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