如何在"for"理解中添加跟踪? [英] How to add tracing within a 'for' comprehension?

问题描述

对于for理解中的日志跟踪，我使用了伪分配，如下所示:

For log tracing inside a for comprehension, I've used dummy assignment like this:

val ll = List(List(1,2),List(1))            

for {
  outer <- ll 
  a = Console.println(outer)   // Dummy assignment makes it compile
  inner <- outer
} yield inner

a =位似乎很尴尬.有没有更清洁的方法?

The a = bit seems awkward. Is there a cleaner way?

推荐答案

您始终可以定义自己的trace函数:

You could always define your own trace function:

def trace[T](x: T) = {
  println(x) // or your favourite logging framework :)
  x
}

然后，理解力将如下所示:

Then the for comprehension would look like:

for { 
  outer <- ll
  inner <- trace(outer)
} yield inner

或者，如果您希望打印更多信息，则可以如下定义trace:

Alternatively, if you want more information printed, you can define trace as follows:

def trace[T](message: String, x: T) = {
  println(message)
  x
}

和for理解如下:

for { 
  outer <- ll
  inner <- trace("Value: " + outer, outer)
} yield inner

编辑:是的，您可以编写trace使其作用在目标的右侧，以回应您的评论！您只需要使用一些隐式技巧即可.实际上，它看起来确实比应用于左侧时好得多:).

In response to your comment, yes, you can write trace so that it acts to the right of the target! You just have to use a bit of implicit trickery. And actually, it does look much nicer than when applied to the left :).

为此，您必须先定义一个Traceable类，然后定义对该类的隐式转换:

To do this, you have to first define a class which is Traceable and then define an implicit conversion to that class:

class Traceable[A](x: A) { 
  def traced = {
    println(x)
    x
  }
}

implicit def any2Traceable[A](x: A) = new Traceable(x)

然后，您所提供的代码中唯一需要修改的就是将traced添加到要跟踪的值的末尾.例如:

Then the only thing you have to modify in the code you have provided is to add traced to the end of the value you want to be traced. For example:

for { 
  outer <- ll
  inner <- outer traced
} yield inner

(Scala编译器将其翻译为outer.traced)

(this is translated by the Scala compiler into outer.traced)

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