elixir.cursorrules

# Elixir Cursor Rules

You are an expert Elixir developer. Follow these rules:

## Pattern Matching
- Pattern match in function heads — avoid conditional logic in body
- Multi-clause functions ordered from specific to general
- Use guards (when) for type/value constraints
- Pin operator (^) when matching against existing values
- Destructure in function arguments: def handle({:ok, data}), not def handle(result)

## Pipelines
- Pipe operator (|>) for data transformation chains
- First argument flows through — design functions for piping
- Keep pipeline steps small and focused
- Kernel functions (elem, put_in) for inline transforms
- Extract complex transformations into named private functions

## OTP & Processes
- GenServer for stateful processes. Agent for simple state wrappers
- Supervisor trees: let processes crash and restart
- DynamicSupervisor for runtime-spawned children
- Task for one-off async work. Task.Supervisor for fault-tolerant tasks
- Never catch exits — let the supervisor handle failures

## Modules & Functions
- One public API per module — internal functions are defp
- Use @moduledoc and @doc — ExDoc is your friend
- @spec typespecs on all public functions
- Behaviours for polymorphism, protocols for data-type dispatch
- Avoid large modules: split by responsibility

## Phoenix-Specific
- Contexts for business logic boundaries — controllers are thin
- Ecto changesets for validation, not custom validation functions
- LiveView for real-time UI. Minimize handle_event complexity
- PubSub for cross-process communication
- Use verified routes: ~p"/users/#{user}"

## Testing
- ExUnit with describe blocks grouping related tests
- Mox for mocking — define behaviours first
- Use setup blocks for shared test state
- Property-based testing with StreamData for edge cases