Dotty Documentation

0.17.0-bin-SNAPSHOT

Implicit By-Name Parameters

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Implicit parameters can be declared by-name to avoid a divergent inferred expansion. Example:

trait Codec[T] {
  def write(x: T): Unit
}

delegate intCodec for Codec[Int] = ???

delegate optionCodec[T] for Codec[Option[T]] given (ev: => Codec[T]) {
  def write(xo: Option[T]) = xo match {
    case Some(x) => ev.write(x)
    case None =>
  }
}

val s = the[Codec[Option[Int]]]

s.write(Some(33))
s.write(None)

As is the case for a normal by-name parameter, the argument for the implicit parameter ev is evaluated on demand. In the example above, if the option value x is None, it is not evaluated at all.

The synthesized argument for an implicit parameter is backed by a local val if this is necessary to prevent an otherwise diverging expansion.

The precise steps for synthesizing an argument for an implicit by-name parameter of type => T are as follows.

  1. Create a new delegate for type T:

    delegate lv for T = ???

    where lv is an arbitrary fresh name.

  2. This delegate is not immediately available as candidate for argument inference (making it immediately available could result in a loop in the synthesized computation). But it becomes available in all nested contexts that look again for an argument to an implicit by-name parameter.

  3. If this search succeeds with expression E, and E contains references to the delegate lv, replace E by

    { delegate lv for T = E; lv }

    Otherwise, return E unchanged.

In the example above, the definition of s would be expanded as follows.

val s = the[Test.Codec[Option[Int]]](
  optionCodec[Int](intCodec))

No local delegate was generated because the synthesized argument is not recursive.

Reference

For more info, see Issue #1998 and the associated Scala SIP.