Scala Implicit Conversion Gotchas

Question

EDIT
OK, @Drexin brings up a good point re: loss of type safety/surprising results when using implicit converters.

How about a less common conversion, where conflicts with PreDef implicits would not occur? For example, I'm working with JodaTime (great project!) in Scala. In the same controller package object where my implicits are defined, I have a type alias:

type JodaTime = org.joda.time.DateTime

and an implicit that converts JodaTime to Long (for a DAL built on top of ScalaQuery where dates are stored as Long)

implicit def joda2Long(d: JodaTime) = d.getMillis

Here no ambiguity could exist between PreDef and my controller package implicits, and, the controller implicits will not filter into the DAL as that is in a different package scope. So when I do

dao.getHeadlines(articleType, Some(jodaDate))

the implicit conversion to Long is done for me, IMO, safely, and given that date-based queries are used heavily, I save some boilerplate.

Similarly, for str2Int conversions, the controller layer receives servlet URI params as String -> String. There are many cases where the URI then contains numeric strings, so when I filter a route to determine if the String is an Int, I do not want to stringVal.toInt everytime; instead, if the regex passes, let the implicit convert the string value to Int for me. All together it would look like:

implicit def str2Int(s: String) = s.toInt
get( """/([0-9]+)""".r ) {
  show(captures(0)) // captures(0) is String
}
def show(id: Int) = {...}

In the above contexts, are these valid use cases for implicit conversions, or is it more, always be explicit? If the latter, then what are valid implicit conversion use cases?

ORIGINAL
In a package object I have some implicit conversions defined, one of them a simple String to Int:

implicit def str2Int(s: String) = s.toInt

Generally this works fine, methods that take an Int param, but receive a String, make the conversion to Int, as do methods where the return type is set to Int, but the actual returned value is a String.

Great, now in some cases the compiler errors with the dreaded ambiguous implicit:

both method augmentString in object Predef of type (x: String) scala.collection.immutable.StringOps and method str2Int(s: String) Int are possible conversion functions from java.lang.String to ?{val toInt: ?}

The case where I know this is happening is when attempting to do manual inline String-to-Int conversions. For example, val i = "10".toInt

My workaround/hack has been to create an asInt helper along with the implicits in the package object: def asInt(i: Int) = i and used as, asInt("10")

So, is implicit best practice implicit (i.e. learn by getting burned), or are there some guidelines to follow so as to not get caught in a trap of one's own making? In other words, should one avoid simple, common implicit conversions and only utilize where the type to convert is unique? (i.e. will never hit ambiguity trap)

Thanks for the feedback, implicits are awesome...when they work as intended ;-)

Answer 1

I think you're mixing two different use cases here.

In the first case, you're using implicit conversions used to hide the arbitrary distinction (or arbitrary-to-you, anyway) between different classes in cases where the functionality is identical. The JodaTime to Long implicit conversion fits in that category; it's probably safe, and very likely a good idea. I would probably use the enrich-my-library pattern instead, and write

class JodaGivesMS(jt: JodaTime) { def ms = jt.getMillis }
implicit def joda_can_give_ms(jt: JodaTime) = new JodaGivesMS(jt)

and use .ms on every call, just to be explicit. The reason is that units matter here (milliseconds are not microseconds are not seconds are not millimeters, but all can be represented as ints), and I'd rather leave some record of what the units are at the interface, in most cases. getMillis is rather a mouthful to type every time, but ms is not too bad. Still, the conversion is reasonable (if well-documented for people who may modify the code in years to come (including you)).

In the second case, however, you're performing an unreliable transformation between one very common type and another. True, you're doing it in only a limited context, but that transformation is still liable to escape and cause problems (either exceptions or types that aren't what you meant). Instead, you should write those handy routines that you need that correctly handle the conversion, and use those everywhere. For example, suppose you have a field that you expect to be "yes", "no", or an integer. You might have something like

val Rint = """(\d+)""".r
s match {
  case "yes" => println("Joy!")
  case "no" => println("Woe!")
  case Rint(i) => println("The magic number is "+i.toInt)
  case _ => println("I cannot begin to describe how calamitous this is")
}

But this code is wrong, because "12414321431243".toInt throws an exception, when what you really want is to say that the situation is calamitous. Instead, you should write code that matches properly:

case object Rint {
  val Reg = """([-]\d+)""".r
  def unapply(s: String): Option[Int] = s match {
    case Reg(si) =>
      try { Some(si.toInt) }
      catch { case nfe: NumberFormatException => None }
    case _ => None
  }
}

and use this instead. Now instead of performing a risky and implicit conversion from String to Int, when you perform a match it will all be handled properly, both the regex match (to avoid throwing and catching piles of exceptions on bad parses) and the exception handling even if the regex passes.

If you have something that has both a string and an int representation, create a new class and then have implicit conversions to each if you don't want your use of the object (which you know can safely be either) to keep repeating a method call that doesn't really provide any illumination.

Answer 2

I try not to convert anything implicitly just to convert it from one type to another, but only for the pimp my library pattern. It can be a bit confusing, when you pass a String to a function that takes an Int. Also there is a huge loss of type safety. If you would pass a string to a function that takes an Int by mistake the compiler could not detect it, as it assumes you want to do it. So always do type conversion explicitly and only use implicit conversions to extend classes.

edit:

To answer your updated question: For the sake of readability, please use the explicit getMillis. In my eyes valid use cases for implicits are "pimp my library", view/context bounds, type classes, manifests, builders... but not being too lazy to write an explicit call to a method.