What is pattern matching in Erlang?

Pattern matching is Erlang's core mechanism for both destructuring data and controlling program flow. The = operator isn't assignment in the usual sense — it asserts that the left-hand pattern must match the right-hand value, binding any unbound variables in the process.

{ok, Value} = {ok, 42},      % Value becomes 42
{ok, Value2} = {error, oops} % fails: patterns don't match -> exception

The same mechanism drives function clause selection and case expressions: Erlang tries each clause's pattern in order and runs the first one that matches, extracting variables as it goes.

describe(0) -> "zero";
describe(N) when N > 0 -> "positive";
describe(_) -> "negative".

This removes the need for most manual type-checking or field-extraction code you'd otherwise write by hand.

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