Elixir Thinking

Mental shifts required before writing Elixir. These contradict conventional OOP patterns.

The Iron Law

NO PROCESS WITHOUT A RUNTIME REASON

Before creating a GenServer, Agent, or any process, answer YES to at least one:

1. Do I need mutable state persisting across calls?
2. Do I need concurrent execution?
3. Do I need fault isolation?

All three are NO? Use plain functions. Modules organize code; processes manage runtime.

The Three Decoupled Dimensions

OOP couples behavior, state, and mutability together. Elixir decouples them:

Pick only what you need. "I only need data and functions" = no process needed.

"Let It Crash" = "Let It Heal"

The misconception: Write careless code. The truth: Supervisors START processes.

• Handle expected errors explicitly ({:ok, _} / {:error, _})
• Let unexpected errors crash → supervisor restarts

Control Flow

Pattern matching first:

• Match on function heads instead of if/else or case in bodies
• %{} matches ANY map—use map_size(map) == 0 guard for empty maps
• Avoid nested case—refactor to single case, with, or separate functions

Error handling:

• Use {:ok, result} / {:error, reason} for operations that can fail
• Avoid raising exceptions for control flow
• Use with for chaining {:ok, _} / {:error, _} operations

Polymorphism

Behaviors = default for module polymorphism (very cheap at runtime) Protocols = only when composing data types, especially built-ins Message passing = only when stateful by design (IO, file handles)

Use the simplest abstraction: pattern matching → anonymous functions → behaviors → protocols → message passing. Each step adds complexity.

When justified: Library extensibility, multiple implementations, test swapping. When to stay coupled: Internal module, single implementation, pattern matching handles all cases.

Data Modeling Replaces Class Hierarchies

OOP: Complex class hierarchy + visitor pattern. Elixir: Model as data + pattern matching + recursion.

{:sequence, {:literal, "rain"}, {:repeat, {:alternation, "dogs", "cats"}}}

def interpret({:literal, text}, input), do: ...
def interpret({:sequence, left, right}, input), do: ...
def interpret({:repeat, pattern}, input), do: ...

Defaults and Options

Use /3 variants (Keyword.get/3, Map.get/3) instead of case statements branching on nil:

# WRONG
case Keyword.get(opts, :chunker) do
  nil -> chunker()
  config -> parse_chunker_config(config)
end

# RIGHT
Keyword.get(opts, :chunker, :default) |> parse_chunker_config()

Don't create helper functions to merge config defaults. Inline the fallback:

# WRONG
defp merge_defaults(opts), do: Keyword.merge([repo: Application.get_env(:app, :repo)], opts)

# RIGHT
def some_function(opts) do
  repo = opts[:repo] || Application.get_env(:app, :repo)
end

Idioms

• Process dictionary is typically unidiomatic—pass state explicitly
• Reserve is_thing names for guards only
• Use structs over maps when shape is known: defstruct [:name, :age]
• Prepend to lists [new | list] not list ++ [new]
• Use dbg/1 for debugging—prints formatted value with context

Testing

Test behavior, not implementation. Test use cases / public API. Refactoring shouldn't break tests.

Keep tests async. async: false means you've coupled to global state. Fix the coupling:

Common Rationalizations

Red Flags - STOP and Reconsider

• Creating process without answering the three questions
• Using GenServer for stateless operations
• Wrapping a library in a process "for safety"
• One process per entity without runtime justification
• Reaching for protocols when pattern matching works

Any of these? Re-read The Iron Law.