Atoms

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.8.0", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Setup

Ensure you type the ea keyboard shortcut to evaluate all Elixir cells before starting. Alternatively, you can evaluate the Elixir cells as you read.

Review Questions

Upon completing this lesson, a student should be able to answer the following questions.

• What is an atom and how might we use them in our programs?
• Why might we use atoms for the sake of comparing constant values rather than strings?

Atoms

Atoms are named constants. In other words, their name is their value.

Atoms are often used to represent common constants in your program. For example, they often represent the :success or :error of a program.

:success
:error

They might also be used to represent the state of some action. For example:

:completed
:in_progress
:starting

You might wonder why we have both strings and atoms because they seem to do the same thing. One primary reason is performance. Atoms are stored in an atom table and can be referenced by a key. This makes it way faster to check if two atoms are equal.

What is an atom table? Imagine it like an excel spreadsheet. Each value has a key. For the computer, comparing these keys in the table is much more performance that checking if every character in a string is equal.

It’s kind of like checking if 1 == 2 instead of checking if"HeLloWorld == HeLloWorLd".

Atoms are defined using a colon : and a series of letters, digits, and certain valid symbols.

Rules For Naming Atoms

There are specific rules for naming atoms, but fortunately you don’t need to memorize them! Instead, pay attention to the colors in your code. If you define an atom incorrectly, you’ll notice that it’s no longer blue.

Here are some valid atoms:

:hello
:my_atom1

Here are some invalid atoms, notice they are mostly white:

:$this_is_invalid
:!this_is_invalid
:2
:@
:$
:?
:invalid # spaces are not valid

Sometimes languages establish conventions. Conventions are common patterns of doing things which the community agrees upon. For example, In Elixir it’s conventional for atoms to have lowercase names separated by underscores.

While the following will work:

:MY_ATOM!

It’s usually unconventional to name an atom with capital letters except in specific circumstances.

You can use quotes with atoms to avoid name rules.

:"$Now we don't have to follow any rules. This is a valid atom (though unconventional)"

Secret Atoms

nil, false, and true are all actually atoms.

We can use === to verify that :nil and nil are equivalent.

nil === :nil

The same goes for true, and false. They are all atoms but omit the colon :.

true === :true
false === :false

Your Turn

In the Elixir cell below, use === to check if nil is equal to :nil.

Check if true is equal to :true.

Check if false is equal to :false.

Further Reading

Consider the following resource(s) to deepen your understanding of the topic.

• Elixir School: Atoms
• HexDocs: Atoms
• Exercism: Atoms

Mark As Completed

file_name = Path.basename(Regex.replace(~r/#.+/, __ENV__.file, ""), ".livemd")

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "atoms_reading"
    "exercises" -> "atoms_exercise"
  end

progress_path = __DIR__ <> "/../progress.json"
existing_progress = File.read!(progress_path) |> Jason.decode!()

default = Map.get(existing_progress, save_name, false)

form =
  Kino.Control.form(
    [
      completed: input = Kino.Input.checkbox("Mark As Completed", default: default)
    ],
    report_changes: true
  )

Task.async(fn ->
  for %{data: %{completed: completed}} <- Kino.Control.stream(form) do
    File.write!(
      progress_path,
      Jason.encode!(Map.put(existing_progress, save_name, completed), pretty: true)
    )
  end
end)

form

Commit Your Progress

Run the following in your command line from the curriculum folder to track and save your progress in a Git commit. Ensure that you do not already have undesired or unrelated changes by running git status or by checking the source control tab in Visual Studio Code.

$ git checkout -b atoms-reading
$ git add .
$ git commit -m "finish atoms reading"
$ git push origin atoms-reading

Create a pull request from your atoms-reading branch to your solutions branch. Please do not create a pull request to the DockYard Academy repository as this will spam our PR tracker.

DockYard Academy Students Only:

Notify your teacher by including @BrooklinJazz in your PR description to get feedback. You (or your teacher) may merge your PR into your solutions branch after review.

If you are interested in joining the next academy cohort, sign up here to receive more news when it is available.

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