c9f66fd425
This is the result of running `dprint fmt` after removing `src/` from
the list of excluded directories.
This also reformats the Rust code: we might want to tweak this a bit in
the future since some of the changes removes the hand-formatting. Of
course, this formatting can be seen as a mis-feature, so maybe this is
good overall.
Thanks to mdbook-i18n-helpers 0.2, the POT file is nearly unchanged
after this, meaning that all existing translations remain valid! A few
messages were changed because of stray whitespace characters:
msgid ""
"Slices always borrow from another object. In this example, `a` has to remain "
-"'alive' (in scope) for at least as long as our slice. "
+"'alive' (in scope) for at least as long as our slice."
msgstr ""
The formatting is enforced in CI and we will have to see how annoying
this is in practice for the many contributors. If it becomes annoying,
we should look into fixing dprint/check#11 so that `dprint` can annotate
the lines that need fixing directly, then I think we can consider more
strict formatting checks.
I added more customization to `rustfmt.toml`. This is to better emulate
the dense style used in the course:
- `max_width = 85` allows lines to take up the full width available in
our code blocks (when taking margins and the line numbers into account).
- `wrap_comments = true` ensures that we don't show very long comments
in the code examples. I edited some comments to shorten them and avoid
unnecessary line breaks — please trim other unnecessarily long comments
when you see them! Remember we're writing code for slides 😄
- `use_small_heuristics = "Max"` allows for things like struct literals
and if-statements to take up the full line width configured above.
The formatting settings apply to all our Rust code right now — I think
we could improve this with https://github.com/dprint/dprint/issues/711
which lets us add per-directory `dprint` configuration files. However,
the `inherit: true` setting is not yet implemented (as far as I can
tell), so a nested configuration file will have to copy most or all of
the top-level file.
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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.
Example
use std::error::Error;
use std::fmt::{self, Display, Formatter};
use std::fs::File;
use std::io::{self, Read};
#[derive(Debug)]
enum ReadUsernameError {
IoError(io::Error),
EmptyUsername(String),
}
impl Error for ReadUsernameError {}
impl Display for ReadUsernameError {
fn fmt(&self, f: &mut Formatter) -> fmt::Result {
match self {
Self::IoError(e) => write!(f, "IO error: {e}"),
Self::EmptyUsername(path) => write!(f, "Found no username in {path}"),
}
}
}
impl From<io::Error> for ReadUsernameError {
fn from(err: io::Error) -> Self {
Self::IoError(err)
}
}
fn read_username(path: &str) -> Result<String, ReadUsernameError> {
let mut username = String::with_capacity(100);
File::open(path)?.read_to_string(&mut username)?;
if username.is_empty() {
return Err(ReadUsernameError::EmptyUsername(String::from(path)));
}
Ok(username)
}
fn main() {
//fs::write("config.dat", "").unwrap();
let username = read_username("config.dat");
println!("username or error: {username:?}");
}
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.