Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

refutable pattern

A pattern that might not match.

A refutable pattern can fail to match (Some(x)); irrefutable patterns always match (x, (a, b)).

Examples

let Some(x) = opt; fails with E0005 because Some(x) might not match; let (a, b) = pair; is irrefutable and fine.