This doc page is specific to features shipped in Scala 2, which have either been removed in Scala 3 or replaced by an alternative. Unless otherwise stated, all the code examples in this page assume you are using Scala 2.
In Scala 3, implicit classes are still supported for compatibility reasons but the recommended way to achieve the same result is to use extension methods.
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’s 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 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.time.LocalDate) // OK!
implicit class Indexer[T](collection: Seq[T], index: Int) // BAD!
implicit class Indexer[T](collection: 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. The implicit def introduced by implicit class must not be ambiguous with respect to other term members.
Note: This means an implicit class cannot be a case class, since the implicit def would be ambiguous with the companion apply.
object Bar
implicit class Bar(x: Int) // BAD!
val x = 5
implicit class x(y: Int) // BAD!
implicit case class Baz(x: Int) // BAD!