Implicit Classes

Josh Suereth

Introduction

Scala 2.10 introduced a new feature called implicit classes. An implicit class is a class marked with the implicit keyword. This keyword makes the class’ primary constructor available for implicit conversions when the class is in scope.

Implicit classes were proposed in SIP-13.

Usage

To create an implicit class, simply place the implicit keyword in front of an appropriate class. Here’s an example:

object Helpers {
  implicit class IntWithTimes(x: Int) {
    def times[A](f: => A): Unit = {
      def loop(current: Int): Unit =
        if(current > 0) {
          f
          loop(current - 1)
        }
      loop(x)
    }
  }
}

This example creates the implicit class IntWithTimes. This class wraps an Int value and provides a new method, times. To use this class, just import it into scope and call the times method. Here’s an example:

scala> import Helpers._
import Helpers._

scala> 5 times println("HI")
HI
HI
HI
HI
HI

For an implicit class to work, its name must be in scope and unambiguous, like any other implicit value or conversion.

Restrictions

Implicit classes have the following restrictions:

1. They must be defined inside of another trait/class/object.

object Helpers {
   implicit class RichInt(x: Int) // OK!
}
implicit class RichDouble(x: Double) // BAD!

2. They may only take one non-implicit argument in their constructor.

implicit class RichDate(date: java.util.Date) // OK!
implicit class Indexer[T](collecton: Seq[T], index: Int) // BAD!
implicit class Indexer[T](collecton: Seq[T])(implicit index: Index) // OK!

While it’s possible to create an implicit class with more than one non-implicit argument, such classes aren’t used during implicit lookup.

3. There may not be any method, member or object in scope with the same name as the implicit class.

Note: This means an implicit class cannot be a case class.

object Bar
implicit class Bar(x: Int) // BAD!

val x = 5
implicit class x(y: Int) // BAD!

implicit case class Baz(x: Int) // BAD!

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