GATs 泛型关联类型
GATs(generic associated types) allows types in trait to be generic.
In some reason, you wnat to let map() method returns Option<U> or Result<U>.
Declare trait:
trait Mappable {
type Item;
type Result<U>;
fn map<U, P: FnMut(Self::Item) -> U>(self, f: P)
-> Self::Result<U>;
}Implement it for both Option and Result:
impl<T> Mappable for Option<T> {
type Item = T;
type Result<U> = Option<U>;
fn map<U, P: FnMut(Self::Item) -> U>(self, f: P) -> Option<U> {
self.map(f)
}
}
impl<T, E> Mappable for Result<T, E> {
type Item = T;
type Result<U> = Result<U, E>;
fn map<U, P: FnMut(Self::Item) -> U>(self, f: P) -> Result<U, E> {
self.map(f)
}
}Finally, you get the ability to implemnet function which is generic over generic like this:
fn zero_to_42<C: Mappable<Item = i32>>(c: C) -> <C as Mappable>::Result<i32> {
c.map(|x| if x == 0 { 42 } else { x })
}