A special class used to implement negation in implicit search.
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:
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 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, 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.
- Companion
- object