Scala运算符＃>导致编译错误，但无法&> - 为什么？ [英] Scala operator #> causes compilation error but not &> - why?

问题描述

我有一个类型推理问题，并要求帮助此处。
最初的问题是由于重载。一旦纠正I
仍然有问题。

I have a type inference issue and asked for help here. The initial problem was due to overload. Once corrected I still had problems.

这里是代码：

class DPipe[ A ]( a: A ) {
  def !>[ B ]( f: A => B ) = Try(f( a ))
  def #>[ B, C ]( f: B => C )(implicit ev: A =:= Try[B]) : Try[C] = a.map(f)
  //def &>[ B, C ]( f: B => C )( implicit ev: A =:= Try[ B ] ) =  a.map( f )
}

object DPipe {
  def apply[ A ]( v: A ) = new DPipe( v )
}

object DPipeOps {
  implicit def toDPipe[ A ]( a: A ): DPipe[ A ] = DPipe( a )
}

这里是测试：

object DPipeDebug {

 def main( args: Array[ String ] ) {

    import DPipeOps._

    val r8 = 100.0 !> {x : Double => x / 0.0}  
    println(r8)
    val r9 = r8 #> {x:Double => x* 3.0} 
    println(r9)
    /*
    val r8 = 100.0 !> { x: Double => x / 0.0 }
    println( r8.get )
    val r9 = r8 &> { x: Double => x * 3.0 }
    println( r9 )*/

    val r10 = (100.0 !> {x : Double => x / 0.0}) #> {x:Double => x* 3.0} 
   //val r10 = ( 100.0 !> { x: Double => x / 0.0 } ) &> { x: Double => x * 3.0 }

    val r11 = 100.0 !> {x : Double => x / 0.0} #> {x:Double => x* 3.0} 
    //val r11 = 100.0 !> { x: Double => x / 0.0 } &> { x: Double => x * 3.0     }
  }

}

我们在最后一行代码中有以下错误：

As it stands we have the following error in the last code line:

Cannot prove that Double => Double =:= scala.util.Try[Double].
val r11 = 100.0 !> {x : Double => x / 0.0} #> {x:Double => x* 3.0} 
                                           ^

请注意，在第二个代码行中，添加
圆括号以强制执行左侧左侧相关性
（Scala默认值）。看起来像＃> 运算符尝试
使用函数 {x：Double => x / 0.0} ，这的确是一个
Double。

Notice that in the second last code line, I need only add the parenthesis to enforce left left-hand associativity (Scala default). It seems like the #> operator tries to use the function {x : Double => x / 0.0}, which indeed is a Double.

但是，如果我使用&>运算符，则不会发生错误。在下面的
测试代码中，只需翻转注释。所以我的问题是，为什么
是这种情况发生。这是Scala 2.12.0的新功能吗？

If however I use the "&>" operator, no error occurs. In the test code below, just flip the comments. So my question is, why is this happening. Is this something new to Scala 2.12.0?

TIA

推荐答案

问题是与运算符优先级。以下是更多详细信息： Scala中的运算符优先级

The problem is with the operator precedence. Here are more details: Operator precedence in Scala

这取决于运算符的第一个字符。 ＃的优先级高于！和！ & 。

It depends on the first character of the operator. # has higher precedence than ! and ! has higher precedence than &.

因此您会得到

100.0 !> ({x : Double => x / 0.0} #> {x:Double => x* 3.0})

b $ b

而不是

instead of

(100.0 !> {x : Double => x / 0.0}) #> {x:Double => x* 3.0}

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