How to add tracing within a 'for' comprehension?

Question

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

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

for {
  outer <         


        

Answer 1

Flaviu's answer inspired me to try playing with implicits. The idea is to see if the trace looks better with the 'trace' further to the right on the line:

import Trace._

object Main {  
  def main(args:Array[String])  {
    val listList = List(List(1,2,3), List(3,4))    
    for {
      list <- trace1(listList, "lList is: %s", listList)  // trace() 
      item <- list traced("list is: %s", list)            // implicit         
    } yield item

I also wanted to try mixing in error logging in the same comprehension. Error logging seems to look best mixed with Daniel's approach:

    val optOpt:Option[Option[Int]] = Some(Some(1))
    for {
      opt <- optOpt;
      _ = trace2("opt found: %s", opt)   // trying Daniel's suggestion
      int <- opt orElse 
        err("num not found in: %s", opt)   // together with error logging
    } yield int
  }
}

Here's the supporting code for both experiments:

object Trace {
  def trace1[T](any:T, message:String, params:AnyRef*):T = {
    Console println String.format("TRA: " + message, params:_*)
    any
  }

  def trace2[T](message:String, params:AnyRef*) {
    Console println String.format("TRA: " + message, params:_*)
  }

  def err[T](message:String, params:AnyRef*):Option[T] = {
    Console println String.format("ERR: " + message, params:_*)
    None
  }

  implicit def anyRefToTraceable[T](anyRef:T):Traceable[T] = {
    new Traceable(anyRef)
  }

  class Traceable[T](val self:T) {
    def traced(message:String, params:AnyRef*):T = {
      Console println String.format("TRA: " + message, params:_*)
      self
    }  
  }  
}