Rust by Example

Defining an error type

Sometimes it simplifies the code to mask all of the different errors with a single type of error. We'll show this with a custom error.

Rust allows us to define our own error types. In general, a "good" error type:

• Represents different errors with the same type

• Presents nice error messages to the user

• Is easy to compare with other types

• Good: Err(EmptyVec)

• Bad: Err("Please use a vector with at least one element".to_owned())

• Can hold information about the error

• Good: Err(BadChar(c, position))

• Bad: Err("+ cannot be used here".to_owned())

• Composes well with other errors

use std::fmt;

type Result = std::result::Result;

// Define our error types. These may be customized for our error handling cases.

// Now we will be able to write our own errors, defer to an underlying error

// implementation, or do something in between.

#[derive(Debug, Clone)]

struct DoubleError;

// Generation of an error is completely separate from how it is displayed.

// There's no need to be concerned about cluttering complex logic with the display style.

//

// Note that we don't store any extra info about the errors. This means we can't state

// which string failed to parse without modifying our types to carry that information.

impl fmt::Display for DoubleError {

fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {

write!(f, "invalid first item to double")

}

}

fn double_first(vec: Vec<&str>) -> Result {

vec.first()

// Change the error to our new type.

.ok_or(DoubleError)

.and_then(|s| {

s.parse::()

// Update to the new error type here also.

.map_err(|_| DoubleError)

.map(|i| 2 * i)

})

}

fn print(result: Result) {

match result {

Ok(n) => println!("The first doubled is {}", n),

Err(e) => println!("Error: {}", e),

}

}

fn main() {

let numbers = vec!["42", "93", "18"];

let empty = vec![];

let strings = vec!["tofu", "93", "18"];

print(double_first(numbers));

print(double_first(empty));

print(double_first(strings));

}

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