Rust by Example

// Error 1: the input vector is empty

println!("The first doubled is {}", double_first(strings));

// Error 2: the element doesn't parse to a number

}

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Over the next sections, we'll see several strategies for handling these kind of problems.

Pulling Result s out of Option s

The most basic way of handling mixed error types is to just embed them in each other.

use std::num::ParseIntError;

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

vec.first().map(|first| {

first.parse::().map(|n| 2 * n)

})

}

fn main() {

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

let empty = vec![];

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

println!("The first doubled is {:?}", double_first(numbers));

println!("The first doubled is {:?}", double_first(empty));

// Error 1: the input vector is empty

println!("The first doubled is {:?}", double_first(strings));

// Error 2: the element doesn't parse to a number

}

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There are times when we'll want to stop processing on errors (like with ?) but keep going when the Option is None. A couple of combinators come in handy to swap the Result and Option.

use std::num::ParseIntError;

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

let opt = vec.first().map(|first| {

first.parse::().map(|n| 2 * n)

});

opt.map_or(Ok(None), |r| r.map(Some))

}

fn main() {

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

let empty = vec![];

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

println!("The first doubled is {:?}", double_first(numbers));

println!("The first doubled is {:?}", double_first(empty));

println!("The first doubled is {:?}", double_first(strings));

}