Result
Result is a richer version of the Option type that
describes possible error instead of possible absence.
That is, Result<T, E> could have one of two outcomes:
Ok(T): An elementTwas foundErr(E): An error was found with elementE
By convention, the expected outcome is Ok while the unexpected outcome is Err.
Like Option, Result has many methods associated with it. unwrap(), for
example, either yields the element T or panics. For case handling,
there are many combinators between Result and Option that overlap.
In working with Rust, you will likely encounter methods that return the
Result type, such as the parse() method. It might not always
be possible to parse a string into the other type, so parse() returns a
Result indicating possible failure.
Let's see what happens when we successfully and unsuccessfully parse() a string:
fn multiply(first_number_str: &str, second_number_str: &str) -> i32 { // Let's try using `unwrap()` to get the number out. Will it bite us? let first_number = first_number_str.parse::<i32>().unwrap(); let second_number = second_number_str.parse::<i32>().unwrap(); first_number * second_number } fn main() { let twenty = multiply("10", "2"); println!("double is {}", twenty); let tt = multiply("t", "2"); println!("double is {}", tt); }
In the unsuccessful case, parse() leaves us with an error for unwrap()
to panic on. Additionally, the panic exits our program and provides an
unpleasant error message.
To improve the quality of our error message, we should be more specific about the return type and consider explicitly handling the error.
Using Result in main
The Result type can also be the return type of the main function if
specified explicitly. Typically the main function will be of the form:
fn main() { println!("Hello World!"); }
However main is also able to have a return type of Result. If an error
occurs within the main function it will return an error code and print a debug
representation of the error (using the Debug trait). The following example
shows such a scenario and touches on aspects covered in the following section.
use std::num::ParseIntError; fn main() -> Result<(), ParseIntError> { let number_str = "10"; let number = match number_str.parse::<i32>() { Ok(number) => number, Err(e) => return Err(e), }; println!("{}", number); Ok(()) }