محاولات التحويل (Try Conversions)
The effective expansion of ?
is a little more complicated than previously indicated:
expression?
works the same as
match expression {
Ok(value) => value,
Err(err) => return Err(From::from(err)),
}
The From::from
call here means we attempt to convert the error type to the type returned by the function. This makes it easy to encapsulate errors into higher-level errors.
مثال (Example)
Speaker Notes
This slide should take about 5 minutes.
The ?
operator must return a value compatible with the return type of the function. For Result
, it means that the error types have to be compatible. A function that returns Result<T, ErrorOuter>
can only use ?
on a value of type Result<U, ErrorInner>
if ErrorOuter
and ErrorInner
are the same type or if ErrorOuter
implements From<ErrorInner>
.
A common alternative to a From
implementation is Result::map_err
, especially when the conversion only happens in one place.
There is no compatibility requirement for Option
. A function returning Option<T>
can use the ?
operator on Option<U>
for arbitrary T
and U
types.
A function that returns Result
cannot use ?
on Option
and vice versa. However, Option::ok_or
converts Option
to Result
whereas Result::ok
turns Result
into Option
.