Elixir Guidelines

This skill provides Elixir language best practices to help you write correct, idiomatic Elixir code.

List Access

Elixir lists do not support index based access via the access syntax.

Never do this (invalid):

i = 0
mylist = ["blue", "green"]
mylist[i]  # This will NOT work

Always do this instead:

i = 0
mylist = ["blue", "green"]
Enum.at(mylist, i)  # Use Enum.at, pattern matching, or List module

Variable Rebinding in Block Expressions

Elixir variables are immutable, but can be rebound. For block expressions like if, case, cond, etc., you must bind the result of the expression to a variable if you want to use it. You cannot rebind the result inside the expression.

Invalid approach:

# INVALID: rebinding inside the `if` - result never gets assigned
if connected?(socket) do
  socket = assign(socket, :val, val)
end

Valid approach:

# VALID: rebind the result of the `if` to a new variable
socket =
  if connected?(socket) do
    assign(socket, :val, val)
  end

Module Organization

• Never nest multiple modules in the same file as it can cause cyclic dependencies and compilation errors
• Each module should be in its own file

Struct Access

• Never use map access syntax (changeset[:field]) on structs as they do not implement the Access behaviour by default
• For regular structs, you must access fields directly: my_struct.field
• Use higher level APIs when available: Ecto.Changeset.get_field/2 for changesets

Date and Time Handling

• Elixir's standard library has everything necessary for date and time manipulation
• Familiarize yourself with Time, Date, DateTime, and Calendar interfaces
• Never install additional dependencies unless asked or for date/time parsing (use date_time_parser package if needed)

Atom Safety

• Never use String.to_atom/1 on user input (creates memory leak risk)
• Atoms are not garbage collected, so dynamic atom creation can exhaust memory

Naming Conventions

• Predicate function names should not start with is_ and should end in a question mark
• Names like is_thing should be reserved for guards
• Example: active?/1 not is_active/1

OTP Primitives

Elixir's builtin OTP primitives like DynamicSupervisor and Registry require names in the child spec:

{DynamicSupervisor, name: MyApp.MyDynamicSup}

Then use:

DynamicSupervisor.start_child(MyApp.MyDynamicSup, child_spec)

Concurrent Processing

• Use Task.async_stream(collection, callback, options) for concurrent enumeration with back-pressure
• Most times you'll want to pass timeout: :infinity as an option to avoid timeouts on long-running tasks

Task.async_stream(items, &process_item/1, timeout: :infinity)
|> Enum.to_list()

Mix Guidelines

Reading Documentation

• Read the docs and options before using tasks: mix help task_name
• Mix help provides comprehensive information about available options

Debugging Test Failures

• Run tests in a specific file: mix test test/my_test.exs
• Run all previously failed tests: mix test --failed
• Run a specific test by line number: mix test test/my_test.exs:42

Dependency Management

• mix deps.clean --all is almost never needed
• Avoid using it unless you have good reason (e.g., corrupted build artifacts)
• Usually mix deps.get and mix deps.compile are sufficient