usage_rules

# 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
- Elixir has no `return` statement, nor early returns. The last expression in a block is always returned.
- 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 or `Enum` functions
- Prefer `Enum` functions like `Enum.reduce` over recursion
- When recursion is necessary, prefer to use pattern matching in function heads for base case detection
- Using the process dictionary is typically a sign of unidiomatic code
- Only use macros if explicitly requested
- There are many useful standard library functions, prefer to use them where possible

## 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`
- Predicate function names should not start with `is` and should end in a question mark. 
- Names like `is_thing` should be reserved for guards

## 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]`

## Mix Tasks

- Use `mix help` to list available mix tasks
- Use `mix help task_name` to get docs for an individual task
- Read the docs and options fully before using tasks

## Testing
- Run tests in a specific file with `mix test test/my_test.exs` and a specific test with the line number `mix test path/to/test.exs:123`
- Limit the number of failed tests with `mix test --max-failures n`
- Use `@tag` to tag specific tests, and `mix test --only tag` to run only those tests
- Use `assert_raise` for testing expected exceptions: `assert_raise ArgumentError, fn -> invalid_function() end`
- Use `mix help test` to for full documentation on running tests

## Debugging

- Use `dbg/1` to print values while debugging. This will display the formatted value and other relevant information in the console.