Ruby Operator Lookup

The underscore symbol (_) is used in a few different contexts in Ruby.

Readable Number Notation

Glance at the following number real quick. How long does it take to figure out how big it is?

> 10000000
=> 10000000

We can make it easier for ourselves and future readers of our code with some underscores to separate the thousands.

> 10_000_000
=> 10000000

Implicit Positional Arguments in a Block

We can use _1, _2, etc. to refer to the first, second, etc. positional arguments in a block.

> def method_using_block(a, b)
  yield(a, b) if block_given?
end
=> :method_using_block
> method_using_block(4,5) { _2 ** _1 }
=> 625

Unused Variable

This is purely a convention. If you have a positional argument for a method that you don't care about and won't use, you can indicate that it is unused with a leading underscore.

def method_with_unused_argument(_, b)
  b
end

A practical example of this is from Noel Rappin's article on Keyword Arguments ↗ where the concept of a dual interface is introduced.

> def dual_interface(_name = nil, _state = nil, name: _name, state: _state)
  p "#{name}: #{state}"
end

=> :dual_interface
> dual_interface("Taco", "Bell")
=> "Taco: Bell"
> dual_interface(name: "Taco", state: "Bell")
=> "Taco: Bell"
> dual_interface("Taco", state: "Bell")
=> "Taco: Bell"
> dual_interface()
=> ": "

Method and File Names

In Ruby, we snake_case all over the place.

Methods names: ActiveRecord::Base.sanitize_sql_array
File names: app/controllers/reading_statuses_controller.rb

Double Underscore Keywords

There are a few keywords ↗ in Ruby that use (double) underscores, such as __FILE__ and __LINE__.

puts <<-`HEREDOC`
  cat #{__FILE__}
HEREDOC

There is also the embedded data ↗ feature that relies on __END__.