Binding with @

Level: 201 → 301 · deep dive

One line: n @ 1..=9 tests the value and keeps it — the pattern on the right decides whether the arm fires, the name on the left binds what matched, and without the @ you have to choose between the two.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• The problem it solves: 1..=9 => … matched but has no name for the number; n => … has the name but tests nothing. n @ 1..=9 is both
• Reading it aloud — "call it n, and it must look like this" — which is the whole syntax
• On a nested pattern: msg @ Message::Resize { .. }, keeping the whole value while testing its shape
• Its interaction with ownership: @ binds by move unless the inner pattern is Copy or you are matching a reference — the same rule as every other binding
• @ with | inside it (n @ (1 | 9)), stabilised later than the rest
• The other two places a name shows up in a pattern, so the three are not confused: a plain binding, and the field shorthand in a struct pattern

The trap it exists for

Reaching for a guard where @ is what you wanted — n if (1..=9).contains(&n) => … does the same job, opts the arm out of exhaustiveness checking, and reads worse. If the condition is expressible as a pattern, make it a pattern.

See also

• Match guards — the tool this one frequently replaces, and what it costs
• One arm, many values — the ranges and | that go on the right of the @
• Destructuring enums — where msg @ Variant { .. } earns its keep
• @ bindings ↗ · Comprehensive Rust: Pattern Matching ↗

Po polsku

Znak @ rozwiązuje wybór, przed którym stawia zwykły wzorzec: 1..=9 sprawdza wartość, ale nie zostawia dla niej nazwy, a samo n nazywa, lecz niczego nie sprawdza — n @ 1..=9 robi jedno i drugie, i czyta się to „nazwij to n, ale tylko jeśli ma taki kształt”. Tak samo działa na całym wariancie: msg @ Message::Resize { .. } bada kształt, a zachowuje wartość w komplecie. Nawyk wart wyrobienia jest jednak inny niż sama składnia: kiedy pojawia się ochota na strażnika (guard) w rodzaju n if (1..=9).contains(&n), to prawie zawsze znak, że chodziło o @ — strażnik robi to samo, czyta się gorzej, a przy okazji wypisuje ramię z kontroli kompletności dopasowania. Jeśli warunek da się wyrazić wzorcem, wyraź go wzorcem.

Szukaj po polsku: wiązanie we wzorcu · dopasowanie wzorców · rust @ bindings pattern · rust bind and test value in match