Elixir: use vs import

import Module brings all the Functions and Macros of Module un-namespaced into your module.

require Module allows you to use macros of Module but does not import them. (Functions of Module are always available namespaced.)

use Module first requires module and then calls the __using__ macro on Module.

Consider the following:

defmodule ModA do
  defmacro __using__(_opts) do
    IO.puts "You are USING ModA"
  end

  def moda() do
    IO.puts "Inside ModA"
  end
end

defmodule ModB do
  use ModA

  def modb() do
    IO.puts "Inside ModB"
    moda()     # <- ModA was not imported, this function doesn't exist
  end
end

This will not compile as ModA.moda() has not been imported into ModB.

The following will compile though:

defmodule ModA do
  defmacro __using__(_opts) do
    IO.puts "You are USING ModA"
    quote do          # <--
      import ModA     # <--
    end               # <--
  end

  def moda() do
    IO.puts "Inside ModA"
  end
end

defmodule ModB do
  use ModA

  def modb() do
    IO.puts "Inside ModB"
    moda()            # <-- all good now
  end
end

As when you used ModA it generated an import statement that was inserted into ModB.

use is intended for injecting code into the current module, while import is used to, well, import functions for use. You can build a use implementation which automatically imports functions for example, as I do with Timex when you add use Timex to a module, take a look at timex.ex if you want to know what I mean, it's a very simple example of how to build a module which can be use'd

See «alias, require, and import» page from the elixir official getting started guide:

# Ensure the module is compiled and available (usually for macros)
require Foo

# Import functions from Foo so they can be called without the `Foo.` prefix
import Foo

# Invokes the custom code defined in Foo as an extension point
use Foo

Require

Elixir provides macros as a mechanism for meta-programming (writing code that generates code).

Macros are chunks of code that are executed and expanded at compilation time. This means, in order to use a macro, we need to guarantee its module and implementation are available during compilation. This is done with the require directive.

In general a module does not need to be required before usage, except if we want to use the macros available in that module.

Import

We use import whenever we want to easily access functions or macros from other modules without using the fully-qualified name. For instance, if we want to use the duplicate/2 function from the List module several times, we can import it:

iex> import List, only: [duplicate: 2]
List
iex> duplicate :ok, 3
[:ok, :ok, :ok]

In this case, we are importing only the function duplicate (with arity 2) from List.

Note that importing a module automatically requires it.

Use

Although not a directive, use is a macro tightly related to require that allows you to use a module in the current context. The use macro is frequently used by developers to bring external functionality into the current lexical scope, often modules.

Behind the scenes, use requires the given module and then calls the __using__/1 callback on it allowing the module to inject some code into the current context. Generally speaking, the following module:

defmodule Example do
  use Feature, option: :value
end

is compiled into

defmodule Example do
  require Feature
  Feature.__using__(option: :value)
end