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:

• Two variants: Some(T) or None.
• Used when absence is normal, not exceptional.
• No error information — just "value here" or "not here".
• Rich combinator API: .map(), .and_then(), .unwrap_or().

1fn find_user(id: u32) -> Option<String> {
2    match id {
3        1 => Some("Alice".to_string()),
4        2 => Some("Bob".to_string()),
5        _ => None,
6    }
7}
8
9// Chaining
10let greeting = find_user(1)
11    .map(|name| format!("Hello, {}!", name))
12    .unwrap_or("User not found".to_string());

Result:

• Two variants: Ok(T) or Err(E).
• 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(path: &str) -> Result<String, Box<dyn std::error::Error>> {
4    let content = fs::read_to_string(path)?;
5    Ok(content)
6}
7
8// Chaining with map_err
9fn parse(s: &str) -> Result<i32, String> {
10    s.parse::<i32>()
11        .map_err(|e| format!("Parse error: {}", e))
12}

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 on None/Err.
• Forgetting to propagate errors — use ? or match.
• Using Option when the error reason matters — use Result.