Rust Foundations·Ownership as a design tool

Result, Option and the ? operator

Expected failures are values, not exceptions.

Intermediate 14 min read Lesson 8 of 300 Challenge included

What you will be able to do

Return Result for operations that can fail.
Use ? to propagate errors that the caller should handle.
Add context with anyhow or thiserror when building applications.

In plain English

Result<T, E> represents success or failure; Option<T> represents presence or absence. The ? operator propagates errors with a clear return type. Use unwrap only when a panic is truly a bug, not for user input.

files.rsRust

use std::fs;
use std::io;

fn read_notes() -> Result<String, io::Error> {
    let contents = fs::read_to_string("notes.txt")?;
    Ok(contents)
}

fn main() {
    match read_notes() {
        Ok(notes) => println!("{notes}"),
        Err(error) => eprintln!("Could not read notes: {error}"),
    }
}

Your turn

Read a file with ?

Medium

Write a function that reads notes.txt and returns Result<String, io::Error>.

RUST · starter

fn read_notes() -> Result<String, std::io::Error> {
    let value = String::new();
    Ok(value)
}

Check your understanding before moving on.

Knowledge checkQuestion 1

What does the ? operator do?

Select an answer to continue

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Next: Collections and iterators

Lesson 8 of 300 · 0% of course