Extension Methods

Note The syntax described in this section is currently under revision. Here is the new version which will be implemented in Dotty 0.20.

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.

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

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

We can make the extension method available by defining a given StringSeqOps instance, like this:

given ops1: StringSeqOps


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

is legal everywhere ops1 is available. 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 a given instance i in either the current scope or in the implicit 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 given (i.e. it is visible at the point of call or it is defined in the companion object of Circle).

Given Instances for Extension Methods

Given instances that define extension methods can also be defined without a parent. E.g.,

given stringOps: {
  def (xs: Seq[String]) longestStrings: Seq[String] = {
    val maxLength = xs.map(_.length).max
    xs.filter(_.length == maxLength)

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

If such given instances are anonymous (as in the second clause), their name is synthesized from the name of the first defined extension method.

Given Instances with Collective Parameters

If a given instance has no parent but several extension methods one can pull out the left parameter section as well as any type parameters of these extension methods into the given instance itself. For instance, here is a given instance with two extension methods.

given listOps: {
  def [T](xs: List[T]) second: T = xs.tail.head
  def [T](xs: List[T]) third: T = xs.tail.tail.head

The repetition in the parameters can be avoided by hoisting the parameters up into the given instance itself. The following version is a shorthand for the code above.

given listOps: [T](xs: List[T]) {
  def second: T = xs.tail.head
  def third: T = xs.tail.tail.head

This syntax just adds convenience at the definition site. Applications of such extension methods are exactly the same as if their parameters were repeated in each extension method. Examples:

val xs = List(1, 2, 3)


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

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

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

If an extension method has type parameters, they come immediately after the def and are followed by the extended parameter. When calling a generic extension method, any explicitly given type arguments follow the method name. So the second method can be instantiated as follows:

List(1, 2, 3).second[Int]


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

DefSig            ::=  ...
                    |  ExtParamClause [nl] id DefParamClauses
GivenDef          ::=  ...
                       [GivenSig ‘:’] [ExtParamClause] TemplateBody
ExtParamClause    ::=  [DefTypeParamClause] ‘(’ DefParam ‘)’ {GivenParamClause}