Elixir

Recommended Readings

• Good and Bad Elixir

Erlang/OTP

Erlang was proprietary software within Ericsson, which is a telecom company. The software was maintained by the Open Telecom Platform (OTP) product unit, who released Erlang/OTP as open-source in the late 1990s.

The release comes with many handy tools and libraries for development…

• Erlang compiler;

• Test framework;

• Profiler;

• (etc.)

Naming Conventions

https://hexdocs.pm/elixir/master/naming-conventions.html

Casing

Variables, Function Names, Module Attributes, etc.

snake_case is enforced.

Atoms

snake_case is recommended, or PascalCase is also acceptable.

Module Names and Aliases

PascalCase is enforced.

(In fact, the aliases are converted to atoms during compilation, because the modules are always represented by atoms in the Erlang VM.)

Filenames

snake_case is recommended.

Underscore (_foo)

Variables

A value that will not be used must be assigned to _ or to a variable starting with underscore (_foo).

Function Names

If the function name starts with underscore, the function will not be imported by default.

Trailing bang (foo!)

The trailing bang signifies a function or macro where failure cases raise an exception.

The version without ! is preferred when you want to handle different outcomes using pattern matching.

However, if you expect the outcome to always be successful, the bang variation is more convenient because it raises a more helpful error message on failure.

Trailing question mark (foo?)

Functions, except those who are also valid guards, that return a boolean are named with a trailing question mark.

is_ prefix (is_foo)

Type checks and other boolean checks that are allowed in guard clauses are named with the is_ prefix, instead of a trailing question mark.

Special names

length and size

If the function name contains size, the operation runs in O(1) time.

If the function name contains length, the operation runs in O(n) time.

Lists or Tuples?

Lists are stored in memory as linked lists. This means accessing the length of a list is a linear operation.

Tuples are stored contiguously in memory. Getting the tuple size or accessing an element by index is fast. However, updating or adding elements to tuples is expensive because it requires creating a new tuple in memory.

Keyword Lists

Keyword list represents the key-value pairs in a list of 2-item tuples, where the key is always an atom.

## Since the usage of keyword lists is very common
[{:a, 1}, {:b, 2}]

## can be shortened as
[a: 1, b: 2]

When the keyword list is the last argument of a function, the square brackets are optional.

## Therefore
if false, [do: :this, else: :that]

## is equivalent to
if false, do: :this, else: :that

Why Atoms?

Atoms are similar to enum. They are mapped to distinct integers behind the scene.

• Comparing atoms is more efficient than comparing string;

• Declaring the same atom multiple times won’t consume more memory;

• Using atoms (instead of integers) improves code readability.

Multiple Clauses and Guards

Case

case {1, 2, 3} do
  {4, 5, 6} ->
    "This clause won't match"

  {1, x, 3} ->
    "This clause will match and bind x to 2 in this clause"

  _ ->
    "This clause would match any value"
end

Cond

n = 10
cond do
  n % 3 == 0 ->
    "n is a multiple of 3"

  n % 2 == 0 ->
    "n is a even number"

  true ->
    "n is none of the above"
end

With

with {:ok, date} <- next_sunday(),
     {:ok, weather} <- weather_forecast(date) do
  IO.puts("Next sunday is a #{weather} day")
else
  {:error, :date_out_of_range} ->
    IO.puts("Cannot obtain weather forecast")

  error ->
    IO.puts("Unknown error: #{inspect(error)}")
end

Anonymous Function

f = fn
  x, y when x > 0 -> x + y
  x, y -> x * y
end

## [NOTE]
## The number of arguments in each clause needs to be the same, otherwise an error is raised.

How to build an infinite loop in Elixir?

By Recursion

This enables the application to store state as well. Below is an example implementation of a simple key-value store.

defmodule KV do
  def start_link do
    Task.start_link(fn -> loop(%{}) end)
  end

  defp loop(map) do
    receive do
      {:get, key, caller} ->
        send caller, Map.get(map, key)
        loop(map)
      {:put, key, value} ->
        loop(Map.put(map, key, value))
    end
  end
end

Why doesn’t the recursion result in stack overflow?

Because the compiler will apply the Tail Call Optimization (TCO).

https://medium.com/fave-engineering/recursion-tail-call-optimization-and-recursion-ac54b01e3b18

When the function calls itself as the final statement, instead of adding a new stack frame to the call stack, it does a goto.

import and use

import

We import a module to access its functions and macros without referencing its fully-qualified name.

⚠️ The use of import is discouraged because you cannot directly see in what module a function is defined.

require

We require a module to opt-in using the macros.

use

We use a module to allow the module to inject any code in the current module.

💡 Using a module is the same as requiring it and then calling its __using__ function.

Plug

Just another name for “middleware”. There are two implementations.

The Function Implementation

It takes the Plug.Conn struct, optionally modifies it, and returns it.

def hello_world_plug(conn, _opts) do
  conn
  |> put_resp_content_type("text/plain")
  |> send_resp(200, "Hello world")
end

The Module Implementation

The init/1 function initialises the options.

The call/2 function takes the Plug.Conn struct, optionally modifies it, and returns it.

defmodule MyPlug do
  def init([]), do: false
  def call(conn, _opts), do: conn
end