rwdaigle/elixir-directives


Lightning talk about the various Elixir directives and their purpose(s)

Host: GitHub


Making Sense of Elixir Directives

This is a lightning talk I gave at the Triangle |> Elixir group discussing the purpose of the various Elixir directives (which I find to be confusing for new Elixirists).


What are Directives?

  • Way to declare, import and reference other Modules
  • Elixir has four of them
  • But, but, why?

^ Think require in Ruby, import in Python

^ Elixir has four, all with slightly different purposes. Based on conversation I was having I find them confusing so I dug in a bit

^ Wrote up my gripes, presented them to Jose & Chris McCord. Summary is they agree it's confusing, but have a hard time figuring out how to change things already set in motion. So, this is my effort to add some clarity to the situation.


Elixir's Directives

  • alias
  • import
  • require
  • use

^ What are Elixir's directives?

^ Last one might not technically be a directive, but it's so closely tied to the purpose of directives that I'm going to abuse the term and include it

alias

Used to reference a module in short-hand

alias Foo.Bar, as: Bar

# Can now do
Bar.function

^ Can also rename a module to avoid conflicts or make clearer to your context

import

Used to easily access functions or macros from other modules

import List, only: [duplicate: 2]

# Can now do
duplicate("hello", 3)

^ Can import all or some functions

^ Seems like this is just special form of alias, yeah?

require

Used to make sure module is compiled and available

require Foo

Foo.a_macro

^ Used a lot for macros which need to be present at compile time

^ Ok, but when don't I want the thing to be compiled and available?

^ Turns out, so you can have circular dependencies, so use alias to soft-reference modules by default

use

Callback that injects code into current context

use ExUnit.Case, async: true

test "now have testing macros" do
	assert true
end

^ Hook point that lets modules bring in external functionality

^ Invokes Module.__using__/1

^ Automatically requires module

Summary

  • alias to more conveniently reference modules
  • import to more conveniently reference functions
  • require to ensure macros are available and compiled
  • use to give module opportunity to "hook" into your code

^ All are lexically scoped

^ Biggest beef is that directives are implementation specific

^ Have to know if what you're using is a macro or just function to know how to invoke it

^ Have to know if module needs to hook into your code to know to use use

^ Shouldn't have to think about these things as a user

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