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Chaining results using match can get pretty untidy; luckily, the ? operator can be used to make things pretty again. ? is used at the end of an expression returning a Result, and is equivalent to a match expression, where the Err(err) branch expands to an early return Err(From::from...
...ReverseSearcher<&'a Self>;
pub fn trim_matches<'a, P>(&'a self, pat: P) -> &'a Self
where
P: Pattern<&'a Self>,
P::Searcher: DoubleEndedSearcher<&'a Self>;
pub fn trim_left_matches<'a, P>(&'a self, pat: P) -> &'a Self
where
P: Pattern<&'a Self>;
pub fn trim_right_matches<'a, P...
...The exception is that the outer delimiters for the matcher will match any pair of delimiters. Thus, for instance, the matcher (()) will match {()} but not {{}}. The character $ cannot be matched or transcribed literally.
Forwarding a matched fragment
When forwarding a matched fragment to another macro-by-example, matchers in the...
Rust’s match statement can do a lot of things, even C-style fallthough to the next branch, despite having no real support for it. It turns out to be a “shallow” feature, where the C to Rust translation is easily done, without needing to understand the code itself. The...
...IS EMPTY matches existing attributes with empty value while IS NULL matches fields with a null value.
Here’s an example considering the following documents:
[
{
"id": 0,
"color": []
},
{
"id": 1,
"color": null
},
{
"id": 2,
}
]
The new filter operators work like this:
color IS EMPTY: matches document 0
color IS NULL...
...fn env_get(k: &str, env: &RispEnv) -> Option<RispExp> {
match env.data.get(k) {
Some(exp) => Some(exp.clone()),
None => {
match &env.outer {
Some(outer_env) => env_get(k, &outer_env),
None => None
}
}
}
}
fn eval(exp: &RispExp, env: &mut RispEnv) -> Result<RispExp, RispErr> {
match exp {
RispExp::Symbol(k) =>
env_get...
...fn read_username_from_file() -> Result<String, io::Error> {
let f = File::open("username.txt");
let mut f = match f {
Ok(file) => file,
Err(e) => return Err(e),
};
let mut s = String::new();
match f.read_to_string(&mut s) {
Ok(_) => Ok(s),
Err(e) => Err(e),
}
}
Enter fullscreen mode...
...Write,
{
match var {
Var::One => serde_json::to_writer(&mut writer, &Foo),
Var::Two => serde_json::to_writer(&mut writer, &Bar),
}
}
fn write_pretty<W>(var: Var, mut writer: W) -> serde_json::Result<()>
where
W: Write,
{
match var {
Var::One => serde_json::to_writer_pretty(&mut writer, &Foo),
Var::Two...
...One macro I really like is matches!.
It's basically a single-arm match expression, but you can readably use it in if statements.
So, this:
match data_type.id {
ParsedDataTypeId::Standard(ref data_type_id) if data_type_id == "data-trait-name" => {
// ...
}
_ => {}
}
Can be written as:
if matches!(data...
...This is a powerful construct in that it can match multiple patterns that you provide in a single
scan. Using this idea, I thought it would be fun to write a router with a RegexSet matching
requests under the hood.
The result of this is reroute. This crate provides a...
...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...
...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...
...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...
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...
"In Rust it’s the compiler that complains, with C++ it’s the colleagues"