How do Option, Result and the question mark operator work?
Assesses fundamental understanding of Rust conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Option<T> models presence with Some and None, replacing null. Result<T, E> models success or failure with Ok and Err, and is how Rust does recoverable errors.
fn parse(s: &str) -> Result<i32, std::num::ParseIntError> {
let n: i32 = s.trim().parse()?; // returns early on Err
Ok(n * 2)
}
The ? operator unwraps Ok and returns the error early, converting through the From trait, which keeps propagation clean. Combinators such as map, and_then, unwrap_or and ok_or let you work without a full match.
unwrap() and expect() panic on failure and belong in tests or when failure is truly impossible. For libraries, define an error enum or use thiserror; for applications anyhow provides convenient boxed errors. This design makes failure explicit in the type system, so callers cannot forget to consider it, unlike exceptions that may be silently swallowed.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.