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package scala.implicits
/** A special class used to implement negation in implicit search.
*
* Consider the problem of using implicit `i1` for a query type `D` if an implicit
* for some other class `C` is available, and using an implicit `i2` if no implicit
* value of type `C` is available. If we do not want to prioritize `i1` and `i2` by
* putting them in different traits we can instead define the following:
*
* implicit def i1: D(implicit ev: C) = ...
* implicit def i2: D(implicit ev: Not[C]) = ...
*
* `Not` is treated specially in implicit search, similar to the way logical negation
* is treated in Prolog: The implicit search for `Not[C]` succeeds if and only if the implicit
* search for `C` fails.
*
* In Scala 2 this form of negation can be simulated by setting up a conditional
* ambiguous implicit and an unconditional fallback, the way it is done with the
* `default`, `amb1` and `amb2` methods below. Due to the way these two methods are
* defined, `Not` is also usable from Scala 2.
*
* In Dotty, ambiguity is a global error, and therefore cannot be used to implement negation.
* Instead, `Not` is treated natively in implicit search.
*/
final class Not[+T] private ()
trait LowPriorityNot {
/** A fallback method used to emulate negation in Scala 2 */
implicit def default[T]: Not[T] = Not.value
}
object Not extends LowPriorityNot {
/** A value of type `Not` to signal a successful search for `Not[C]` (i.e. a failing
* search for `C`). A reference to this value will be explicitly constructed by Dotty's
* implicit search algorithm
*/
def value: Not[Nothing] = new Not[Nothing]()
/** One of two ambiguous methods used to emulate negation in Scala 2 */
implicit def amb1[T](implicit ev: T): Not[T] = ???
/** One of two ambiguous methods used to emulate negation in Scala 2 */
implicit def amb2[T](implicit ev: T): Not[T] = ???
}
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