Dorian Dorian - 1 year ago 51
Ruby Question

What does double splat (**) argument mean in this code example and why use it?

So I've been over Traiblazer and Reform documentation and I often see this kind of code

class AlbumForm < Reform::Form
collection :songs, populate_if_empty: :populate_songs! do
property :name

def populate_songs!(fragment:, **)
Song.find_by(name: fragment["name"]) or

Notice the
def populate_songs!(fragment:, **)

I'm well aware of double splat named arguments (like
) that capture all other keyword arguments. But I've never seen
alone, without a name.

So my 2 questions are:

  1. what does ** mean in the block above?

  2. why use this syntax?

Answer Source

what does ** mean in the block above?

It's a kwsplat, but it's not assigned a name. So this method will accept arbitrary set of keyword arguments and ignore all but :fragment.

why use this syntax?

To ignore arguments you're not interested in.

A little demo

class Person
  attr_reader :name, :age

  def initialize(name:, age:)
    @name = name
    @age = age

  def description
    "name: #{name}, age: #{age}"

class Rapper < Person
  def initialize(name:, **)
    name = "Lil #{name}" # amend one argument
    super # send name and the rest (however many there are) to super
end 'John', age: 25).description # => "name: John, age: 25" 'John', age: 25).description # => "name: Lil John, age: 25"
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