Rust Tutorial (13) Results & Options

24 days from Node.js to Rust

preface

In tutorial 8, we mentioned options when we introduced HashMap. When we queried the data corresponding to the key value of the HashMap, we could not guarantee the existence of the corresponding data, so the return result must be “empty”. There is no such thing as undefined or null in Rust, and in order to safely represent “null”, Option is needed to solve this problem

“Empty” is like an expected error. In this case, either the Result is correct or the Result is “empty”, which means failure. In order to deal with the expected error processing, Result is created. Result and Option often occur together, are handled similarly, and can be converted to each other if necessary

The body of the

reading

The Rust Book: ch 06 – Enums
Rust by Example: Enums
The Rust Reference: Enumerations
Rust by Example: match
Rust by Example: if let
Rust docs: Result
Rust docs: Option

Option

If you haven’t studied Enums thoroughly, read the articles listed above

Rust’s enums differs from many other languages in that the Option enums is defined as follows:

pub enum Option<T> {
    /// No value
    None./// Some value `T`
    Some(T),
}
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The value can be either Option::None for “empty” or Option::Some(T) for Some value. We’ve touched on generics before, and you can read more about them in the reading list for tutorial 10

Creating and using options in Rust is simple:

fn main() {
    let some = returns_some();
    println!("{:? }", some);
    let none = returns_none();
    println!("{:? }", none);
}

fn returns_some() - >Option<String> {
    Some("my string".to_owned())
}

fn returns_none() - >Option<String> {
    None
}
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When returning None, we also need to specify T in Option

Note: We can substitute Some and None for Option::Some() and Option::None because they were pre-introduced by Rust, read STD :: Prelude for information.

Result

The value of the Result::Err variable usually follows some convention, but if you look at the implementation, there are actually no constraints

pub enum Result<T, E> {
  Ok(T),
  Err(E),
}
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Is there anything special about the creation of Ok() and Err()

fn main() {
    let value = returns_ok();
    println!("{:? }", value);

    let value = returns_err();
    println!("{:? }", value);
}

fn returns_ok() - >Result<String, MyError> {
    Ok("This turned out great!".to_owned())
}

fn returns_err() - >Result<String, MyError> {
    Err(MyError("This failed horribly.".to_owned()))
}

#[derive(Debug)]
struct MyError(String);
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Struct is used here to indicate that Err can contain any value, such as struct, String, HashMap, or whatever

.unwrap()

Option and Result look straightforward, but are they really?

The confusion comes from how to get the value. You’ve already encountered.unwrap() in this tutorial, and if you use.unwrap() for None or Err, you’ll get an error. With Rust, if you pay enough attention to warnings, you can reduce exceptions by 99% and achieve faster, more stable results

How to get a value

.unwrap()

Use.unwrap() only if you make sure Some() or Ok() is used as an example of the previous IP address:

let ip_address = std::net::Ipv4Addr::from_str("127.0.0.1").unwrap();
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Unwrap () is based on the coder’s knowledge of the code and needs to be able to verify its value before using it. For example, we can use.contains_key() to verify that HashMaps contain the specified key, and if so we can safely use.unwrap()

.unwrap_or()

.unwrap_or() will provide a default value for failure. The default value needs to be of the same type as the T for Ok(T) and Some(T) :

fn main() {
    let value = returns_ok();
    println!("{:? }", value.unwrap());

    let value = returns_err();
    // This must be of type String
    println!("{:? }", value.unwrap_or("It really failed.".to_owned()));
}

fn returns_ok() - >Result<String.String> {
    Ok("Success!".to_owned())
}

fn returns_err() - >Result<String.String> {
    Err("Fail!".to_owned())
}

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.unwrap_or_else(|| {})

.unwrap_or_else() takes a function and will take its return value in the case of None or Err. This can be used if the default value is expensive to calculate and there is no value in calculating it in advance

As with.unwrap_or(), the return type of the function needs to be the same as T

let unwrap_or_else = returns_none()
  .unwrap_or_else(|| format!("Default value from a function at time {:? }", Instant::now()));

println!(
  "returns_none().unwrap_or_else(|| {{... }}) : {:? }",
  unwrap_or_else
);
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| |… Closure syntax in Rust, more on this later

unwrap_or_default()

Unwrap_or_default () follows the Default value of the specified type if it does not exist. Default is a trait like Debug or Display.

In TypeScript you might write code like this:

et my_string = maybe_undefined || "";
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In Rust it looks like this:

let my_string = maybe_none.unwrap_or_default(); // Assuming `T` is `String`.
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You can implement Default like this:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() - >Self { Kind::A }
}
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Match expression

You can use match expressions to get values:

let match_value = match returns_some() {
  Some(val) => val,
  None= >"My default value".to_owned(),
};

println!("match {{... }}, {:? }", match_value);
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If let expression

You can apply a conditional judgment to the specified variant of an enumeration:

if let Some(val) = returns_some() { println! ("if let : {:? }", val); }Copy the code

If the Option returned by returns_some() is Some(), its internal value will be bound to the identifier val, which is a bit of a strange syntax, but useful

? The operator

What does the following code show? Operator tips:

use std::fs::read_to_string;

fn main() - >Result<(), std::io::Error> {
    let html = render_markdown(".. /README.md")? ;println!("{}", html);
    Ok(())}fn render_markdown(file: &str) - >Result<String, std::io::Error> {
    letsource = read_to_string(file)? ;Ok(markdown::to_html(&source))
}
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First of all we use we use in number three-> Result<(), std::io::Error>willmain()Change the return value type toResult, pay attention to(a)isunit type“This is another way of saying empty.-> Result<(), ... >Means return null or fail
Second, in line 10 we usestd::fs::read_to_string(), it takes a path and returnsResult<String, std::io::Error>That means it will returnStringType of file content orstd::io::ErrorType error
And then line 10?Operator automaticallyunwrapIf the result is failure then?The operator returns the result directly to the callermain()function
We continue in line 4?Operator to do automaticunwrapBecause ofmain()The function has no caller, so if there is a failure, the program terminates
Due to themain()Is the return type ofResult<(), ... >So we need to write another one at the endOk(())

? With the try!

In some old articles, you’ll read about try! The contents of the macro, try! In fact, right? No matter who’s good or bad, it’s always good to know more about the pioneers of operators. Try! The implementation of the

Macros are not the focus of this tutorial, but take a look at try! Implementation of the source code, I believe it is not difficult to understand:

macro_rules! r#try{ ($expr:expr $(,)?) = > {match $expr {
            $crate::result::Result: :Ok(val) => val,
            $crate::result::Result: :Err(err) => {
                return $crate::result::Result: :Err($crate::convert::From::from(err)); }}}; }Copy the code

try! The macro receives an expression and uses it in the match statement, returning its value if the expression is Ok, or returning it early and converting the return value to Result Error if it fails

Error handling

Finally, let’s look at the difference if the file path is retrieved from an environment variable:

use std::fs::read_to_string;

fn main() - >Result<(), std::io::Error> {
    lethtml = render_markdown()? ;println!("{}", html);
    Ok(())}fn render_markdown() - >Result<String, std::io::Error> {
    let file = std::env::var("MARKDOWN")? ;letsource = read_to_string(file)? ;Ok(markdown::to_html(&source))
}
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We added a line of code to get the file path from an environment variable named “MARKDOWN”. If this environment variable is not present, the program will report an error. Expression to short-circuit, and now we get a compilation error:? Could not convert the error to STD :: IO :: error…

Now that you know Option and Result, that’s not the most difficult part, but how do you handle different errors, which we’ll talk about in the next tutorial

conclusion

Options and results are ubiquitous in Rust, and you need to understand them correctly. Enums are also ubiquitous, and you often use Enums rather than specific strings, numbers, or Booleans

In the next article, we’ll introduce you to Err

More and more

Rust tutorial (1) From NVM to Rust
Rust tutorial (2) from NPM to Cargo for the front end
Configure Visual Studio Code in Rust (3)
Rust tutorial (4) Hello World
Rust Tutorial for the front end (5) Borrowing & Ownership
Part 1 of Rust Tutorial (6) String
Rust Tutorial (7)
Rust Tutorial (8)
Rust Tutorial (9)
Rust tutorial for the front end (10) from Mixins to Traits
Rust Tutorial (11) Module for the front-end
Part 2 of Rust Tutorial (12) String
Rust Tutorial (13) Results & Options

Rust Tutorial (13) Results & Options

preface

The body of the

reading

Option

Result

.unwrap()

How to get a value

.unwrap()

.unwrap_or()

.unwrap_or_else(|| {})

unwrap_or_default()

Match expression

If let expression

? The operator

? With the try!

Error handling

conclusion

More and more

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