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Erlang Reference Manual
User's Guide
Version 5.9

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# 3 Pattern Matching

### 3.1 Pattern Matching

Variables are bound to values through the pattern matching mechanism. Pattern matching occurs when evaluating a function call, case- receive- try- expressions and match operator (=) expressions.

In a pattern matching, a left-hand side pattern is matched against a right-hand side term. If the matching succeeds, any unbound variables in the pattern become bound. If the matching fails, a run-time error occurs.

Examples:

    1> X.
    ** 1: variable 'X' is unbound **
    2> X = 2.
    2
    3> X + 1.
    3
    4> {X, Y} = {1, 2}.
    ** exception error: no match of right hand side value {1,2}
    5> {X, Y} = {2, 3}.
    {2,3}
    6> Y.
    3