elixir-pattern-matching

Pattern matching in Elixir lets you define multiple function clauses that execute based on argument structure. Write separate clauses with different patterns—tuples, lists, maps, or structs—and Elixir selects the matching clause automatically. This enables clean control flow for handling success/error results, routing data, and validation.

Function clauses pattern-match at definition time, executing the first matching clause body. Case statements pattern-match at runtime within a single function, testing multiple patterns sequentially. Both use the same syntax for tuples, lists, maps, and structs, but function clauses separate logic across clauses while case keeps it in one expression.

Destructuring extracts values from nested structures by matching their shape in function heads, case branches, or with blocks. Use pattern syntax for tuples `{a, b}`, lists `[h | t]`, maps `%{key: value}`, or structs `%Module{field: x}`. Unmatched patterns fail; use the pin operator `^var` to match against existing values.

Guards add conditional logic beyond structure matching—use them to test values with `when` clauses after pattern syntax. Guards filter clauses by numeric comparisons, type checks, or custom predicates, while patterns match data shape. Combine both: pattern matches the structure, guards validate the values inside.

Yes. The `with` statement chains pattern-matched steps, stopping at the first non-match and returning a failure value. Each line destructures the previous result; if a pattern fails, execution jumps to the `else` block. This creates clean, linear flows for multi-step validations and transformations.

A non-matching pattern raises a `FunctionClauseError` in functions or a `CaseClauseError` in case statements. In `with` blocks, non-matching patterns skip to the `else` clause without raising. Use the underscore `_` to catch any value and prevent match failures in catch-all clauses.