Atoms are constants whose values are their own name.

They are often useful to enumerate over distinct values, such as:

iex> :apple
:apple
iex> :orange
:orange
iex> :watermelon
:watermelon

Atoms are equal if their names are equal.

iex> :apple == :apple
true
iex> :apple == :orange
false

Often they are used to express the state of an operation, by using values such as :ok and :error.

The booleans true and false are also atoms:

iex> true == :true
true
iex> is_atom(false)
true
iex> is_boolean(:false)
true

Elixir allows you to skip the leading : for the atoms false, true, and nil.

Atoms must be composed of Unicode characters such as letters, numbers, underscore, and @. If the keyword has a character that does not belong to the category above, such as spaces, you can wrap it in quotes:

iex> :"this is an atom with spaces"
:"this is an atom with spaces"