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priv/builtins/elixir.md
# Elixir Core Usage Rules
## Pattern Matching
- Use pattern matching over conditional logic when possible
- Prefer to match on function heads instead of using `if`/`else` or `case` in function bodies
## Error Handling
- Use `{:ok, result}` and `{:error, reason}` tuples for operations that can fail
- Avoid raising exceptions for control flow
- Use `with` for chaining operations that return `{:ok, _}` or `{:error, _}`
## Common Mistakes to Avoid
- Don't use `Enum` functions on large collections when `Stream` is more appropriate
- Avoid nested `case` statements - refactor to a single `case`, `with` or separate functions
- Don't use `String.to_atom/1` on user input (memory leak risk)
- Lists and enumerables cannot be indexed with brackets. Use pattern matching, `Enum` functions or `hd/1` and `tl/1`.
## Function Design
- Use guard clauses: `when is_binary(name) and byte_size(name) > 0`
- Prefer multiple function clauses over complex conditional logic
- Name functions descriptively: `calculate_total_price/2` not `calc/2`
## Data Structures
- Use structs over maps when the shape is known: `defstruct [:name, :age]`
- Prefer keyword lists for options: `[timeout: 5000, retries: 3]`
- Use maps for dynamic key-value data
- Prefer to prepend to lists `[new | list]` not `list ++ [new]`