Elixir/Phoenix Guidelines

Critical Elixir Gotchas

List Access

Lists do NOT support index-based access syntax. Never use list[i].

# ❌ 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:

# ❌ 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:

# ❌ 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:

# 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:

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