elixir-patterns/smells/anti-patterns.md

# Anti-Patterns

Things the Elixir and Phoenix source deliberately avoids — and why you should too.

## 1. GenServer for Pure Functions

**Source:** `lib/elixir/lib/gen_server.ex:533-575` ("When (not) to use a GenServer")

The GenServer docs explicitly say:

> If you don't need a process, then you don't need a process.

**What they avoid:** Creating a GenServer to wrap stateless computation.

**Why:** A process adds message passing overhead, serialization (one request at a time), and supervision complexity — all unnecessary for pure functions.

**Do this instead:** A plain module with functions:
```elixir
# Good: pure function, no process needed
defmodule MyApp.Calculator do
  def add(a, b), do: a + b
end

# Bad: unnecessary process
defmodule MyApp.Calculator do
  use GenServer
  def add(a, b), do: GenServer.call(__MODULE__, {:add, a, b})
  def handle_call({:add, a, b}, _from, state), do: {:reply, a + b, state}
end
```

---

## 2. Dynamic Atoms for Process Names

**Source:** `lib/elixir/lib/registry.ex:28-60` (Registry as alternative to atoms)

The Registry module exists specifically because dynamic atom creation is dangerous:

> atoms are never garbage collected

**What they avoid:** `String.to_atom("worker_#{id}")`

**What they do instead:**
```elixir
{:via, Registry, {MyApp.Registry, "worker-#{id}"}}
```

---

## 3. Broad `import` Without `:only`

**Source:** `lib/elixir/lib/enum.ex:250`

```elixir
import Kernel, except: [max: 2, min: 2]
```

Even within the standard library, imports are scoped. Enum explicitly excludes the specific Kernel functions it replaces.

**What they avoid:** `import Kernel` without qualification when they define conflicting names.

**Exception:** Phoenix Router (`lib/phoenix/router.ex:274-276`) imports broadly — but it's a DSL where usability trumps explicitness.

---

## 4. Exceptions for Control Flow

**Source:** `lib/elixir/lib/task.ex:455-460` (async documentation on error handling)

> an asynchronous task should be thought of as an extension of the
> caller process rather than a mechanism to isolate it from all errors.

The Task documentation advises returning `{:ok, val} | :error` for normal flow, NOT using try/rescue:

> For example, to either return `{:ok, val} | :error` results or,
> in more extreme cases, by using `try/rescue`

**What they avoid:** Using `try/rescue` around expected failure cases.

**Why:** Pattern matching on tagged tuples is more explicit, composable (works with `with`), and doesn't hide the error path.

---

## 5. Trapping Exits in Normal Code

**Source:** `lib/elixir/lib/task.ex:469-477` (explicit warning)

> Setting `:trap_exit` to `true` - trapping exits should be used only in special
> circumstances as it would make your process immune to not only exits from the
> task but from any other processes.
>
> Moreover, even when trapping exits, calling `await` will still exit if the
> task has terminated without sending its result back.

**What they avoid:** `Process.flag(:trap_exit, true)` in normal application code.

**Why:** Trapping exits breaks the supervision contract. A supervisor expects to be able to kill its children — if they trap exits, shutdown semantics change unpredictably.

---

## 6. Expensive Work in `init/1`

**Source:** `lib/elixir/lib/gen_server.ex:127-145` (handle_continue pattern)

```elixir
# What NOT to do — blocks the supervisor
def init(url) do
  data = HTTP.get!(url)  # BAD: blocks here
  {:ok, data}
end

# What TO do — return immediately, do work later
def init(url) do
  {:ok, :unset, {:continue, {:fetch, url}}}
end

def handle_continue({:fetch, url}, _state) do
  {:noreply, HTTP.get!(url)}
end
```

**What they avoid:** Network calls, disk I/O, or any slow operation in `init/1`.

**Why:** `init/1` blocks `start_link`, which blocks the supervisor. If your init takes 5 seconds, the entire supervision tree startup stalls.

---

## 7. Unlinking Task Processes

**Source:** `lib/elixir/lib/task.ex:478-482`

> Unlinking the task process started with `async`/`await`. If you unlink the
> processes and the task does not belong to any supervisor, you may leave
> dangling tasks in case the caller process dies.

**What they avoid:** `Process.unlink/1` on task processes.

**Why:** The link is a safety mechanism. If the caller dies, the task should die too (since nobody will read the result). Unlinking creates orphan processes.

---

## 8. Blocking the Agent with Expensive Computation

**Source:** `lib/elixir/lib/agent.ex:62-82` (client vs server computation)

```elixir
# BAD: blocks the agent, other callers queue up
def get_something(agent) do
  Agent.get(agent, fn state -> do_something_expensive(state) end)
end

# GOOD: copies state to caller, work happens in caller's process
def get_something(agent) do
  Agent.get(agent, & &1) |> do_something_expensive()
end
```

**What they avoid:** Running expensive operations inside the Agent's process.

**Why:** The Agent is a single process. While it's computing, ALL other get/update/cast operations queue up. Move computation to the caller unless atomicity is required.

---

## 9. Raw `spawn` Instead of Supervised Processes

**Source:** `lib/elixir/lib/task.ex:24-26` (why Task over spawn)

> Compared to plain processes, started with `spawn/1`, tasks include monitoring
> metadata and logging in case of errors.

**Source:** `lib/elixir/lib/task.ex:100-115`

> We encourage developers to rely on supervised tasks as much as possible.
> Supervised tasks improve the visibility of how many tasks are running
> at a given moment and enable a variety of patterns that give you
> explicit control on how to handle the results, errors, and timeouts.

**What they avoid:** `spawn/1` and `spawn_link/1` in application code.

**Why:** Unsupervised processes are invisible to the system. They don't appear in observer, don't get logged on crash, and can't be gracefully shut down.