Is it possible to pass “this” as implicit paramete

问题:

Suppose I want to wrap code that can throw exceptions with a try-catch block that logs the exception and continues. Something like:

loggingExceptions {
  // something dangerous
}

Ideally, I would like to use for logging the Logger defined on the calling object, if any (and if none, get a compile-time error). I'd love to define something like this:

def loggingExceptions[L <: { def logger: Logger }](work: => Unit)(implicit objectWithLogger: L): Unit = {
  try {
    work
  } catch {
    case t: Exception => objectWithLogger.logger.error(t.getMessage)
  }
}

where objectWithLogger would somehow "magically" expand to "this" in client code. Is this (or a similar thing) possible?

回答1:

It can in fact be done just as you want. The other answerers surrendered too quickly. No white flags!

package object foo {
  type HasLogger = { def logger: Logger }
  implicit def mkLog(x: HasLogger) = new {
    def loggingExceptions(body: => Unit): Unit =
      try body
      catch { case ex: Exception => println(ex) }
  }
}

package foo {
  case class Logger(name: String) { }

  // Doesn't compile:
  // class A {
  //   def f = this.loggingExceptions(println("hi"))
  // }
  // 1124.scala:14: error: value loggingExceptions is not a member of foo.A
  //         def f = this.loggingExceptions(println("hi"))
  //                      ^
  // one error found  

  // Does compile
  class B {
    def logger = Logger("B")
    def f = this.loggingExceptions(println("hi"))
    def g = this.loggingExceptions(throw new Exception)
  }
}

object Test {
  def main(args: Array[String]): Unit = {
    val b = new foo.B
    b.f
    b.g
  }
}

// output
//
// % scala Test
// hi
// java.lang.Exception

回答2:

Debilski's answer will work, but I'm not sure I see a good reason to use a structural type (i.e. { def logger: Logger }) here. Doing so will incur extra runtime overhead whenever logger is invoked, since the implementation of structural types relies on reflection. The loggingExceptions method is closely tied to logging, so I would just make it part of a Logging trait:

trait Logging {
   def logger: Logger

   final def loggingExceptions(body: => Unit) =
      try body catch { case e: Exception => logger.error(e.getMessage) }
}

trait ConcreteLogging extends Logging { 
   val logger = // ...
}

object MyObject extends SomeClass with ConcreteLogging {
   def main {
      // ...
      loggingExceptions { 
         // ...
      }
   }
}

回答3:

You could add a trait to all classes which want to use def loggingExceptions and in this trait add a self-type which expects def logger: Logger being available.

trait LoggingExceptions {
    this: { def logger: Logger } =>
  def loggingExceptions(work: => Unit) {
    try { work }
    catch { case t: Exception => logger.error(t.getMessage) }
  }
}

object MyObjectWithLogging extends OtherClass with LoggingExceptions {
  def logger: Logger = // ...

  def main {
    // ...
    loggingExceptions { // ...
    }
  }
}