Protocols

Remarks#

A note on structs

Instead of sharing protocol implementation with maps, structs require their own protocol implementation.

Introduction

Protocols enable polymorphism in Elixir. Define protocols with defprotocol:

defprotocol Log do
  def log(value, opts)
end

Implement a protocol with defimpl:

require Logger
# User and Post are custom structs

defimpl Log, for: User do
  def log(user, _opts) do
    Logger.info "User: #{user.name}, #{user.age}"
  end
end

defimpl Log, for: Post do
  def log(user, _opts) do
    Logger.info "Post: #{post.title}, #{post.category}"
  end
end

With the above implementations, we can do:

iex> Log.log(%User{name: "Yos", age: 23})
22:53:11.604 [info]  User: Yos, 23
iex> Log.log(%Post{title: "Protocols", category: "Protocols"})
22:53:43.604 [info]  Post: Protocols, Protocols

Protocols let you dispatch to any data type, so long as it implements the protocol. This includes some built-in types such as Atom, BitString, Tuples, and others.