Optional syntax sheet

Source: Elixir guide, Optional syntax sheet.

Elixir lets you omit a few delimiters. Four rules explain all of them, and they keep the core of the language tiny.

Start from this:

if variable? do
  Call.this()
else
  Call.that()
end

Now take the conveniences away, one at a time:

Each step undoes one convenience. The last line is what the compiler sees.

1. do-end blocks are keywords: if variable?, do: Call.this(), else: Call.that()
2. A keyword list as the last argument needs no square brackets.
3. A keyword list is a list of two-element tuples: [{:do, …}, {:else, …}]
4. Parentheses are optional on function calls.

Why it matters

if, def and defmodule are not special keywords with their own grammar. They are ordinary calls built from these four rules. So this:

defmodule Math do
  def add(a, b) do
    a + b
  end
end

is just:

defmodule(Math, [
  {:do, def(add(a, b), [{:do, a + b}])}
])

That's what lets you extend the language with the same constructs it is built from (see meta-programming).

You don't have to apply these rules by hand: mix format does. It always adds parentheses to calls unless configured otherwise.