Rust Search
Find the best content on Rust, curated by the community; a
search engine for Rustaceans.
...Understanding match in Rust
Feel free to skip to the next section if you already understand pattern matching.
Before exploring how that’s even possible, let’s understand Rust’s idea of pattern matching. Here’s a scenario:
A hungry customer asks for a meal from our Korean street food...
...i32 = match some_bool {
true => 23,
false => panic!("aaah!"), // an expression of type `!`, gets cast to `i32`
}
match break {
() => 23, // matching with a `()` forces the match argument to be cast to type `()`
}
These casts can be implemented by having the compiler assign a fresh, diverging
type variable to any expression...
...u32 = match <u32 as TryFrom<u8>>::try_from(42) { | Ok(it) => it, | Err(unreachable) => unreachable, // Error, expected `u32`, found `Infallible` }; but the following doesn't! use ::never_say_never::Never; let x: u32 = match Ok::<_, Never>(42) { | Ok(it) => it, | Err(unreachable) => unreachable, };
...i32 = match some_bool {
true => 23,
false => panic!("aaah!"), // an expression of type `!`, gets cast to `i32`
}
match break {
() => 23, // matching with a `()` forces the match argument to be cast to type `()`
}
```
These casts can be implemented by having the compiler assign a fresh, diverging
type variable to any expression...
...Debug;
fn start_state() -> Self::State;
fn next_state(&self, state: &Self::State, input: &Input) -> Self::State;
fn get_match(&self, state: &Self::State, text_offset: usize) -> Option<Match<Payload>>;
fn find<'i, 'a>(&'a self, s: &'i [Input]) -> Matches<'i, 'a, Input, Payload, Self>
where Self: Sized
{
Matches {
aut...
...Some libraries (like Python's `re` and RE2/C++) distinguish between matching an
expression against an entire string and matching an expression against part of
the string. My implementation favors simplicity: matching the entirety of a
string requires using the `^` and/or `$` anchors. In all cases, an implicit
`.*?` is added...
No More Nulls Subject: Enumerated (enum) types, pattern matching, and meaningful return types. Notes Today’s episode discusses, in order: Enumerated types, with an eye to the difference between structs and enums, and to...
...When match is involved, you can usually arrange for the misbehaving borrow to be performed outside of the match statement, in a new, non-overlapping lexical scope. This is easy when the relevant branch does not rely on any variables from the pattern-match by using short-circuiting control operators...
...The second arm of the outer match stays the same, so the program panics on any error besides the missing file error.
Alternatives to Using match with Result<T, E>
That’s a lot of match! The match expression is very useful but also very much a primitive. In Chapter...
...io::Result<()> {
match OpenOptions::new().create(true).write(true).open(path) {
Ok(_) => Ok(()),
Err(e) => Err(e),
}
}
fn main() {
println!("`mkdir a`");
// Create a directory, returns `io::Result<()>`
match fs::create_dir("a") {
Err(why) => println!("! {:?}", why.kind()),
Ok(_) => {},
}
println!("`echo hello > a/b.txt`");
// The previous match can be...
...use croner::Cron; use chrono::Local; fn main() { // Parse cron expression let cron_all = Cron::from_str("18 * * * 5") .expect("Couldn't parse cron string"); // Compare cron pattern with current local time let time = Local::now(); let matches_all = cron_all.is_time_matching(&time).unwrap(); // Get next match let...
gruebelinchen.wordpress.com
...It uses a placeholder, which we call ActualT, to specify the type of the value being matched. Rust provides two ways to express this. One can define a generic trait with a type parameter:
123trait Matcher<ActualT> { fn matches(&self, actual: &ActualT) -> bool;}
Or one can add an associated type...
...However, we still have to pattern match for dynamic behavior at runtime. We can’t pattern match on the BufferArrayData itself but instead have to pattern match on the separate type metadata.
fn add(a: &ArrayData, a_type: DataType, b: &ArrayData, b_type: DataType) -> Rc<ArrayData> {
match (a_type, b...
...2, ..Default::default() };
Pattern match guards
Sometimes when pattern matching, the cases you want to handle
don’t map exactly to the patterns of the data you’re matching
on. For instance, you might write some code like this:
fn divide_opt(x: Option<i32>, y: Option<i32>) -> Option<i32...
...f64) -> f64 {
// This is a three level match pyramid!
match checked::div(x, y) {
Err(why) => panic!("{:?}", why),
Ok(ratio) => match checked::ln(ratio) {
Err(why) => panic!("{:?}", why),
Ok(ln) => match checked::sqrt(ln) {
Err(why) => panic!("{:?}", why),
Ok(sqrt) => sqrt,
},
},
}
}
fn main() {
// Will this fail?
println!("{}", op(1.0...
tuples
Tuples can be destructured in a match as follows:
fn main() {
let triple = (0, -2, 3);
// TODO ^ Try different values for `triple`
println!("Tell me about {:?}", triple);
// Match can be used to destructure a tuple
match triple {
// Destructure the second and third elements
(0, y, z) => println!("First is...
...Consider what happens
with matches. Consider:
1
2
3
4
match ... {
x => { ... }
y => { ... }
}
Is x or y a variable, or a variant? There’s no way to know unless you
perform name resolution, otherwise known as the resolve pass in the compiler.
Unfortunately though, there’s no way for Stateful...
...let role = Student;
match stage {
// Note the lack of scoping because of the explicit `use` above.
Beginner => println!("Beginners are starting their learning journey!"),
Advanced => println!("Advanced learners are mastering their subjects..."),
}
match role {
// Note again the lack of scoping.
Student => println!("Students are acquiring knowledge!"),
Teacher => println!("Teachers are...
...The innermost expression that encloses both of these
expressions is the match itself (as depicted above), and hence the
borrow is considered to extend until the end of the
match. Unfortunately, the match encloses not only the Some branch,
but also the None branch, and hence when we go to...
...String) -> Result<i32, exitcode::ExitCode> { match s.parse::<i32>() { Ok(i) => Ok(i), Err(_) => Err(exitcode::USAGE) } } pub fn main() { match parse_int_or_return_error_exitcode("123".to_string()) { Ok(i) => println!("Parsed: {}", i), Err(code) => { println!("Parse error. Exiting with code: {}", code); process::exit(code); } } match parse_int...
"Congrats to the #Rustlang and #Rust-for-#Linux community: the #LinuxKernel now contains the first useful thing built using Rust!"