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<Result<i32, ParseIntError>> {
vec.first().map(|first| {
first.parse::<i32>().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
}
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<Option<i32>, ParseIntError> {
let opt = vec.first().map(|first| {
first.parse::<i32>().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));
}