Rust: What is the difference between Option and Result?

Option — value may or may not exist (no error info). Result — operation succeeded or failed (carries error info).

Option:

• Some(T) for present values, None for absent.
• Used when absence is expected, not exceptional.
• No error information — just "value here" or "not here".
• Methods: .unwrap_or(), .map(), .and_then(), .filter().

1fn find_by_key(key: &str) -> Option<String> {
2    let map = std::collections::HashMap::from([
3        ("name".to_string(), "Alice".to_string()),
4    ]);
5    map.get(key).cloned()
6}
7
8// Chaining with Option
9let name = find_by_key("name")
10    .map(|s| s.to_uppercase())
11    .unwrap_or("Unknown".to_string());

Result:

• Ok(T) for success, Err(E) for failure.
• Used for fallible operations (I/O, parsing, network).
• Carries an error type for debugging and handling.
• The ? operator propagates errors up the call stack.

1use std::fs;
2
3fn read_config() -> Result<String, Box<dyn std::error::Error>> {
4    let content = fs::read_to_string("config.toml")?;
5    Ok(content)
6}
7
8// Using ? for error propagation
9fn parse_config() -> Result<u32, Box<dyn std::error::Error>> {
10    let content = fs::read_to_string("config.toml")?;
11    let value: u32 = content.trim().parse()?;
12    Ok(value)
13}

When to use:

• Option for: lookups, optional fields, nullable values, iterators.
• Result for: file I/O, network calls, parsing, database queries.

Common mistakes:

• Using unwrap() in production — can panic.
• Forgetting to handle errors — use ? or match.
• Confusing Option and Result — Result needs an error type.