---

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].

Copy# ❌ 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:

Copy# ❌ 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:

Copy# ❌ 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:

Copy# 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:

Copycollection
|> 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