elixir-anti-patterns

Elixir Anti-Patterns Detection and Refactoring

You are an expert at identifying Elixir anti-patterns and suggesting idiomatic refactorings. Use this knowledge to analyze code, suggest improvements, and help developers write better Elixir.

Code-Related Anti-Patterns

1. Comments Overuse

Problem: Excessive or self-explanatory comments reduce readability rather than enhance it.

Detection:

• Inline comments explaining obvious code
• Comments for every function line
• Comments duplicating what code already says clearly

Refactoring:

• Use clear function and variable names instead of explanatory comments
• Replace inline comments with @doc and @moduledoc for documentation
• Use module attributes for configuration values

Example:

# Bad
def calculate() do
  # Get current time
  now = DateTime.utc_now()
  # Add 5 minutes
  DateTime.add(now, 5 * 60, :second)
end

# Good
@minutes_to_add 5

def timestamp_five_minutes_from_now do
  now = DateTime.utc_now()
  DateTime.add(now, @minutes_to_add * 60, :second)
end

2. Complex else Clauses in with

Problem: Flattening all error handling into a single complex else block obscures which clause produced which error.

Detection:

• Large else blocks with many pattern match clauses
• Difficulty determining error sources
• Complex error handling logic in else

Refactoring:

• Normalize return types in private functions
• Handle errors closer to their source
• Let with focus on success paths

Example:

# Bad
def read_config(path) do
  with {:ok, content} <- File.read(path),
       {:ok, decoded} <- Jason.decode(content) do
    {:ok, decoded}
  else
    {:error, :enoent} -> {:error, :file_not_found}
    {:error, %Jason.DecodeError{}} -> {:error, :invalid_json}
    {:error, reason} -> {:error, reason}
  end
end

# Good
def read_config(path) do
  with {:ok, content} <- read_file(path),
       {:ok, config} <- parse_json(content) do
    {:ok, config}
  end
end

defp read_file(path) do
  case File.read(path) do
    {:ok, content} -> {:ok, content}
    {:error, :enoent} -> {:error, :file_not_found}
    error -> error
  end
end

defp parse_json(content) do
  case Jason.decode(content) do
    {:ok, data} -> {:ok, data}
    {:error, _} -> {:error, :invalid_json}
  end
end

3. Complex Extractions in Clauses

Problem: Extracting values across multiple clauses and arguments makes it unclear which variables serve pattern/guard purposes versus function body usage.

Detection:

• Many variable extractions in function heads
• Mixed guard and body variable usage
• Unclear variable purposes

Refactoring:

• Extract only pattern/guard-related variables in function signatures
• Use capture patterns like %User{age: age} = user
• Extract body variables inside the clause

Example:

# Bad
def process(%User{age: age, name: name, email: email} = user) when age >= 18 do
  # Only using name and email in body, not age
  send_email(email, "Hello #{name}")
end

# Good
def process(%User{age: age} = user) when age >= 18 do
  send_email(user.email, "Hello #{user.name}")
end

4. Dynamic Atom Creation

Problem: Atoms aren't garbage-collected and are limited to ~1 million. Uncontrolled dynamic atom creation poses memory and security risks.

Detection:

• String.to_atom/1 with untrusted input
• Converting user input directly to atoms
• Unbounded atom creation in loops

Refactoring:

• Use explicit mappings via pattern-matching
• Use String.to_existing_atom/1 with pre-defined atoms
• Keep strings when atom conversion isn't necessary

Example:

# Bad - Security risk!
def set_role(user, role_string) do
  %{user | role: String.to_atom(role_string)}
end

# Good
def set_role(user, role) when role in [:admin, :editor, :viewer] do
  %{user | role: role}
end

# Or with pattern matching
def set_role(user, "admin"), do: %{user | role: :admin}
def set_role(user, "editor"), do: %{user | role: :editor}
def set_role(user, "viewer"), do: %{user | role: :viewer}
def set_role(_user, invalid), do: {:error, "Invalid role: #{invalid}"}

5. Long Parameter List

Problem: Functions with excessive parameters become confusing and error-prone to use.

Detection:

• Functions with 4+ parameters
• Parameters that are conceptually related
• Difficult to remember parameter order

Refactoring:

• Group related parameters into maps or structs
• Use keyword lists for optional parameters
• Create domain objects

Example:

# Bad
def create_loan(user_id, user_name, user_email, book_id, book_title, book_isbn) do
  # ...
end

# Good
def create_loan(user, book) do
  # ...
end

# Or with keyword list for options
def create_loan(user, book, opts \\ []) do
  duration = Keyword.get(opts, :duration, 14)
  renewable = Keyword.get(opts, :renewable, true)
  # ...
end

6. Namespace Trespassing

Problem: Defining modules outside your library's namespace risks conflicts since the Erlang VM loads only one module instance per name.

Detection:

• Library defining modules in common namespaces (e.g., Plug.* when you're not Plug)
• Modules without library prefix
• Potential naming conflicts with other libraries

Refactoring:

• Always prefix modules with your library namespace
• Use clear, unique top-level module names

Example:

# Bad - Library named :plug_auth
defmodule Plug.Auth do
  # This conflicts with the actual Plug library!
end

# Good
defmodule PlugAuth do
  # ...
end

defmodule PlugAuth.Session do
  # ...
end

7. Non-assertive Map Access

Problem: Using dynamic access (map[:key]) for required keys masks missing data, allowing nil to propagate instead of failing fast.

Detection:

• map[:key] for required/expected keys
• Nil checks after map access
• Silent failures from missing keys

Refactoring:

• Use static access (map.key) for required keys
• Pattern-match on struct/map keys
• Reserve dynamic access for optional fields

Example:

# Bad
def distance(point) do
  x = point[:x]  # Returns nil if :x is missing!
  y = point[:y]
  :math.sqrt(x * x + y * y)  # Crashes on nil, but unclear why
end

# Good
def distance(%{x: x, y: y}) do
  :math.sqrt(x * x + y * y)  # Clear error if keys missing
end

# Or with structs
defmodule Point do
  defstruct [:x, :y]
end

def distance(%Point{x: x, y: y}) do
  :math.sqrt(x * x + y * y)
end

8. Non-assertive Pattern Matching

Problem: Writing defensive code that returns incorrect values instead of using pattern matching to assert expected structures causes silent failures.

Detection:

• Defensive nil checks instead of pattern matching
• Functions returning invalid data on unexpected input
• Avoiding crashes when crashes are appropriate

Refactoring:

• Use pattern matching to assert expected structures
• Let functions crash on invalid input
• Trust supervisors to handle failures

Example:

# Bad
def parse_query_param(param) do
  case String.split(param, "=") do
    [key, value] -> {key, value}
    _ -> {"", ""}  # Silent failure!
  end
end

# Good
def parse_query_param(param) do
  [key, value] = String.split(param, "=")
  {key, value}
end
# Crashes with clear error if format is wrong - this is good!

9. Non-assertive Truthiness

Problem: Using truthiness operators (&&, ||, !) when all operands are boolean is unnecessarily generic and unclear.

Detection:

• &&, ||, ! with boolean expressions
• Comparisons like is_binary(x) && is_integer(y)
• Mixing boolean and truthy logic

Refactoring:

• Use and, or, not for boolean-only operations
• Reserve &&, ||, ! for truthy/falsy logic

Example:

# Bad
def valid_user?(name, age) do
  is_binary(name) && is_integer(age) && age >= 18
end

# Good
def valid_user?(name, age) do
  is_binary(name) and is_integer(age) and age >= 18
end

# Truthy operators are OK for nil/value checks
def get_name(user) do
  user[:name] || "Anonymous"
end

10. Structs with 32 Fields or More

Problem: Structs with 32+ fields switch from Erlang's efficient flat-map representation to hash maps, increasing memory usage.

Detection:

• Struct definitions with 32+ fields
• Large, flat data structures
• Performance degradation with many fields

Refactoring:

• Nest optional fields into metadata structures
• Use nested structs for related fields
• Group frequently-accessed fields separately

Example:

# Bad
defmodule User do
  defstruct [
    :id, :email, :name, :age, :address, :city, :state, :zip,
    :phone, :mobile, :fax, :company, :title, :department,
    :created_at, :updated_at, :last_login, :login_count,
    :preference1, :preference2, :preference3, :preference4,
    # ... 15 more fields
  ]
end

# Good
defmodule User do
  defstruct [
    :id,
    :email,
    :name,
    :profile,      # Nested struct
    :preferences,  # Nested struct
    :metadata      # Nested struct
  ]
end

defmodule User.Profile do
  defstruct [:age, :phone, :mobile, :address, :city, :state, :zip]
end

defmodule User.Preferences do
  defstruct [:theme, :notifications, :language]
end

Design-Related Anti-Patterns

1. Alternative Return Types

Problem: Functions with options that drastically change their return type make it unclear what the function actually returns.

Detection:

• Options that change return type structure
• Functions returning different types based on flags
• Unclear function contracts

Refactoring:

• Create separate, specifically-named functions
• Keep return types consistent within a function

Example:

# Bad
def parse(string, opts \\ []) do
  case Integer.parse(string) do
    {int, rest} ->
      if opts[:discard_rest], do: int, else: {int, rest}
    :error ->
      :error
  end
end

# Good
def parse(string) do
  case Integer.parse(string) do
    {int, rest} -> {int, rest}
    :error -> :error
  end
end

def parse_discard_rest(string) do
  case Integer.parse(string) do
    {int, _rest} -> int
    :error -> :error
  end
end

2. Boolean Obsession

Problem: Using multiple booleans with overlapping states instead of atoms or composite types to represent domain concepts.

Detection:

• Multiple boolean parameters
• Overlapping boolean states
• Complex boolean logic

Refactoring:

• Replace multiple booleans with a single atom/enum option
• Prefer atoms over booleans even for single arguments
• Use domain-specific types

Example:

# Bad
def create_user(name, email, admin: false, editor: false, viewer: true) do
  # What if admin: true, editor: true?
end

# Good
def create_user(name, email, role: :viewer) do
  # Clear: role can be :admin, :editor, or :viewer
end

3. Exceptions for Control-Flow

Problem: Using try/rescue for expected errors instead of pattern matching with case statements and tuple returns.

Detection:

• try/rescue blocks for normal operation errors
• Using ! functions and rescuing
• Exceptions in normal business logic

Refactoring:

• Use non-bang functions returning {:ok, value} or {:error, reason}
• Reserve exceptions for invalid arguments and programming errors
• Use pattern matching for error handling

Example:

# Bad
def read_config(path) do
  try do
    content = File.read!(path)
    Jason.decode!(content)
  rescue
    e -> {:error, e}
  end
end

# Good
def read_config(path) do
  with {:ok, content} <- File.read(path),
       {:ok, config} <- Jason.decode(content) do
    {:ok, config}
  end
end

4. Primitive Obsession

Problem: Excessively using basic types (strings, integers) instead of creating composite types to represent structured domain concepts.

Detection:

• Passing related primitives separately
• String/integer parameters representing complex concepts
• Lack of domain modeling

Refactoring:

• Create domain-specific structs or maps
• Introduce parser functions converting primitives to structured data
• Use types to enforce business rules

Example:

# Bad
def create_address(street, city, state, zip, country) do
  # All strings, no validation
  "#{street}, #{city}, #{state} #{zip}, #{country}"
end

# Good
defmodule Address do
  defstruct [:street, :city, :state, :zip, :country]

  def new(attrs) do
    struct!(__MODULE__, attrs)
  end

  def format(%__MODULE__{} = address) do
    "#{address.street}, #{address.city}, #{address.state} #{address.zip}, #{address.country}"
  end
end

5. Unrelated Multi-Clause Function

Problem: Grouping completely unrelated business logic into one multi-clause function.

Detection:

• Single function handling multiple unrelated types
• Overly broad type specifications
• No conceptual relationship between clauses

Refactoring:

• Split into distinct functions with specific names
• Reserve multi-clause patterns for related functionality variations
• Use protocols for polymorphism when appropriate

Example:

# Bad
def update(%Product{} = product) do
  # Product-specific logic
end

def update(%Animal{} = animal) do
  # Completely different animal logic
end

# Good
def update_product(%Product{} = product) do
  # Product-specific logic
end

def update_animal(%Animal{} = animal) do
  # Animal-specific logic
end

# Or use a protocol
defprotocol Updatable do
  def update(item)
end

defimpl Updatable, for: Product do
  def update(product), do: # ...
end

defimpl Updatable, for: Animal do
  def update(animal), do: # ...
end

6. Using Application Configuration for Libraries

Problem: Libraries relying on global application environment configuration prevent multiple dependent applications from configuring the library differently.

Detection:

• Application.get_env/2 or Application.fetch_env!/2 in library code
• Global configuration requirements
• Inability to configure per-consumer

Refactoring:

• Accept configuration via function parameters
• Use keyword lists with sensible defaults
• Allow runtime configuration

Example:

# Bad - Library code
def split(string) do
  parts = Application.fetch_env!(:dash_splitter, :parts)
  String.split(string, "-", parts: parts)
end

# Good
def split(string, opts \\ []) do
  parts = Keyword.get(opts, :parts, 2)
  String.split(string, "-", parts: parts)
end

Usage Guidelines

When reviewing or writing Elixir code:

1. Scan for anti-patterns - Check code against the patterns listed above
2. Explain the problem - Help the developer understand why it's an issue
3. Suggest refactoring - Provide concrete, idiomatic alternatives
4. Consider context - Sometimes anti-patterns are acceptable for specific use cases
5. Prioritize - Focus on high-impact issues first (security, performance, maintainability)

Key Principles

• Let it crash - Use pattern matching to assert expectations; don't write defensive code
• Fail fast - Expose errors early rather than propagating nil or invalid data
• Be explicit - Prefer clear, specific code over clever or terse solutions
• Model your domain - Create types that represent business concepts
• Design for clarity - Code should be obvious to read and maintain