Understanding the & (capture operator) in Elixir

By: Daniel Xu

Understanding the & (capture operator) in Elixir

& is the capture operator in Elixir, it is used to capture and create anonymous functions.

Anonymous functions and arity

Before going into details about the capture operator, let’s get familiar with anonymous functions and arity first.

Given the following example:

add_one = fn x -> x + 1 end

we defined a function, but it isn’t bound to a global name, so it is an anonymous functions or a lambda.

This function takes one argument, so its arity is 1.

How to use &

capture function

Let’s first talk about capturing function. Capture means & can turn a function into an anonymous functions which can be passed as arguments to other function or be bound to a variable.

& can capture two types of functions:

  • function with given name and arity from a module

The notation is: &(module_name.function_name/arity), e.g.

speak = &(IO.puts/1)
speak.("hello")  # hello

We capture puts function from IO module and bind it with a local name speak.

  • local function

In the following example, put_in_columns and put_in_one_row are defined in the same module, so we can capture put_in_one_row by &put_in_one_row/1, notice that we don’t include the module name here.

defmodule Issues.TableFormatter do
  def put_in_columns(data_by_columns, format) do
     Enum.each(data_by_columns, &put_in_one_row/1)
  end

  def put_in_one_row(fields) do
     # Do some things...
  end
end

create anonymous functions

The capture operator can also be used to create anonymous functions, for example:

add_one = &(&1 + 1)
add_one.(1) # 2

is the same with:

add_one = fn x -> x + 1 end
add_one.(1) # 2

You might notice that &1 is used in the above example. That’s called a value placeholder, and it identifies the nth argument of the function

In addition, as {} and [] are also operators in Elixir, & can work with them too.

return_list = &[&1, &2]
return_list.(1, 2) # [1, 2]

return_tuple = &{&1, &2}
return_tuple.(1, 2) # {1, 2}

It’s hard to comprehend at first, we just need to think about it from another perpective:

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