Option & Result as Enums

Option<T> models presence or absence. Result<T, E> models success or recoverable failure. Together they replace null and exceptions.

Recipe

fn divide(a: f64, b: f64) -> Option<f64> {
    if b == 0.0 { None } else { Some(a / b) }
}
 
fn read_port(s: &str) -> Result<u16, std::num::ParseIntError> {
    s.parse()
}
 
fn main() {
    println!("{:?} {:?}", divide(10.0, 2.0), read_port("8080"));
}

When to reach for this: Any optional value (Option) and any fallible operation (Result).

Working Example

fn first_line(path: &str) -> Result<Option<String>, std::io::Error> {
    let mut lines = std::fs::read_to_string(path)?.lines();
    Ok(lines.next().map(|s| s.to_string()))
}
 
fn main() {
    match first_line("Cargo.toml") {
        Ok(Some(l)) => println!("{l}"),
        Ok(None) => println!("empty"),
        Err(e) => eprintln!("{e}"),
    }
}

What this demonstrates:

• Result for I/O failure
• Option inside Ok for empty file
• ? propagates io::Error
• Nested enums compose cleanly

Deep Dive

Definitions

enum Option<T> { None, Some(T) }
enum Result<T, E> { Ok(T), Err(E) }

Conversions

• Option -> Result: ok_or, ok_or_else
• Result -> Option: ok, err
• ? on Result in functions returning Result

Option is not error handling

Use Result when caller needs failure reason. Option for legitimately missing data.

Gotchas

• unwrap in library code - Panics caller. Fix: Return Result or Option.
• None for errors - Loses context. Fix: Result with error type.
• Mixing ? on Option in Result fn - Needs From or ok_or. Fix: ok_or(MyError::Missing)? or ? in Option returning fn only.
• Huge error types in Result - Allocation cost. Fix: Box<dyn Error> or thiserror enums in apps.
• Chaining too deep without ? - Nested match hell. Fix: ? and combinators.

Alternatives

FAQs

Related

• Error Handling Basics
• The ? Operator
• Option Combinators
• Pattern Matching with match

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+.