묵시적 형변환

실제로 ?가 적용되는 과정은 아까 설명한 것 보다 좀 더 복잡합니다:

expression?

위 표현은 아래와 같습니다

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.

예제

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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>.

From 구현의 일반적인 대안은 특히 변환이 한 곳에서만 발생하는 경우 Result::map_err입니다.

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.