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.
1.5 KiB
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Dynamic Error Types
Sometimes we want to allow any type of error to be returned without writing our
own enum covering all the different possibilities. The std::error::Error trait
makes it easy to create a trait object that can contain any error.
use std::error::Error;
use std::fs;
use std::io::Read;
fn read_count(path: &str) -> Result<i32, Box<dyn Error>> {
let mut count_str = String::new();
fs::File::open(path)?.read_to_string(&mut count_str)?;
let count: i32 = count_str.parse()?;
Ok(count)
}
fn main() {
fs::write("count.dat", "1i3").unwrap();
match read_count("count.dat") {
Ok(count) => println!("Count: {count}"),
Err(err) => println!("Error: {err}"),
}
}
The read_count function can return std::io::Error (from file operations) or
std::num::ParseIntError (from String::parse).
Boxing errors saves on code, but gives up the ability to cleanly handle
different error cases differently in the program. As such it's generally not a
good idea to use Box<dyn Error> in the public API of a library, but it can be
a good option in a program where you just want to display the error message
somewhere.
Make sure to implement the std::error::Error trait when defining a custom
error type so it can be boxed. But if you need to support the no_std
attribute, keep in mind that the std::error::Error trait is currently
compatible with no_std in
nightly only.