Function enum_err 

pub fn enum_err<'a, T, E>() -> EnumKpType<'a, Result<T, E>, E>

Extract from Result<T, E> - Err variant

Example

let result: Result<String, i32> = Err(42);
let err_kp = enum_err();
assert_eq!(err_kp.get(&result), Some(&42));