Optional syntax sheet

Intro

In this guide so far, we learned that the Elixir syntax allows developers to omit delimiters in a few occasions to make code more readable. For example, we learned that parentheses are optional:

length([1, 2, 3]) == length([1, 2, 3])

and that do-end blocks are equivalent to keyword lists:

# do-end blocks
if true do
  :this
else
  :that
end

# keyword lists
if true, do: :this, else: :that

Keyword lists use Elixir’s regular notation for separating arguments, where we separate each key-value pair with commas, and each key is followed by :. In the do-blocks, we get rid of the colons, the commas, and separate each keyword by a newline. They are useful exactly because they remove the verbosity when writing blocks of code. Most of the time, we use the block syntax, but it is good to know they are equivalent.

Those conveniences, which we call here “optional syntax”, allow the language syntax core to be small, without sacrificing the readability and expressiveness of your code. In this brief chapter, we will review the four rules provided by the language, using a short snippet as playground.

Walk-through

Take the following code:

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

Now let’s remove the conveniences one by one:

1. do-end blocks are equivalent to keywords:

   if variable?, do: Call.this(), else: Call.that()

2. Keyword lists as last argument do not require square brackets, but let’s add them:

   if variable?, [do: Call.this(), else: Call.that()]

3. Keyword lists are the same as lists of two-element tuples:

   if variable?, [{:do, Call.this()}, {:else, Call.that()}]

4. Finally, parentheses are optional, but let’s add them:

   if(variable?, [{:do, Call.this()}, {:else, Call.that()}])

That’s it! Those four rules outline the optional syntax available in Elixir. Those rules apply everywhere consistently, regardless of the construct you are invoking. Whenever you have any questions, this quick walk-through has you covered.

At the end of the day, those rules are what enables us to write:

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

instead of:

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

Finally, if you are worried about when to apply those rules, keep in mind that those concerns are handled by the Elixir formatter. In our day to day, Elixir developers use the mix format task to format our codebases according to a well-defined set of rules defined by the Elixir team and the community. For instance, mix format will always add parentheses to function calls unless explicitly configured to not do so. This ensures all codebases in your company and in the community follow the same standards.