Result and the question mark in Rust

Ok and Err

Result carries either success or failure. Wrap good values in Ok and problem values in Err, and the type signature tells callers that both outcomes exist.

Keep the error side simple at first, such as a short String message, and refine it once the happy path works.

fn check(n: i32) -> Result<i32, String> { if n >= 0 { Ok(n) } else { Err(String::from("negative")) } }

The check function accepts zero and positive numbers. Negative input returns an Err with a short reason instead of a number.

The ? operator

The question mark returns early on Err and unwraps Ok in one character. Place it after a Result call and the error flows to the caller without manual matching.

The enclosing function must also return a Result, since the operator needs a matching channel for the early return.

fn parse_both(a: &str, b: &str) -> Result<i32, std::num::ParseIntError> { Ok(a.parse::<i32>()? + b.parse::<i32>()?) }

Both parses run in one expression. The first failure returns at once, while two successes add up inside Ok.

Convert with map_err

Map_err rewrites the error side while keeping Ok values untouched. It converts a library error into a message type the rest of the program already uses.

Reach for it at boundaries where a low level failure needs a friendlier or unified error value.

fn parse_age(s: &str) -> Result<i32, String> { s.parse::<i32>().map_err(|e| e.to_string()) }

Good input still yields a number, while bad input yields a String message. The caller sees one error type no matter which parse failed inside.

main returns Result

Main may return a Result, which lets the question mark work at the top level. An Err from main prints a short message and exits with a failure code.

This shape fits small tools that read files or parse arguments and can stop on the first error.

fn main() -> Result<(), std::num::ParseIntError> { let n: i32 = "7".parse()?; println!("ok"); Ok(()) }

The parse either fills n or exits through the Err path. Successful runs print a confirmation and finish with an empty Ok.

Next: Strings advanced

Article author: Arthur Isaev