Dotty Documentation


@implicitNotFound object ValueOf
extends Object with Serializable

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 = Residue10 val nineModTen = Residue10

fiveModTen + nineModTen // OK == Residue10

val fourModEleven = Residue11

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].

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