Elixir Thinking

Resumo (pt-BR): Guia mental para escrever Elixir evitando padrões OOP. Regra de ouro: nenhum processo sem necessidade de runtime (estado mutável, concorrência ou isolamento de falha).

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:

Do I need mutable state persisting across calls?
Do I need concurrent execution?
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:

OOP Dimension	Elixir Equivalent
Behavior	Modules (functions)
State	Data (structs, maps)
Mutability	Processes (GenServer)

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

"Let It Crash" = "Let It Heal"

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.

Be explicit: Avoid _ -> nil catch-alls. Avoid value && value.field nil-punning. Prefer with when combining {:ok, nil} and {:ok, value} cases.

Polymorphism

For Polymorphism Over...	Use
Modules	Behaviors
Data	Protocols
Processes	Message passing

Use the simplest abstraction: pattern matching → anonymous functions → behaviors → protocols → message passing.

Defaults and Options

Use /3 variants (Keyword.get/3, Map.get/3) instead of case statements branching on nil. Don't create helper functions to merge config defaults; inline the fallback.

Idioms

Process dictionary is typically unidiomatic—pass state explicitly
Reserve is_thing names for guards only
Use structs over maps when shape is known
Prepend to lists [new | list] not list ++ [new]
Use dbg/1 for debugging
Use built-in JSON module (Elixir 1.18+) instead of Jason

Testing

Test behavior, not implementation. Test use cases / public API. Test your code, not the framework. Keep tests async; fix coupling if you need async: false.

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.

elixir-thinking