Atoms

What an atom is

An atom is a constant whose name is its value. It starts with a colon, needs no quotes, and compares by identity rather than by content.

status = :ok is_atom(status) # gives true

Atoms never change and take almost no memory, so Elixir uses them as labels everywhere.

true false nil are atoms

The words true, false, and nil look special, but all three are plain atoms. Only their meaning in conditions sets them apart, which the next lesson covers:

is_atom(true) # gives true is_atom(false) # gives true is_atom(nil) # gives true

Aliases are atoms

Module aliases are atoms too, wearing a fancier spelling. Writing IO is shorthand for the atom Elixir.IO, and the runtime can prove it:

is_atom(IO) # gives true Atom.to_string(IO) # gives "Elixir.IO"

This trick explains error messages that mention Elixir followed by a module name.

When atoms win

Atoms shine as tags on small tuples that describe an outcome. By convention, ok tuples carry a result while error tuples carry a reason:

result = {:ok, 42} case result do {:ok, value} -> value {:error, _reason} -> 0 end # gives 42

Matching on the tag first keeps the happy path and the failure path side by side.

Next: Truthiness: only false and nil

Article author: Arthur Isaev