Dotty Documentation


Extension Methods

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Extension methods allow one to add methods to a type after the type is defined. Example:

case class Circle(x: Double, y: Double, radius: Double)

def (c: Circle) circumference: Double = c.radius * math.Pi * 2

Like regular methods, extension methods can be invoked with infix .:

  val circle = Circle(0, 0, 1)

Translation of Extension Methods

Extension methods are methods that have a parameter clause in front of the defined identifier. They translate to methods where the leading parameter section is moved to after the defined identifier. So, the definition of circumference above translates to the plain method, and can also be invoked as such:

def circumference(c: Circle): Double = c.radius * math.Pi * 2

assert(circle.circumference == circumference(circle))

Translation of Calls to Extension Methods

When is an extension method applicable? There are two possibilities.

  • An extension method is applicable if it is visible under a simple name, by being defined or inherited or imported in a scope enclosing the application.
  • An extension method is applicable if it is a member of some implied instance at the point of the application.

As an example, consider an extension method longestStrings on String defined in a trait StringSeqOps.

trait StringSeqOps {
  def (xs: Seq[String]) longestStrings = {
    val maxLength =
    xs.filter(_.length == maxLength)

We can make the extension method available by defining an implied instance of StringSeqOps, like this:

implied ops1 for StringSeqOps


List("here", "is", "a", "list").longestStrings

is legal everywhere ops1 is available as an implied instance. Alternatively, we can define longestStrings as a member of a normal object. But then the method has to be brought into scope to be usable as an extension method.

object ops2 extends StringSeqOps
import ops2.longestStrings
List("here", "is", "a", "list").longestStrings

The precise rules for resolving a selection to an extension method are as follows.

Assume a selection e.m[Ts] where m is not a member of e, where the type arguments [Ts] are optional, and where T is the expected type. The following two rewritings are tried in order:

  1. The selection is rewritten to m[Ts](e).
  2. If the first rewriting does not typecheck with expected type T, and there is an implied instance i in either the current scope or in the implied scope of T, and i defines an extension method named m, then selection is expanded to i.m[Ts](e). This second rewriting is attempted at the time where the compiler also tries an implicit conversion from T to a type containing m. If there is more than one way of rewriting, an ambiguity error results.

So circle.circumference translates to CircleOps.circumference(circle), provided circle has type Circle and CircleOps is an eligible implied instance (i.e. it is visible at the point of call or it is defined in the companion object of Circle).

Implied Instances for Extension Methods

Implied instances that define extension methods can also be defined without an of clause. E.g.,

implied StringOps {
  def (xs: Seq[String]) longestStrings: Seq[String] = {
    val maxLength =
    xs.filter(_.length == maxLength)

implied ListOps {
  def (xs: List[T]) second[T] = xs.tail.head

If such implied instances are anonymous (as in the examples above), their name is synthesized from the name of the first defined extension method.


The extension method syntax also applies to the definition of operators. In each case the definition syntax mirrors the way the operator is applied. Examples:

  def (x: String) < (y: String) = ...
  def (x: Elem) +: (xs: Seq[Elem]) = ...

  "ab" + "c"
  1 +: List(2, 3)

The two definitions above translate to

  def < (x: String)(y: String) = ...
  def +: (xs: Seq[Elem])(x: Elem) = ...

Note that swap of the two parameters x and xs when translating the right-binding operator +: to an extension method. This is analogous to the implementation of right binding operators as normal methods.

Generic Extensions

The StringSeqOps examples extended a specific instance of a generic type. It is also possible to extend a generic type by adding type parameters to an extension method. Examples:

def (xs: List[T]) second [T] =

def (xs: List[List[T]]) flattened [T] =
  xs.foldLeft[List[T]](Nil)(_ ++ _)

def (x: T) + [T : Numeric](y: T): T =
  implicitly[Numeric[T]].plus(x, y)

As usual, type parameters of the extension method follow the defined method name. Nevertheless, such type parameters can already be used in the preceding parameter clause.


The required syntax extension just adds one clause for extension methods relative to the current syntax.

DefSig            ::=  ...
                    |  ‘(’ DefParam ‘)’ [nl] id [DefTypeParamClause] DefParamClauses