alias, require, import, and use

Source: Elixir guide, alias, require, import, and use.

Three directives and one macro for reusing code. The first three have lexical scope.

Directive	What it does
alias Foo.Bar, as: Bar	Refer to Foo.Bar as Bar
require Foo	Allow using Foo's macros
import Foo	Call Foo's functions without the Foo. prefix
use Foo	Let Foo inject code into your module

alias

alias Math.List is short for alias Math.List, as: List: the alias defaults to the last part of the name. The original stays reachable as Elixir.List.

Lexical scope

Each directive is valid only from where it appears to the end of its enclosing block. Put it inside a function and only that function sees it.

An alias inside plus/2 doesn't reach minus/2.

require

Macros are expanded at compile time, so you opt in to a module's macros by requiring it.

Integer.is_odd(3)
#=> ** (UndefinedFunctionError) ... there is a macro with the same name and arity. Be sure to require Integer
require Integer
Integer.is_odd(3)   #=> true

import

import lets you call public functions or macros without the prefix. Prefer :only (or :except) to keep the scope small:

import List, only: [duplicate: 2]
duplicate(:ok, 3)   #=> [:ok, :ok, :ok]

Private functions can't be imported. In your own code prefer alias, which keeps it clear where a function comes from.

use

use is an extension point. It requires the module, then calls its __using__/1 macro, which can inject anything. use ExUnit.Case, use GenServer and use Supervisor work this way.

use is require plus a call to __using__/1.

use can run arbitrary code, so read the docs of what you use, and don't reach for it where import or alias would do.

Several at once

alias MyApp.{Foo, Bar, Baz}