Rust
? operator
Propagate errors automatically to the caller with ?.
The ? operator
? unpacks a Result: if it's Ok(v) it gives v, if it's Err(e) it immediately returns Err(e) from the function. It reduces error-propagation code to one character:
use std::num::ParseIntError;
fn topla(a: &str, b: &str) -> Result<i32, ParseIntError> {
let x: i32 = a.parse()?;
let y: i32 = b.parse()?;
Ok(x + y)
}
? can only be used in functions that return Result (or Option).
? and From
Before returning the error, ? converts it to the function's error type with From::from. This way different error types can be gathered into one type:
use std::error::Error;
fn oku(s: &str) -> Result<f64, Box<dyn Error>> {
let n: i32 = s.parse()?; // ParseIntError → Box<dyn Error>
let k: f64 = "2.5".parse()?; // ParseFloatError → Box<dyn Error>
Ok(n as f64 * k)
}
In a function returning Option, ? propagates None upward.
Exercise Add texts
Write the function fn topla(a: &str, b: &str) -> Result<i32, ParseIntError> above with ?. Print the results of topla("2", "3") and topla("2", "x") with {:?}, separated by a space.
Show example solution
use std::num::ParseIntError;
fn topla(a: &str, b: &str) -> Result<i32, ParseIntError> {
let x: i32 = a.parse()?;
let y: i32 = b.parse()?;
Ok(x + y)
}
fn main() {
println!("{:?} {:?}", topla("2", "3"), topla("2", "x"));
}
Test yourself
-
Why doesn't <code>fn main() { let n: i32 = "5".parse()?; }</code> compile?
- "5" is invalid
- It compiles
- main returns (); ? needs it to return a Result
- parse is misused
Show answer
main returns (); ? needs it to return a Result — main can return a Result too: fn main() -> Result<(), Box<dyn std::error::Error>> { ...; Ok(()) }.
-
Propagate the error
fn sayi(s: &str) -> Result<i32, std::num::ParseIntError> {
let n: i32 = s.parse()____;
Ok(n * 2)
}
Show answer
fn sayi(s: &str) -> Result<i32, std::num::ParseIntError> {
let n: i32 = s.parse()?;
Ok(n * 2)
}
-
Syntax and meaning
- x?
- Unpack Ok, return on Err
- Box<dyn Error>
- Any error type
- From::from
- Error conversion
- opt?
- Propagates None
Next lesson: match and Result
