Elixir and Phoenix development guidelines with critical language-specific gotchas. Use when writing Elixir code, Phoenix applications, or Mix tasks. Prevents common mistakes with list access, variable rebinding, struct access, and OTP patterns that cause subtle bugs or compilation errors.
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---
name: elixir-phoenix
description: Elixir and Phoenix development guidelines with critical language-specific gotchas. Use when writing Elixir code, Phoenix applications, or Mix tasks. Prevents common mistakes with list access, variable rebinding, struct access, and OTP patterns that cause subtle bugs or compilation errors.
---
# Elixir/Phoenix Guidelines
## Critical Elixir Gotchas
### List Access
Lists do NOT support index-based access syntax. Never use `list[i]`.
```elixir
# ❌ INVALID - will not compile
mylist = ["blue", "green"]
mylist[0]
# ✅ CORRECT
Enum.at(mylist, 0)
```
### Block Expression Rebinding
Variables are immutable but can be rebound. Block expressions (`if`, `case`, `cond`) must bind results externally:
```elixir
# ❌ INVALID - rebinding inside block has no effect outside
if connected?(socket) do
socket = assign(socket, :val, val)
end
# ✅ CORRECT - bind the block result
socket =
if connected?(socket) do
assign(socket, :val, val)
end
```
### Struct Access
Never use map access syntax on structs—they don't implement Access behaviour:
```elixir
# ❌ INVALID
changeset[:field]
# ✅ CORRECT
Ecto.Changeset.get_field(changeset, :field)
my_struct.field
```
### Module Nesting
Never nest multiple modules in the same file—causes cyclic dependencies and compilation errors.
## Naming Conventions
- Predicate functions: end with `?`, not `is_` prefix
- `valid?` not `is_valid`
- Reserve `is_thing` for guards only
- Avoid `String.to_atom/1` on user input (memory leak risk)
## OTP Patterns
Primitives like `DynamicSupervisor` and `Registry` require names in child specs:
```elixir
# In supervision tree
{DynamicSupervisor, name: MyApp.MyDynamicSup}
# Then use the name
DynamicSupervisor.start_child(MyApp.MyDynamicSup, child_spec)
```
## Concurrency
Use `Task.async_stream/3` for concurrent enumeration with back-pressure:
```elixir
collection
|> Task.async_stream(&process/1, timeout: :infinity)
|> Enum.to_list()
```
## Date/Time
Use standard library (`Time`, `Date`, `DateTime`, `Calendar`). Only add `date_time_parser` for parsing if needed.
## Mix Guidelines
- Read task docs first: `mix help task_name`
- Debug test failures: `mix test test/my_test.exs` or `mix test --failed`
- Avoid `mix deps.clean --all` unless absolutely necessary
