The & Operator in Elixir

28 Feb 2017

Writing short helpers functions in Elixir is a natural behavior since Elixir is a functional programming language and thankfully Elixir provides a shortcut to save us time: The & operator.

Create shortcut anonymous functions

Before we get some explanations let’s see what it looks like writing shortcuts anonymous functions.

fn x -> x + 1 end is same as &(&1 + 1) 
fn x -> x + x end is same as &(&1 + &1)
fn x, y -> x + y end is same as &(&1 + &2)

Now what happens is that the & operator converts the expression inside the parentheses into an anonymous function where &1, &2 and so on correspond to the parameters given to the function. So &(IO.puts &1 <> " " <> &2) is equivalent to fn hello, world -> IO.puts hello <> " " <> world end. Look at the length of each version and judge by yourself.

A particular use case where the & operator is useful is when we have to pass a function to another function (a common task in Elixir when dealing with data structures).

For example, we want to get each atom from the following list [19, :elixir, 77, :phoenix, 64, {:one, "one"}, :erlang] returned as a string.

list = [19, :elixir, 77, :phoenix, 64, {:one, "one"}, :erlang]
list 
|> Enum.filter(&(is_atom(&1))) 
|> Enum.map(&(to_string(&1)))

When run, the above code outputs: ["elixir", "phoenix", "erlang"] which is exactly what we expected.

Capture named function

Another feature of the & operator is to capture named functions. Capturing mean & can turn a named function into an anonymous function.
The above example could be then refactored capturing is_atom and to_string as following:

list = [19, :elixir, 77, :phoenix, 64, {:one, "one"}, :erlang]
list 
|> Enum.filter(&is_atom/1) 
|> Enum.map(&to_string/1)

And the output remains the same.
This is an optimization since instead of creating an anonymous function and allocating new memory, Elixir maintains a direct reference to the original function. Hope you’ll find this helpful and now that you get it go and save your precious time.

Originally published at prima.engineering on February 28, 2017.