scala.ValueOf.scala Maven / Gradle / Ivy
/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
package scala
/**
* `ValueOf[T]` provides the unique value of the type `T` where `T` is a type which has a
* single inhabitant. Eligible types are singleton types of the form `stablePath.type`,
* Unit and singleton types corresponding to value literals.
*
* Instances of `ValueOf[T]` are provided implicitly for all eligible types. Typically
* an instance would be required where a runtime value corresponding to a type level
* computation is needed.
* For example, we might define a type `Residue[M <: Int]` corresponding to the group of
* integers modulo `M`. We could then mandate that residues can be summed only when they
* are parameterized by the same modulus,
*
* {{{
* case class Residue[M <: Int](n: Int) extends AnyVal {
* def +(rhs: Residue[M])(implicit m: ValueOf[M]): Residue[M] =
* Residue((this.n + rhs.n) % valueOf[M])
* }
*
* val fiveModTen = Residue[10](5)
* val nineModTen = Residue[10](9)
*
* fiveModTen + nineModTen // OK == Residue[10](4)
*
* val fourModEleven = Residue[11](4)
*
* fiveModTen + fourModEleven // compiler error: type mismatch;
* // found : Residue[11]
* // required: Residue[10]
* }}}
*
* Notice that here the modulus is encoded in the type of the values and so does not
* incur any additional per-value storage cost. When a runtime value of the modulus
* is required in the implementation of `+` it is provided at the call site via the
* implicit argument `m` of type `ValueOf[M]`.
*/
@scala.annotation.implicitNotFound(msg = "No singleton value available for ${T}.")
final class ValueOf[T](val value: T) extends AnyVal