ValueOf
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.
The value itself can conveniently be retrieved with Predef#valueOf, which requires a ValueOf
to be available in implicit scope.
The compiler provides instances of ValueOf[T]
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]
.
Attributes
- Source
- ValueOf.scala
- Graph
-
- Supertypes