Error Handling Basics

8 examples - 5 basic, 3 intermediate - covering Option, Result, and when to panic.

Prerequisites

Option & Result as Enums

Basic Examples

1. `Option` for Missing Values

fn find(ids: &[u32], target: u32) -> Option<usize> {
    ids.iter().position(|&id| id == target)
}

None means not found without error detail
Some(i) carries index
No null pointers

2. `Result` for Failures

use std::fs;
 
fn read(path: &str) -> Result<String, std::io::Error> {
    fs::read_to_string(path)
}

Err includes error reason
Ok on success
Recoverable errors use Result

3. `match` on `Result`

fn main() {
    match read("Cargo.toml") {
        Ok(body) => println!("{body}"),
        Err(e) => eprintln!("{e}"),
    }
}

4. `unwrap` and `expect` (prototypes)

let n: u32 = "42".parse().expect("valid port");

Panics on Err - binaries/prototypes only
Libraries should return Result

5. `?` Propagation Preview

fn load() -> Result<String, std::io::Error> {
    let s = std::fs::read_to_string("config.toml")?;
    Ok(s)
}

Related: The ? Operator

Intermediate Examples

6. Combine `Option` and `Result`

fn first_user(names: &[&str]) -> Result<&str, &'static str> {
    names.first().ok_or("empty list")
}

7. `map` and `and_then`

let port: Option<u16> = Some("8080")
    .and_then(|s| s.parse().ok());

8. Custom Error Enum Preview

enum AppError { Io(std::io::Error), Parse(std::num::ParseIntError) }

Related: Error Handling Best Practices

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+.

Prerequisites

Option & Result as Enums

Basic Examples

1. `Option` for Missing Values

fn find(ids: &[u32], target: u32) -> Option<usize> {
    ids.iter().position(|&id| id == target)
}

None means not found without error detail
Some(i) carries index
No null pointers

2. `Result` for Failures

use std::fs;
 
fn read(path: &str) -> Result<String, std::io::Error> {
    fs::read_to_string(path)
}

Err includes error reason
Ok on success
Recoverable errors use Result

3. `match` on `Result`

fn main() {
    match read("Cargo.toml") {
        Ok(body) => println!("{body}"),
        Err(e) => eprintln!("{e}"),
    }
}

4. `unwrap` and `expect` (prototypes)

let n: u32 = "42".parse().expect("valid port");

Panics on Err - binaries/prototypes only
Libraries should return Result

5. `?` Propagation Preview

fn load() -> Result<String, std::io::Error> {
    let s = std::fs::read_to_string("config.toml")?;
    Ok(s)
}

Related: The ? Operator

Intermediate Examples

6. Combine `Option` and `Result`

fn first_user(names: &[&str]) -> Result<&str, &'static str> {
    names.first().ok_or("empty list")
}

7. `map` and `and_then`

let port: Option<u16> = Some("8080")
    .and_then(|s| s.parse().ok());

8. Custom Error Enum Preview

enum AppError { Io(std::io::Error), Parse(std::num::ParseIntError) }

Related: Error Handling Best Practices

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+.

Error Handling Basics

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Error Handling Basics