Ruby Data Class: Syntax

July 21, 2025

This content originally appeared on DEV Community and was authored by Kelly Popko

What is Ruby’s Data class?

The Data class was introduced to Ruby in Ruby 3.2, to:

‘define simple classes for value-alike’
store immutable atomic values

Create the blueprint (instance of Data class)

We call Data.define and pass keyword arguments.

Book = Data.define(:title, :author, :year)

We can define methods as well if we pass it a block.

Book = Data.define(:title, :author, :year) do
  SUMMARY = '%<title>s was written by %<author>s in %<year>i.'

  def to_s
    SUMMARY % {title:, author:, year:}
  end
end

hobbit = Book.new("The Hobbit", "J.R.R. Tolkien", 1937)
puts hobbit
# => nil
'The Hobbit was written by J.R.R. Tolkien in 1937.'

Building from the blueprint

Call .new or use [] notation
Use keyword or positional arguments

`.new` + keyword arguments

hobbit = Book.new(title: "The Hobbit", author: "J. R. R. Tolkien", year: 1937)
# => <data Book title="The Hobbit", author="J. R. R. Tolkien", year=1937>

hobbit.author
# => "J. R. R. Tolkien"

hobbit.year
# => 1937

hobbit.title
# => "The Hobbit"

Bracket Notation + Positional arguments

Location = Data.define(:latitude, :longitude)
# => Location

We can use positional arguments but need to take care with the order.

philadelphia = Location[39.9526, -75.1652]
# => #<data Location latitude=39.9526, longitude=-75.1652>

philadelphia.latitude
# => 39.9526

philadelphia = Location[-75.1652, 39.9526]
# => #<data Location latitude=-75.1652, longitude=39.9526>

philadelphia.latitude
# => -75.1652

Bracket Notation + Keyword Arguments

Note that with keyword arguments, we have flexibility in the order we pass arguments.

philadelphia = Location[latitude: 39.9526, longitude:-75.1652]
# => #<data Location latitude=39.9526, longitude=-75.1652>

philadelphia.latitude
# => 39.9526

philadelphia = Location[longitude:-75.1652, latitude: 39.9526]
# => #<data Location latitude=39.9526, longitude=-75.1652>

philadelphia.latitude
# => 39.9526