Implicit Conversions

Implicit conversions are defined by given instances of the scala.Conversion class. This class is defined in package scala as follows:

abstract class Conversion[-T, +U] extends (T => U)

For example, here is an implicit conversion from String to Token:

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

Using an alias this can be expressed more concisely as:

given Conversion[String, Token] = new KeyWord(_)

An implicit conversion is applied automatically by the compiler in three situations:

  1. If an expression e has type T, and T does not conform to the expression's expected type S.
  2. In a selection e.m with e of type T, but T defines no member m.
  3. In an application e.m(args) with e of type T, if T does define some member(s) named m, but none of these members can be applied to the arguments args.

In the first case, the compiler looks for a given scala.Conversion instance that maps an argument of type T to type S. In the second and third case, it looks for a given scala.Conversion instance that maps an argument of type T to a type that defines a member m which can be applied to args if present. If such an instance C is found, the expression e is replaced by C.apply(e).


  1. The Predef package contains "auto-boxing" conversions that map primitive number types to subclasses of java.lang.Number. For instance, the conversion from Int to java.lang.Integer can be defined as follows:
given int2Integer as Conversion[Int, java.lang.Integer] =
  1. The "magnet" pattern is sometimes used to express many variants of a method. Instead of defining overloaded versions of the method, one can also let the method take one or more arguments of specially defined "magnet" types, into which various argument types can be converted. E.g.
object Completions {

  // The argument "magnet" type
  enum CompletionArg {
    case Error(s: String)
    case Response(f: Future[HttpResponse])
    case Status(code: Future[StatusCode])
  object CompletionArg {

    // conversions defining the possible arguments to pass to `complete`
    // these always come with CompletionArg
    // They can be invoked explicitly, e.g.
    //   CompletionArg.fromStatusCode(statusCode)

    given fromString     as Conversion[String, CompletionArg]               = Error(_)
    given fromFuture     as Conversion[Future[HttpResponse], CompletionArg] = Response(_)
    given fromStatusCode as Conversion[Future[StatusCode], CompletionArg]   = Status(_)
  import CompletionArg._

  def complete[T](arg: CompletionArg) = arg match {
    case Error(s) => ...
    case Response(f) => ...
    case Status(code) => ...

This setup is more complicated than simple overloading of complete, but it can still be useful if normal overloading is not available (as in the case above, since we cannot have two overloaded methods that take Future[...] arguments), or if normal overloading would lead to a combinatorial explosion of variants.