}

def test(nums: Arrayish): Unit =
  if (nums.length == 100) {
    println( "should be printed" )
    if (nums.length != 100) {
      println("Dead code, so should be warned against")
    }
  }

运行死代码:

scala> test(new Arrayish())
should be printed
Dead code, so should be warned against

---

或者

class NonEqual {
  def ==(x: Int): Boolean = true
  def !=(x: Int): Boolean = true
}

class NaNLength {
  val length: NonEqual = new NonEqual
}

def test2(nums: NaNLength): Unit =
  if (nums.length == 100) {
    println("should be printed")
    if (nums.length != 100) {
      println("Dead code, so should be warned against")
    }
  }

死代码也会运行:

scala> test2(new NaNLength)
should be printed
Dead code, so should be warned against

---

Scalac 编译器不够智能，无法区分表现良好和表现不佳的情况。

---

如果您要提交错误/功能请求，请提及以下示例:

def test3(nums: Array[Int]): Unit = {
  if (true) {
    println("should be printed")
    if (false) {
      println("Dead code, so should be warned against")
    }
  }
}

def test4(nums: Array[Int]): Unit = {
  val hundred = nums.length == 100
  if (hundred) {
    println("should be printed")
    if (!hundred) {
      println("Dead code, so should be warned against")
    }
  }
}

似乎 scalac 死代码报告器并不像例如那样完善 java 。我希望 scalac 能够正确优化这些示例，但它不应该太复杂。