Provides a representation of Generic[T], which has a nested Repr type, as a type with two type parameters instead.
Provides a representation of Generic[T], which has a nested Repr type, as a type with two type parameters instead.
This is useful for two reasons. First, it's surprisingly easy to wind up with a Generic type that has lost the refinement that carries the crucial Generic.Repr type, a problem which Generic.Aux prevents.
More importantly, Aux allows us to write code like this:
def myMethod[T, R]()(implicit eqGen: Generic.Aux[T,R], repEq: Eq[R]) = ???
Here, we specify T, and we find a Generic.Aux[T,R] by implicit search. We then use R in the second argument. Generic.Aux[T, R] is exactly equivalent to Generic[T] { type Repr = R }, but Scala doesn't allow us to write it this way:
def myMethod[T, R]()(eqGen: Generic[T] { Repr = R }, reqEq: Eq[egGen.Repr]) = ???The reason is that we are not allowed to have dependencies between arguments in the same parameter group. So Aux neatly sidesteps this problem.
The "Aux pattern" is now in use in several other libraries as well, and is a useful general technique.
Provides an instance of Generic.
Provides an instance of Generic. Prefer this over finding one with implicitly, or else use the.
Either of these approaches preserves the Repr type refinement, which implicitly will lose.
The companion object for the Generic trait provides a way of obtaining a Generic[T] instance for some T. In addition, it defines Generic.Aux, which is an important implementation technique that can be generally useful.