Relationship with Scala 2 Implicits

Many, but not all, of the new contextual abstraction features in Scala 3 can be mapped to Scala 2's implicits. This page gives a rundown on the relationships between new and old features.

Simulating Contextual Abstraction with Implicits

Given Instances

Given instances can be mapped to combinations of implicit objects, classes and implicit methods.

  1. Given instances without parameters are mapped to implicit objects. E.g.,
    given intOrd: Ord[Int] { ... }
    

    maps to

    implicit object IntOrd extends Ord[Int] { ... }
    
  2. Parameterized given instances are mapped to combinations of classes and implicit methods. E.g.,
      given listOrd[T](given ord: Ord[T]): Ord[List[T]] { ... }
    

    maps to

    class ListOrd[T](implicit ord: Ord[T]) extends Ord[List[T]] { ... }
    final implicit def ListOrd[T](implicit ord: Ord[T]): ListOrd[T] = new ListOrd[T]
    
  3. Alias givens map to implicit methods or implicit lazy vals. If an alias has neither type parameters nor a given clause, it is treated as a lazy val, unless the right hand side is a simple reference, in which case we can use a forwarder to that reference without caching it.

Examples:

given global: ExecutionContext = new ForkJoinContext()

val ctx: Context
given Context = ctx

would map to

final implicit lazy val global: ExecutionContext = new ForkJoinContext()
final implicit def given_Context = ctx

Anonymous Given Instances

Anonymous given instances get compiler synthesized names, which are generated in a reproducible way from the implemented type(s). For example, if the names of the IntOrd and ListOrd givens above were left out, the following names would be synthesized instead:

given given_Ord_Int : Ord[Int] { ... }
given given_Ord_List[T] : Ord[List[T]] { ... }

The synthesized type names are formed from

Tuples are treated as transparent, i.e. a type F[(X, Y)] would get the synthesized name F_X_Y. Directly implemented function types A => B are represented as A_to_B. Function types used as arguments to other type constructors are represented as Function.

Anonymous given instances that define extension methods get their name from the name of the first extension method and the toplevel type constructor of its first parameter. For example, the given instance

given [T] (xs: List[T]) extended with {
  def second = ...
}

gets the synthesized name given_second_of_List_T.

Given Clauses

Given clauses corresponds largely to Scala-2's implicit parameter clauses. E.g.

def max[T](x: T, y: T)(given ord: Ord[T]): T

would be written

def max[T](x: T, y: T)(implicit ord: Ord[T]): T

in Scala 2. The main difference concerns applications of such parameters. Explicit arguments to parameters of given clauses must be written using given, mirroring the definition syntax. E.g, max(2, 3)(given IntOrd). Scala 2 uses normal applications max(2, 3)(IntOrd) instead. The Scala 2 syntax has some inherent ambiguities and restrictions which are overcome by the new syntax. For instance, multiple implicit parameter lists are not available in the old syntax, even though they can be simulated using auxiliary objects in the "Aux" pattern.

The the method corresponds to implicitly in Scala 2. It is precisely the same as the the method in Shapeless. The difference between the (in both versions) and implicitly is that the can return a more precise type than the type that was asked for.

Context Bounds

Context bounds are the same in both language versions. They expand to the respective forms of implicit parameters.

Note: To ease migration, context bounds in Dotty map for a limited time to old-style implicit parameters for which arguments can be passed either with given or with a normal application. Once old-style implicits are deprecated, context bounds will map to given clauses instead.

Extension Methods

Extension methods have no direct counterpart in Scala 2, but they can be simulated with implicit classes. For instance, the extension method

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

could be simulated to some degree by

implicit class CircleDeco(c: Circle) extends AnyVal {
  def circumference: Double = c.radius * math.Pi * 2
}

Extension methods in given instances have no direct counterpart in Scala-2. The only way to simulate these is to make implicit classes available through imports. The Simulacrum macro library can automate this process in some cases.

Typeclass Derivation

Typeclass derivation has no direct counterpart in the Scala 2 language. Comparable functionality can be achieved by macro-based libraries such as Shapeless, Magnolia, or scalaz-deriving.

Implicit Function Types

Implicit function types have no analogue in Scala 2.

Implicit By-Name Parameters

Implicit by-name parameters are not supported in Scala 2, but can be emulated to some degree by the Lazy type in Shapeless.

Simulating Scala 2 Implicits in Dotty

Implicit Conversions

Implicit conversion methods in Scala 2 can be expressed as given instances of the scala.Conversion class in Dotty. E.g. instead of

implicit def stringToToken(str: String): Token = new Keyword(str)

one can write

given stringToToken: Conversion[String, Token] {
  def apply(str: String): Token = new KeyWord(str)
}

Implicit Classes

Implicit classes in Scala 2 are often used to define extension methods, which are directly supported in Dotty. Other uses of implicit classes can be simulated by a pair of a regular class and a given instance of Conversion type.

Implicit Values

Implicit val definitions in Scala 2 can be expressed in Dotty using a regular val definition and an alias given. E.g., Scala 2's

lazy implicit val pos: Position = tree.sourcePos

can be expressed in Dotty as

lazy val pos: Position = tree.sourcePos
given Position = pos

Abstract Implicits

An abstract implicit val or def in Scala 2 can be expressed in Dotty using a regular abstract definition and an alias given. E.g., Scala 2's

implicit def symDeco: SymDeco

can be expressed in Dotty as

def symDeco: SymDeco
given SymDeco = symDeco

Implementation Status and Timeline

The Dotty implementation implements both Scala-2's implicits and the new abstractions. In fact, support for Scala-2's implicits is an essential part of the common language subset between 2.13/2.14 and Dotty. Migration to the new abstractions will be supported by making automatic rewritings available.

Depending on adoption patterns, old style implicits might start to be deprecated in a version following Scala 3.0.