Try Conversions

The effective expansion of ? is a little more complicated than previously indicated:

  1. expression?

works the same as

  1. match expression {
  2.     Ok(value) => value,
  3.     Err(err)  => return Err(From::from(err)),
  4. }

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.

Example

  1. use std::error::Error;
  2. use std::io::Read;
  3. use std::{fmt, fs, io};
  5. #[derive(Debug)]
  6. enum ReadUsernameError {
  7.     IoError(io::Error),
  8.     EmptyUsername(String),
  9. }
  11. impl Error for ReadUsernameError {}
  13. impl fmt::Display for ReadUsernameError {
  14.     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  15.         match self {
  16.             Self::IoError(e) => write!(f, "I/O error: {e}"),
  17.             Self::EmptyUsername(path) => write!(f, "Found no username in {path}"),
  18.         }
  19.     }
  20. }
  22. impl From<io::Error> for ReadUsernameError {
  23.     fn from(err: io::Error) -> Self {
  24.         Self::IoError(err)
  25.     }
  26. }
  28. fn read_username(path: &str) -> Result<String, ReadUsernameError> {
  29.     let mut username = String::with_capacity(100);
  30.     fs::File::open(path)?.read_to_string(&mut username)?;
  31.     if username.is_empty() {
  32.         return Err(ReadUsernameError::EmptyUsername(String::from(path)));
  33.     }
  34.     Ok(username)
  35. }
  37. fn main() {
  38.     //std::fs::write("config.dat", "").unwrap();
  39.     let username = read_username("config.dat");
  40.     println!("username or error: {username:?}");
  41. }

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.