Declaring Implicits

While implicits are easy to use, declaring them properly requires careful consideration. Because there are a few different ways you can declare implicits viz. extension methods, parameter values, type converters and dynamic instances. Each one has a specific purpose. For instance, dynamic instances enable recursive implicit resolution. Understanding the different ways of declaring implicits is critical in choosing the right one for the given scenario. It is also invaluable in troubleshooting subtle bugs and unexpected behavior related to implicit resolution. Let us dive in.

Implicits can be declared in one of the following ways:

• As extension methods (aka Implicit Classes)
• As parameter values
• As type converters
• As dynamic instances

In my opinion, the order listed above relates to its ease of comprehension.

Extension Methods

Extension method is a way to add new functionality to types without modifying its original definition. The decision to not modify the original definition is either intentional or consequential.

• Intentional:
  • The new functionality is not a direct behavior of the original type. But serves as a utility method. Adding it to the original type would pollute the public interface of the type.
  • The new functionality may span and serve type hierarchies instead of a single type. For instance, you want to add a walk method on all Animals , and you would use it on a Dog type.
• Consequential:
  • You don’t have (write) access to the original definition. You may be using the type from one of the dependent libraries.
  • Easier to express certain type constraints as an extension method (e.g Option#flatten).

With that literature of the way, here is how you would define extension methods in Scala (version 2).

package object utils {
  implicit final class StringOps(private val str: String) extends AnyVal {
    def toTitleCase: String =
      // Not the best implementation but we are talking about implicits here. So, shove it under the rug.
      s.split("\\s+").map(_.capitalize).mkString(" ")
  }

  // AI is thinking to help you add more methods here 😀
}

The above extension method is an example of the consequential kind. Anyways, you would bring the extension methods in scope and use it.

import utils._

println("hello world".toTitleCase) // Guess what it prints!

The nice thing about extension methods is you can apply on companion objects too. For instance …

implicit final class StringTypeOps(private val str: String.type) extends AnyVal {
  def isNullOrBlank(s: String): Boolean = s == null || s.isBlank
}

// somewhere else in code ...
val name: String = ... // could be null if interacting with a Java API
String.isNullOrEmpty(name)

Parameter Values

Implicit parameters, which I would say is the more widely known and used, are just like any parameters to a function except the compiler fills it in for you without having to explicitly mention it.

def registerUser(input: RegisterUserInput)(implicit ec: ExecutionContext): Future[User] = ...

The registerUser function is meant to perform a number of tasks to have a user registered. All of it asynchronously in a Future. Running a Future requires an ExecutionContext1``, which if declared in scope (or in one of the places where the compiler will look for2) will be picked up by the compiler and passed implicitly to registerUser.

Here is an example:

import scala.concurrent.ExecutionContext

class UserController @Inject() (...) extends BaseController {
  implicit val ec: ExecutionContext = ExecutionContext.Implicits.global

  // Entry point for /register path
  def registerUserPath() = Action { implicit request =>
    val input = pardonMeButDeserializeRequest() // 🙄
    // ...
    service.registerUser(input) // ec declared above picked up for the implicit parameter
  }
}

Implicit parameters are ubiquitous in almost every bit of Scala code out there although its instantiation has a gotcha.

In the above example, the same instance of ExecutionContext - ec, will be used as the implicit parameter even for another function (say getUserInfo) similar to registerUser. Because ec is declared as a val.

If it was declared a def, every call that needs the implicit parameter would get a new instance. Try this out …

// Not the Show from cats but our version for the post.
trait Show[A] { def show(a: A): String }

object Blah extends App {
  implicit def showInt: Show[Int] = {
    new Show[Int] {
      println("You get a new instance of Show[Int] ...")
      override def show(n: Int): String = s"Int($n)"
    }
  }

  // contrast that with `val showInt: Show[Int] = ...`

  def foo(n: Int)(implicit S: Show[Int]): Unit = {
    println(S.show(n))
  }

  foo(42) // You get a new instance of Show[Int] ...
  foo(10) // You get a new instance of Show[Int] ...
}

I chose a different example for the above so that you can copy the code and run it. If showInt was a val, you would see the message print only once ever because the compiler would create the instance and cache it. Not so in the case of def. In other words, once for implicit vals or each time an implicit value is needed for implicit defs.

You would declare a def …

• if the implicit instance is type parameterized e.g. implicit def foo[A]: XYZ = ...
• if there is some state associated with the implicit instance during its creation.

It is important to know how an implicit parameter value is going to be created. Not only implicit val s and implicit def s but if the implicit is backed by a macro. Macro backed implicits generate code creating the implicit value. The code generation is tied to the number of times the implicit is referenced / evaluated, which directly affects compilation time.

Type converters

Implicit type converters are infamous, and generally speaking, should be avoided. But they do have their place; especially if you are not in the implicits are hard and bad camp.

Compiler is all about types - define, create, manage, derive, convert etc. Say, you have a function def triple(n: Long) . You cannot call the function passing it an Int. You should expect an error because types are different. However, the compiler does not give up in the first go. It is going to look around for a way to convert Int to Long. Perhaps there is an overloaded function (which is not the case here), and so on. In the list of things it will look for is the implicit type conversion function. Something of the sort …

implicit def int2Long(n: Int): Long = n.toLong

The declaration should be self-explanatory - give me Int, take a Long. Just that it is declared implicit, which means the compiler will invoke this function, if in scope, to convert n to Long before calling triple.

In fact, such implicit conversions are declared in the standard library to make life easier … for the meek. For the strong-hearted, there are compiler flags to report such conversions as warnings. Top that with -X:fatal-warnings flag to report warnings as errors. That means such implicit conversions would be prohibited and you have to use .toLong yourself making it explicit and putting the responsibility of the conversion on you, which is right.

Why is implicit type conversion highly discouraged? Because it can kick in unintentionally just because a conversion is in scope. This usually leads to unexpected behavior and bugs that would require few mugs of coffee to troubleshoot/find/fix. Scala 3 has a Conversion typeclass which is the explicit version of the type conversion. It is adding discipline to the conversion and making the it available implicitly. But the actual call to conversion is explicit. In fact, it is not hard at all define such a typeclass if you are using Scala 2.

The type conversion discussed above is not restricted to scalar types. It could be for any two types. When you define a type conversion, you must really know what you are doing. That you really need it and it will not get you in trouble unintentionally.

Here is how an implicit type conversion is used beautifully in cats:

final case class ShowInterpolator(_sc: StringContext) extends AnyVal {
  def show(args: Shown*): String = _sc.s(args: _*)
}

final case class Shown(override val toString: String) extends AnyVal
object Shown {
  // This is where the magic happens! Every type you provide in show"..." is
  // converted to `Shown` as long as there is an implicit Show[A] available.
  // Shown is just a type to denote that the value is stringified.
  implicit def mat[A](x: A)(implicit z: ContravariantShow[A]): Shown = Shown(z.show(x))
}

Read more on Show here.

Generally speaking, if you are about to write a type conversion, think twice. Read about it. Consult with your peers.

Dynamic Instances

This one is my favorite. Because it gives more than what you put in.

// Not the cats.Show but just our own version of Show
implicit val showString:              Show[String] = _.toString
implicit def showNumeric[A: Numeric]: Show[A]      = n => s"${n.getClass.getSimpleName}($n)"

implicit def showOption[A](implicit A: Show[A]): Show[Option[A]] = {
  case None    => "None"
  case Some(a) => s"Some(${A.show(a)})"
}

implicit def showEither[L, R](implicit L: Show[L], R: Show[R]): Show[Either[L, R]] = {
  case Left(l)  => s"Left(${L.show(l)})"
  case Right(r) => s"Right(${R.show(r)})"
}

The above modest piece of implicit code does something wonderful - Recursive implicit resolution.

def showMe[A](a: A)(implicit ev: Show[A]): Unit =
  println(ev.show(a))

showMe(Option(Option(100)))

val e: Either[String, Int] = Right(200)
showMe(e)

val foo: Either[Option[Either[String, Double]], Int] = Left(Some(Left("Success")))
showMe(foo)

val bar: Either[Option[Either[String, Double]], Int] = Left(Some(Right(3.142)))
showMe(bar)

If it is able to find an implicit Show instance for A then showOption would be able to give the string version of the type. Now, A could be anything. Even another Option, which in turn will look for its Show[A] instance. Or it could be an Either. Particularly, see howfoo and bar recursively resolve implicits!

Special thanks to Morgen Peschke for proof-reading and elevating the quality of this post. And in the process improving my understanding³ about implicits.

Featured image courtesy: Unsplash