What are guards used for in Erlang?

A guard is an extra boolean condition attached to a function clause or case branch with the when keyword. It runs after the pattern matches structurally, letting you filter on values rather than just shape.

classify(N) when N < 0 -> negative;
classify(0) -> zero;
classify(N) when N > 0 -> positive.

Guards are intentionally restricted to a small set of safe, side-effect-free built-ins (comparisons, arithmetic, type checks like is_integer/1, and a few guard-only BIFs) — you cannot call arbitrary functions in a guard. This restriction guarantees a guard can never crash or block, so the runtime can always safely evaluate it while deciding which clause to run.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.