Implicit Conversions

Implicit conversions are a powerful Scala feature that allows users to supply an argument of one type as if it were another type, to avoid boilerplate.

Note that in Scala 2, implicit conversions were also used to provide additional members to closed classes (see Implicit Classes). In Scala 3, we recommend to address this use-case by defining extension methods instead of implicit conversions (although the standard library still relies on implicit conversions for historical reasons).

Example

Consider for instance a method findUserById that takes a parameter of type Long:

def findUserById(id: Long): Option[User]

We omit the definition of the type User for the sake of brevity, it does not matter for our example.

In Scala, it is possible to call the method findUserById with an argument of type Int instead of the expected type Long, because the argument will be implicitly converted into the type Long:

val id: Int = 42
findUserById(id) // OK

This code does not fail to compile with an error like “type mismatch: expected Long, found Int” because there is an implicit conversion that converts the argument id to a value of type Long.

Detailed Explanation

This section describes how to define and use implicit conversions.

Defining an Implicit Conversion

import scala.language.implicitConversions

implicit def int2long(x: Int): Long = x.toLong

given int2long: Conversion[Int, Long] with
  def apply(x: Int): Long = x.toLong

given Conversion[Int, Long] with
  def apply(x: Int): Long = x.toLong

given Conversion[Int, Long] = (x: Int) => x.toLong

Using an Implicit Conversion

Implicit conversions are applied in two situations:

1. If an expression e is of type S, and S does not conform to the expression’s expected type T.
2. In a selection e.m with e of type S, if the selector m does not denote a member of S (to support Scala-2-style extension methods).

In the first case, a conversion c is searched for, which is applicable to e and whose result type conforms to T.

In our example above, when we pass the argument id of type Int to the method findUserById, the implicit conversion int2long(id) is inserted.

In the second case, a conversion c is searched for, which is applicable to e and whose result contains a member named m.

An example is to compare two strings "foo" < "bar". In this case, String has no member <, so the implicit conversion Predef.augmentString("foo") < "bar" is inserted. (scala.Predef is automatically imported into all Scala programs.)

How Are Implicit Conversions Brought Into Scope?

When the compiler searches for applicable conversions:

• first, it looks into the current lexical scope
  • implicit conversions defined in the current scope or the outer scopes
  • imported implicit conversions
  • implicit conversions imported by a wildcard import (Scala 2 only)
• then, it looks into the companion objects associated with the argument type S or the expected type T. The companion objects associated with a type X are:
  • the companion object X itself
  • the companion objects associated with any of X’s inherited types
  • the companion objects associated with any type argument in X
  • if X is an inner class, the outer objects in which it is embedded

For instance, consider an implicit conversion fromStringToUser defined in an object Conversions:

import scala.language.implicitConversions

object Conversions {
  implicit def fromStringToUser(name: String): User = (name: String) => User(name)
}

object Conversions:
  given fromStringToUser: Conversion[String, User] = (name: String) => User(name)

The following imports would equivalently bring the conversion into scope:

import Conversions.fromStringToUser
// or
import Conversions._

import Conversions.fromStringToUser
// or
import Conversions.given
// or
import Conversions.{given Conversion[String, User]}

In the introductory example, the conversion from Int to Long does not require an import because it is defined in the object Int, which is the companion object of the type Int.

Further reading: Where does Scala look for implicits? (on Stackoverflow).

Beware the Power of Implicit Conversions