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Thetry!macro
Before there was ?, the same functionality was achieved with the try! macro. The ? operator is now recommended, but you may still find try! when looking at older code. The same multiply function from the previous example would look like this using try!:
// To compile and run this example without errors, while using Cargo, change the value
// of the `edition` field, in the `[package]` section of the `Cargo.toml` file, to "2015".
use std::num::ParseIntError;
fn multiply(first_number_str: &str, second_number_str: &str) -> Result {
let first_number = try!(first_number_str.parse::());
let second_number = try!(second_number_str.parse::());
Ok(first_number * second_number)
}
fn print(result: Result) {
match result {
Ok(n) => println!("n is {}", n),
Err(e) => println!("Error: {}", e),
}
}
fn main() {
print(multiply("10", "2"));
print(multiply("t", "2"));
}
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1
See re-enter ? for more details.
Multiple error types
The previous examples have always been very convenient; Results interact with other Results and Options interact with other Options.
Sometimes an Option needs to interact with a Result, or a Result needs to interact with a Result. In those cases, we want to manage our different error types in a way that makes them composable and easy to interact with.
In the following code, two instances of unwrap generate different error types. Vec::first returns an Option, while parse:: returns a Result:
fn double_first(vec: Vec<&str>) -> i32 {
let first = vec.first().unwrap(); // Generate error 1
2 * first.parse::().unwrap() // Generate error 2
}
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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Over the next sections, we'll see several strategies for handling these kind of problems.
PullingResults out ofOptions
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));
}
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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.