# Module ethcore_util::standard::result 1.0.0 [−] [src]

Error handling with the `Result` type.

`Result<T, E>` is the type used for returning and propagating errors. It is an enum with the variants, `Ok(T)`, representing success and containing a value, and `Err(E)`, representing error and containing an error value.

    enum Result<T, E> {
       Ok(T),
       Err(E),
    }

Functions return `Result` whenever errors are expected and recoverable. In the `std` crate, `Result` is most prominently used for I/O.

A simple function returning `Result` might be defined and used like so:

    #[derive(Debug)]
    enum Version { Version1, Version2 }

    fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
        match header.get(0) {
            None => Err("invalid header length"),
            Some(&1) => Ok(Version::Version1),
            Some(&2) => Ok(Version::Version2),
            Some(_) => Err("invalid version"),
        }
    }

    let version = parse_version(&[1, 2, 3, 4]);
    match version {
        Ok(v) => println!("working with version: {:?}", v),
        Err(e) => println!("error parsing header: {:?}", e),
    }

Pattern matching on `Result`s is clear and straightforward for simple cases, but `Result` comes with some convenience methods that make working with it more succinct.

    let good_result: Result<i32, i32> = Ok(10);
    let bad_result: Result<i32, i32> = Err(10);

    // The `is_ok` and `is_err` methods do what they say.
    assert!(good_result.is_ok() && !good_result.is_err());
    assert!(bad_result.is_err() && !bad_result.is_ok());

    // `map` consumes the `Result` and produces another.
    let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
    let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);

    // Use `and_then` to continue the computation.
    let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));

    // Use `or_else` to handle the error.
    let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));

    // Consume the result and return the contents with `unwrap`.
    let final_awesome_result = good_result.unwrap();

# Results must be used

A common problem with using return values to indicate errors is that it is easy to ignore the return value, thus failing to handle the error. `Result` is annotated with the `#[must_use]` attribute, which will cause the compiler to issue a warning when a Result value is ignored. This makes `Result` especially useful with functions that may encounter errors but don't otherwise return a useful value.

Consider the `write_all` method defined for I/O types by the `Write` trait:

    use std::io;

    trait Write {
        fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
    }

Note: The actual definition of `Write` uses `io::Result`, which is just a synonym for `Result` `<T,` `io::Error` `>`.

This method doesn't produce a value, but the write may fail. It's crucial to handle the error case, and not write something like this:

    use std::fs::File;
    use std::io::prelude::*;

    let mut file = File::create("valuable_data.txt").unwrap();
    // If `write_all` errors, then we'll never know, because the return
    // value is ignored.
    file.write_all(b"important message");

If you do write that in Rust, the compiler will give you a warning (by default, controlled by the `unused_must_use` lint).

You might instead, if you don't want to handle the error, simply assert success with `expect`. This will panic if the write fails, providing a marginally useful message indicating why:

    use std::fs::File;
    use std::io::prelude::*;

    let mut file = File::create("valuable_data.txt").unwrap();
    file.write_all(b"important message").expect("failed to write message");

You might also simply assert success:

    assert!(file.write_all(b"important message").is_ok());

Or propagate the error up the call stack with `try!`:

    fn write_message() -> io::Result<()> {
        let mut file = try!(File::create("valuable_data.txt"));
        try!(file.write_all(b"important message"));
        Ok(())
    }

# The `try!` macro

When writing code that calls many functions that return the `Result` type, the error handling can be tedious. The `try!` macro hides some of the boilerplate of propagating errors up the call stack.

It replaces this:

    use std::fs::File;
    use std::io::prelude::*;
    use std::io;

    struct Info {
        name: String,
        age: i32,
        rating: i32,
    }

    fn write_info(info: &Info) -> io::Result<()> {
        // Early return on error
        let mut file = match File::create("my_best_friends.txt") {
               Err(e) => return Err(e),
               Ok(f) => f,
        };
        if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
            return Err(e)
        }
        if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
            return Err(e)
        }
        if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
            return Err(e)
        }
        Ok(())
    }

With this:

    use std::fs::File;
    use std::io::prelude::*;
    use std::io;

    struct Info {
        name: String,
        age: i32,
        rating: i32,
    }

    fn write_info(info: &Info) -> io::Result<()> {
        let mut file = try!(File::create("my_best_friends.txt"));
        // Early return on error
        try!(file.write_all(format!("name: {}\n", info.name).as_bytes()));
        try!(file.write_all(format!("age: {}\n", info.age).as_bytes()));
        try!(file.write_all(format!("rating: {}\n", info.rating).as_bytes()));
        Ok(())
    }

It's much nicer!

Wrapping an expression in `try!` will result in the unwrapped success (`Ok`) value, unless the result is `Err`, in which case `Err` is returned early from the enclosing function. Its simple definition makes it clear:

    macro_rules! try {
        ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
    }

`try!` is imported by the prelude and is available everywhere, but it can only be used in functions that return `Result` because of the early return of `Err` that it provides.

## Structs

IntoIter | An iterator over the value in a `Ok` variant of a `Result`. This struct is created by the `into_iter` method on `Result` (provided by the `IntoIterator` trait).
Iter | An iterator over a reference to the `Ok` variant of a `Result`.
IterMut | An iterator over a mutable reference to the `Ok` variant of a `Result`.

## Enums

Result | `Result` is a type that represents either success (`Ok`) or failure (`Err`).

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