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
i2 by putting them in different traits we can instead define the following:
given i1: D(using ev: C) = ... given i2: D(using ev: NotGiven[C]) = ...
NotGiven is treated specially in implicit search, similar to the way logical negation is treated in Prolog: The implicit search for
NotGiven[C] succeeds if and only if the implicit search for
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
amb2 methods below. Due to the way these two methods are defined,
NotGiven is also usable from Scala 2.
In Dotty, ambiguity is a global error, and therefore cannot be used to implement negation. Instead,
NotGiven is treated natively in implicit search.