Adding logging if return value is None

Question

Suppose there are two functions findUser(id:String):Option[User] and findAddress(user:User):Option[Address] invoked as follows:

for(user <- findUser(id); address <- findAddress(user)) println(address)

Now I would like to add error logging to this for-comprehension. I would like to call a log(msg:String) function if either user or address is not found.

for(user <- findUser(id) ifNone log("user not found"); 
    address <- findAddress(user) ifNone log("address not found")) 
       println(address)

Can I do it without changing the function signatures?

Answer 1

Maybe

implicit def withIfNone[A](o: Option[A]) = new {
  def ifNone(action: => Unit) = { if (o == None) action; o }
}

You may also consider using Either instead of option (or converting your options to Either). That would not work with a foreach (a for without a yield), but you might do

for(
  a <- option1.toRight("option1 missing").right; 
  b <- option2.toRight("option2 missing").right)
yield f(a,b)

Then you can pattern match on the result with

case Left(error) => log (error)
case Right(result) => // use result

Answer 2

Lift's Box is a more appropriate class for your usage case. A Box is like an Option, but with two empty states: ok and error. You could use it like this:

val addr = for {
  user <- findUser(id) ?~ "user not found"
  address <- findAddress(user) ?~ "address not found"
} yield address

address match {
  case Full(addr) => println(addr)
  case oops: Failure => println(oops.msg) // see Failure for more details
}

See this blog for various suggestions related to your problem.

Answer 3

It might be an overkill, but it looks pretty much like what you wanted ;)

object Extensions {
  // You need a wrapper since Option is sealed
  class OptionWrapper[E](option: Option[E]) {
    def foreach[U](f: E => U) {
      option foreach f
    }
    def isEmpty = option.isEmpty
  }

  // Modification trait for OptionWrapper
  trait ErrorLogging[E] extends OptionWrapper[E] {
    abstract override def foreach[U](f: E => U) {
      if (isEmpty)
        println("error")
      else
        super.foreach(f)
    }
  }

  // Accessor for the new mixin
  def log[E](option: Option[E]) = new OptionWrapper(option) with ErrorLogging[E]
}

object TestingLogger extends App {
  case class User(address: String)
  def findUser(id: Int): Option[User] = if (id == 1) Some(User("address")) else None
  def findAddress(user: User): Option[String] = Some(user.address)

  import Extensions._

  for {
    user <- log(findUser(1)) // prints out address
    address <- log(findAddress(user))
  } println(address)

  for {
    user <- log(findUser(2)) // prints out error
    address <- log(findAddress(user))
  } println(address)
}

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