Rust by Example

// needs to be converted into a `DoubleError`.

impl From for DoubleError {

fn from(err: ParseIntError) -> DoubleError {

DoubleError::Parse(err)

}

}

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

let first = vec.first().ok_or(DoubleError::EmptyVec)?;

// Here we implicitly use the `ParseIntError` implementation of `From` (which

// we defined above) in order to create a `DoubleError`.

let parsed = first.parse::()?;

Ok(2 * parsed)

}

fn print(result: Result) {

match result {

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

Err(e) => {

println!("Error: {}", e);

if let Some(source) = e.source() {

println!(" Caused by: {}", source);

}

},

}

}

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));

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

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This adds a bit more boilerplate for handling errors and might not be needed in all applications. There are some libraries that can take care of the boilerplate for you.

See also:

From::from and Enums

Iterating over Result s

An Iter::map operation might fail, for example:

fn main() {

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

let numbers: Vec<_> = strings

.into_iter()

.map(|s| s.parse::())

.collect();

println!("Results: {:?}", numbers);

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

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Let's step through strategies for handling this.

Ignore the failed items with filter_map()

filter_map calls a function and filters out the results that are None.

fn main() {

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

let numbers: Vec<_> = strings

.into_iter()

.filter_map(|s| s.parse::().ok())

.collect();

println!("Results: {:?}", numbers);

}