The ? Operator

? early-returns Err (or None in Option contexts) and unwraps Ok values, with automatic conversion via From.

Recipe

fn load_config(path: &str) -> Result<String, std::io::Error> {
    let body = std::fs::read_to_string(path)?;
    Ok(body)
}

When to reach for this: Fallible functions returning Result - preferred over nested match.

Working Example

#[derive(Debug)]
enum AppError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
}
 
impl From<std::io::Error> for AppError {
    fn from(e: std::io::Error) -> Self { AppError::Io(e) }
}
impl From<std::num::ParseIntError> for AppError {
    fn from(e: std::num::ParseIntError) -> Self { AppError::Parse(e) }
}
 
fn port_from_file(path: &str) -> Result<u16, AppError> {
    let text = std::fs::read_to_string(path)?;
    let port: u16 = text.trim().parse()?;
    Ok(port)
}

What this demonstrates:

• ? converts io::Error and ParseIntError into AppError via From
• Happy path stays linear
• Return type must implement From for each error source

Deep Dive

? on Result in functions returning Result expands to match with early return. In main, use fn main() -> Result<(), E>.

try_blocks (where stable) group fallible steps - check edition.

Gotchas

• ? in main without return type - Error. Fix: fn main() -> Result<(), Box<dyn Error>>.
• Missing From conversion - Compile error. Fix: impl From or map manually.
• ? on Option in Result fn - Needs compatible types or ok_or. Fix: ok_or(Error::Missing)?.
• Swallowing context - Bare ? loses location. Fix: anyhow::Context or map_err.
• Using ? in closures - Closure must return Result. Fix: Change closure return type.

Alternatives

FAQs

Related

• Error Handling Basics
• Custom Error Types
• anyhow
• Error Context & Backtraces

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.