Functions and Modules

Named Functions

Named functions are defined inside modules using def and defp. They are compiled and can be called from other modules (for def) or only within the defining module (for defp).

def and defp

defmodule Math do
  # Public function
  def add(a, b) do
    a + b
  end

  # Public function with shorthand
  def subtract(a, b), do: a - b

  # Private function
  defp double(x), do: x * 2

  # Private helper used internally
  def quadruple(x), do: double(double(x))
end

iex> Math.add(2, 3)
5
iex> Math.double(4)
** (UndefinedFunctionError) function Math.double/1 is undefined (private)

Default Arguments

Functions can have default values for parameters. Default arguments are evaluated at compile time:

defmodule Greeter do
  def greet(name, greeting \\ "Hello") do
    "#{greeting}, #{name}!"
  end

  # Multiple defaults
  def connect(host, port \\ 80, protocol \\ "http") do
    "#{protocol}://#{host}:#{port}"
  end
end

iex> Greeter.greet("World")
"Hello, World!"
iex> Greeter.greet("World", "Hi")
"Hi, World!"
iex> Greeter.connect("example.com")
"http://example.com:80"
iex> Greeter.connect("example.com", 443, "https")
"https://example.com:443"

When using default arguments, Elixir generates multiple clause heads. This means the function can be called with different arities. Use &Greeter.greet/2 (with 2 args) or &Greeter.greet/1 (with 1 arg).

Function Arity

Every function is identified by both its name and arity (number of arguments). Math.add/2 and Math.add/3 are different functions:

defmodule Example do
  def process(data), do: process(data, [])
  def process(data, opts), do: process(data, opts, %{})
  def process(data, opts, context) do
    # Full implementation
    {data, opts, context}
  end
end

## Arity is part of the function identity
iex> &Example.process/1
#Function<...>
iex> &Example.process/2
#Function<...>
iex> &Example.process/3
#Function<...>

Multiple Clauses and Pattern Matching

Named functions can have multiple clauses with different patterns. Elixir evaluates them top-to-bottom:

defmodule ListUtils do
  def first([]), do: nil
  def first([head | _tail]), do: head

  def classify(0), do: :zero
  def classify(n) when n > 0, do: :positive
  def classify(_n), do: :negative

  def describe({:ok, value}), do: "Success: #{inspect(value)}"
  def describe({:error, reason}), do: "Error: #{inspect(reason)}"
  def describe(:loading), do: "Still loading"
  def describe(other), do: "Unknown: #{inspect(other)}"
end

The Catch-all Clause

A catch-all clause should come last. Using an unbound variable as the catch-all (which will match any value and bind it) is idiomatic, but the underscore-prefixed variable (_other) avoids unused variable warnings:

def handle({:ok, data}), do: process(data)
def handle({:error, _reason}), do: :retry
def handle(_other), do: :unhandled

Anonymous Functions

Anonymous functions (also called lambdas or closures) are first-class values. They are created with fn...end or the capture shorthand &.

fn…end Syntax

## Basic anonymous function
add = fn a, b -> a + b end
iex> add.(2, 3)
5

# Multi-line anonymous function
greet = fn name ->
  greeting = "Hello"
  "#{greeting}, #{name}!"
end
iex> greet.("Alice")
"Hello, Alice!"

# Anonymous function with pattern matching
classify = fn
  n when n > 0 -> :positive
  0 -> :zero
  _ -> :negative
end
iex> classify.(5)
:positive

# Anonymous function with multiple clauses
handle = fn
  {:ok, value} -> {:success, value}
  {:error, reason} -> {:failure, reason}
end
iex> handle.({:ok, 42})
{:success, 42}

Shorthand Capture (&)

The & operator creates anonymous functions by capturing existing functions or creating shorthands:

# Capturing a named function
add = &Kernel.+/2
iex> add.(2, 3)
5

# Capturing a module function
upcase = &String.upcase/1
iex> upcase.("hello")
"HELLO"

# Shorthand: &1, &2, etc. are positional arguments
double = &(&1 * 2)
iex> double.(5)
10

add_one = &(&1 + 1)
iex> add_one.(99)
100

# Multiple arguments
swap = &{&2, &1}
iex> swap.(1, 2)
{2, 1}

# Capturing a local or imported function
map_double = &Enum.map(&1, fn x -> x * 2 end)
# This is equivalent to:
# fn list -> Enum.map(list, fn x -> x * 2 end) end

Capturing Module Functions

The &Module.function/arity syntax captures a named function as a function value:

# From Enum module
Enum.map([1, 2, 3], &Integer.to_string/1)
# ["1", "2", "3"]

# Capturing with &/1, &/2 etc. notation
square = &(&1 * &1)
# This is different from &Mod.fun/arity -- the former is a shorthand,
# the latter captures an existing named function

# Partial application (not built-in, but achievable)
add = fn a, b -> a + b end
add_five = &add.(5, &1)
iex> add_five.(3)
8

Closures

Anonymous functions close over their enclosing scope, capturing variable bindings:

defmodule Counter do
  def make_counter(initial) do
    count = initial

    fn ->
      count = count + 1
      count
    end
  end
end

# Note: the above doesn"t work as expected because Elixir is immutable.
# Each call creates a new binding. For stateful closures, use processes:
defmodule StatefulCounter do
  def start(initial) do
    Agent.start_link(fn -> initial end, name: __MODULE__)
  end

  def increment do
    Agent.update(__MODULE__, &(&1 + 1))
    Agent.get(__MODULE__, & &1)
  end

  def get do
    Agent.get(__MODULE__, & &1)
  end
end

Module Definitions

defmodule

Modules are the basic unit of organization in Elixir. They group related functions, provide namespacing, and enable code reuse through composition.

defmodule MyApp.Calculator do
  @moduledoc """
  A calculator module for basic arithmetic operations.

  ## Examples

      iex> MyApp.Calculator.add(2, 3)
      5
  """

  @doc """
  Adds two numbers.
  """
  def add(a, b), do: a + b

  @doc """
  Subtracts the second number from the first.
  """
  def subtract(a, b), do: a - b

  @doc false
  defp validate_input(n) when is_number(n), do: :ok
  defp validate_input(_), do: {:error, :not_a_number}
end

Module Attributes

Module attributes are compile-time constants stored in the module:

defmodule Config do
  @moduledoc "Module-level documentation"

  @doc "Function-level documentation"
  def my_function, do: :ok

  # Constant attribute
  @version "1.0.0"
  def version, do: @version

  # Compile-time attribute
  @before_compile MyApp.Logger

  # Accumulating attribute
  Module.register_attribute(__MODULE__, :handlers, accumulate: true)

  @handlers :json
  @handlers :xml

  def handlers, do: @handlers  # [:json, :xml]
end

Common module attributes:

Attribute	Purpose
`@moduledoc`	Documentation for the module
`@doc`	Documentation for the next function
`@doc false`	Hide function from documentation
`@spec`	Typespec for a function
`@type`	Type alias
`@callback`	Behaviour callback definition
`@impl`	Mark behaviour implementation
`@behaviour`	Declare a behaviour
`@before_compile`	Callback before module is compiled
`@on_definition`	Callback on every function definition
`@derive`	Auto-implement protocol
`@enforce_keys`	Enforce keys in struct

Nested Modules

defmodule MyApp do
  defmodule Web do
    defmodule Router do
      def match(path) do
        # ...
      end
    end

    defmodule Controller do
      def render(template, assigns) do
        # ...
      end
    end
  end

  defmodule DB do
    def query(sql) do
      # ...
    end
  end
end

# Access with full path
MyApp.Web.Router.match("/users")

# Shorthand with alias
alias MyApp.Web.Router
Router.match("/users")

# Or multiple aliases at once
alias MyApp.{Web.Router, Web.Controller}

alias

alias creates a short name for a module:

# Basic alias
alias MyApp.User
User.find(1)

# Alias with :as option
alias MyApp.Web.{Controller, Router, as: Web}
# Router is available as Web.Router
# Controller is available as Web.Controller

# Alias all modules under a namespace
alias MyApp.Web.{Controller, Router, Middleware}
# Controller, Router, Middleware all accessible

# Scoped alias
alias MyApp.User, as: Person
Person.find(1)

require, use, and import

# require - makes a module's macros available
require Logger
Logger.info("This works")

# use - calls __using__/1 macro on the module
use GenServer
# Equivalent to: require GenServer; GenServer.__using__(__MODULE__)

# use with options
use Ecto.Repo, otp_app: :my_app

# import - brings functions into current scope
import List, only: [flatten: 1, duplicate: 2]
import String, only: :functions
import String, except: [split: 2]
import Enum, only: :macros

Key differences:

Directive	Effect	Common Use
`require`	Makes macros available	`require Logger`
`use`	Calls `__using__/1` macro	`use GenServer`
`import`	Brings functions/macros into scope	`import Enum`
`alias`	Creates short name	`alias MyApp.User`

Structs

defstruct

Structs are maps with a fixed set of fields, default values, and compile-time type checking:

defmodule User do
  @moduledoc "Represents a user in the system"

  @enforce_keys [:id, :name]
  defstruct [:id, :name, :email, age: 0, role: :guest]
end

# Creating structs
iex> %User{id: 1, name: "Alice"}
%User{id: 1, name: "Alice", email: nil, age: 0, role: :guest}

# @enforce_keys ensures these fields must be provided
iex> %User{name: "Alice"}
** (ArgumentError) the following keys must also be given when building struct User: [:id]

# Accessing fields
iex> u = %User{id: 1, name: "Alice", age: 30}
iex> u.name
"Alice"
iex> u[:name]
"Alice"

# Updating structs (creates new struct)
iex> updated = %{u | age: 31}
%User{id: 1, name: "Alice", email: nil, age: 31, role: :guest}

# Pattern matching on structs
defmodule UserService do
  def adult?(%User{age: age}) when age >= 18, do: true
  def adult?(%User{}), do: false

  def greet(%User{name: name, role: :admin}), do: "Welcome, Admin #{name}"
  def greet(%User{name: name}), do: "Hello, #{name}"
end

# Struct comparison
iex> %User{id: 1, name: "A"} == %User{id: 1, name: "A"}
true
iex> %User{id: 1} == %User{id: 2}
false

# Checking struct type
iex> is_struct(%User{})
true
iex> is_struct(%User{}, User)
true
iex> is_struct(%{name: "Alice"}, User)
false

# Map-like operations on structs
iex> Map.keys(%User{id: 1, name: "A"})
[:id, :name, :email, :age, :role]

# Structs are maps under the hood
iex> %User{}.__struct__
User
iex> is_map(%User{})
true

Structs vs Maps

Feature	Struct	Map
Fixed keys	Yes	No
Default values	Yes	No
Required keys (`@enforce_keys`)	Yes	No
Type check (`%ModuleName{}`)	Yes	No
Pattern matching specificity	Yes (specific struct type)	No
Compile-time dispatch	Yes (protocols)	No
Performance	Slightly faster for known keys	General purpose

Protocols

Protocols provide polymorphic dispatch based on the type of the first argument. They are similar to interfaces or type classes.

Defining a Protocol

defprotocol Size do
  @doc "Returns the size of a data structure"
  def size(data)
end

# Implementations for different types
defimpl Size, for: List do
  def size(list), do: length(list)
end

defimpl Size, for: Map do
  def size(map), do: map_size(map)
end

defimpl Size, for: Tuple do
  def size(tuple), do: tuple_size(tuple)
end

defimpl Size, for: BitString do
  def size(binary), do: byte_size(binary)
end

defimpl Size, for: MapSet do
  def size(set), do: MapSet.size(set)
end

Using Protocols

iex> Size.size([1, 2, 3])
3
iex> Size.size(%{a: 1, b: 2})
2
iex> Size.size({:a, :b, :c})
3

# Derive protocol for structs
defstruct [:items]

defimpl Size, for: Collection do
  def size(%Collection{items: items}), do: length(items)
end

# Or derive automatically for structs
defimpl Size, for: Any do
  def size(_), do: 0
end

Built-in Protocols

Elixir ships with several built-in protocols:

Protocol	Purpose	Key Functions
`Inspect`	String representation	`inspect/1`
`String.Chars`	String conversion	`to_string/1`
`Enumerable`	Collection operations	`Enum.reduce/3`, `Enum.member?/2`
`List.Chars`	Charlist conversion	`to_charlist/1`
`Collectable`	Into collection	`Enum.into/2`

@derive

Structs can automatically implement protocols using @derive:

defmodule Point do
  @derive [Inspect, String.Chars]
  defstruct [:x, :y]

  defimpl String.Chars, for: Point do
    def to_string(%Point{x: x, y: y}), do: "(#{x}, #{y})"
  end
end

Behaviours

Behaviours define a set of callbacks that a module must implement. They are similar to interfaces or abstract classes in OOP.

Defining a Behaviour

defmodule Storage do
  @moduledoc "Behaviour for key-value storage backends"

  @type key :: any()
  @type value :: any()
  @type error :: {:error, term()}

  @callback init(opts :: keyword()) :: {:ok, state :: term()} | error()
  @callback put(state :: term(), key(), value()) :: {:ok, term()} | error()
  @callback get(state :: term(), key()) :: {:ok, value()} | {:error, :not_found}
  @callback delete(state :: term(), key()) :: {:ok, term()} | error()
  @callback list(state :: term()) :: [key()]
end

Implementing a Behaviour

defmodule MemoryStorage do
  @behaviour Storage

  @impl true
  def init(_opts), do: {:ok, %{}}

  @impl true
  def put(state, key, value), do: {:ok, Map.put(state, key, value)}

  @impl true
  def get(state, key) do
    case Map.fetch(state, key) do
      {:ok, value} -> {:ok, value}
      :error -> {:error, :not_found}
    end
  end

  @impl true
  def delete(state, key), do: {:ok, Map.delete(state, key)}

  @impl true
  def list(state), do: Map.keys(state)
end

@impl

The @impl attribute marks a function as implementing a behaviour callback:

@impl Storage        # explicitly names the behaviour
def put(state, key, value), do: ...

@impl true           # infers from @behaviour declaration
def get(state, key), do: ...

@impl false          # explicitly NOT a callback
def private_helper, do: ...

Typespecs

Typespecs provide optional type annotations for documentation, static analysis (Dialyzer), and IDE support.

@type, @typep, @opaque

defmodule MyApp.Types do
  # Public type alias
  @type user_id :: String.t()
  @type email :: String.t()
  @type age :: non_neg_integer()

  # Struct type
  @type user :: %{
    id: user_id(),
    name: String.t(),
    email: email() | nil,
    age: age()
  }

  # Private type
  @typep internal_state :: :idle | :running | :stopped

  # Opaque type (type is exported but implementation is hidden)
  @opaque token :: binary()
end

@spec

Function specifications declare the expected types for function parameters and return values:

defmodule Math do
  @spec add(number(), number()) :: number()
  def add(a, b), do: a + b

  @spec safe_div(number(), number()) :: {:ok, float()} | {:error, :division_by_zero}
  def safe_div(_num, 0), do: {:error, :division_by_zero}
  def safe_div(num, denom), do: {:ok, num / denom}

  @spec classify(number()) :: :positive | :zero | :negative
  def classify(n) when n > 0, do: :positive
  def classify(0), do: :zero
  def classify(_n), do: :negative

  @spec fetch(map(), any()) :: {:ok, any()} | {:error, :not_found}
  def fetch(map, key) do
    case Map.fetch(map, key) do
      {:ok, value} -> {:ok, value}
      :error -> {:error, :not_found}
    end
  end
end

Built-in Types

# Basic types
@spec basic_types() :: term()
# term() is the top type (any value)

# Atomic types
@spec atoms() :: atom() | boolean() | nil
# atom() includes all atoms, including :true, :false, :nil

# Numbers
@spec nums() :: number() | integer() | float() | neg_integer() | non_neg_integer() | pos_integer()

# Collections
@spec collections() ::
  list(integer()) |           # list of integers
  [integer()] |               # same as list(integer())
  map(atom(), String.t()) |   # map with atom keys and string values
  {:ok, String.t()} |         # tuple
  String.t()                  # String.t() is an alias for binary()

@callback with Typespecs

defmodule Handler do
  @type event :: map()
  @type result :: {:ok, map()} | {:error, String.t()}

  @callback handle(event()) :: result()
  @callback init(keyword()) :: {:ok, term()} | {:error, String.t()}
end

Comprehensions

for Comprehensions

# Basic comprehension
iex> for x <- [1, 2, 3], do: x * 2
[2, 4, 6]

# Multiple generators
iex> for x <- [1, 2], y <- [:a, :b], do: {x, y}
[{1, :a}, {1, :b}, {2, :a}, {2, :b}}

# With filter
iex> for x <- 1..10, rem(x, 2) == 0, do: x
[2, 4, 6, 8, 10]

# With :into (build maps, structsets, etc.)
iex> for {k, v} <- [a: 1, b: 2], into: %{}, do: {k, v}
%{a: 1, b: 2}

iex> for x <- [1, 2, 3, 2, 1], into: MapSet.new(), do: x
MapSet.new([1, 2, 3])

# With :reduce (folding)
iex> for x <- [1, 2, 3], reduce: 0, do: (acc -> acc + x)
6

# The reduce option transforms the comprehension into a reducer.
# Syntax: for pattern <- enumerable, reduce: acc, do: (acc -> new_acc)

# Practical: building a map from a list
users = [%{id: 1, name: "Alice"}, %{id: 2, name: "Bob"}]
for user <- users, into: %{}, do: {user.id, user}
# %{1 => %{id: 1, name: "Alice"}, 2 => %{id: 2, name: "Bob"}}

# Practical: filtering and transforming
for {_key, value} <- %{a: 1, b: 2, c: 3}, value > 1, into: [], do: value
# [2, 3]

Exception Handling

raise and try/rescue

# Defining a custom exception
defmodule MyApp.Error do
  defexception [:message, :code]

  @impl true
  def message(%{message: msg, code: code}) do
    "[#{code}] #{msg}"
  end
end

# Raising exceptions
raise "something went wrong"
raise MyApp.Error, message: "invalid input", code: :bad_input

# Handling exceptions
try do
  risky_operation()
rescue
  RuntimeError -> :handled_runtime_error
  ArithmeticError -> :handled_arithmetic_error
  %MyApp.Error{code: code} -> {:error, code}
  error in [ArgumentError, TypeError] ->
    {:error, Exception.message(error)}
else
  result -> {:ok, result}
after
  cleanup()
end

try/rescue vs pattern matching

In Elixir, exceptions are used sparingly. The preferred pattern is to return {:ok, result} or {:error, reason} tuples and use pattern matching:

# Preferred pattern
case File.read("data.txt") do
  {:ok, content} -> process(content)
  {:error, :enoent} -> IO.puts("File not found")
  {:error, reason} -> IO.puts("Error: #{reason}")
end

# vs. exception handling (avoid unless interfacing with Erlang)
try do
  :gen_tcp.send(socket, data)
rescue
  _ -> :error
end

Intuition

Modules are labeled drawers, protocols are universal adapters: A module is like a labeled drawer in a workshop — it groups related tools (functions) together so you can find them. defp is a drawer with a lock: only tools inside that workshop can use it. Structs are molds for casting consistent shapes — every User struct has the same slots. Protocols are universal power adapters: they let you plug any type into a standard interface, so the Size protocol works on lists, maps, tuples, and your custom types without them sharing a common ancestor.

Why it matters: The module + struct + protocol pattern gives Elixir ad-hoc polymorphism without class hierarchies. You define behavior once (the protocol) and implement it for each type independently — no need for the types to know about each other.

The key insight: In Elixir, you don’t need inheritance to share behavior — protocols let any type opt into any interface, and behaviours let any module conform to any contract.

Summary

Functions and modules are the organizational backbone of Elixir:

def for public functions, defp for private
Anonymous functions with fn...end or & shorthand
Modules provide namespacing with defmodule, alias, require, use, import
Structs add structure and type safety to maps with defstruct
Protocols provide polymorphism based on type dispatch
Behaviours define interfaces with @callback and @impl
Typespecs with @type and @spec add optional static type checking

Common Mistakes

Confusing import, alias, and require: import brings functions into scope; alias creates a shorthand name; require makes macros available. Using the wrong one causes “undefined function” or “unused import” errors.

Forgetting @enforce_keys in structs: Without @enforce_keys, struct fields default to nil silently. Always enforce required fields to catch missing data at compile time.

Using exceptions for control flow: Elixir favors {:ok, result} / {:error, reason} tuples over raise/rescue. Exceptions are for truly exceptional cases, not expected error paths.