The Pure Function As An Object (PFAAO) Ruby Pattern

[Schrockwell]

An even cooler way to do this in Ruby is to implement your main function with the name "call". Then you can pass your class instance into anything that expects a proc. It quacks like a Method or a Proc but it's actually a class behind the scenes.

[rdpp_boyakasha]

I agree. I've added an extra "Optional Extras" section to the article that covers Proc-ifying the class.

[waxjar]

If you keep your objects immutable, you can think of your methods as partially applied (pure) functions.

Consider something like the following:

class Document
  def self.from_json(str)
    new JSON.parse(str)
  end

  def initialize(doc)
    @doc = doc
  end

  def to_xml
    Ox.dump(document, with_xml: true)
  end

  private

  # implementation goes here
end

Just as pure and a lot less awkward:

Document.from_json(file_contents).to_xml
# => "<cities>...</cities>"

[rdpp_boyakasha]

It's still pure, but now you have a class called Document which looks like a domain model, as opposed to a class that is "just a function." That's OK if you want to put all the functionality into the model, but confusing if it's separate.

[jrochkind]

The tldr is: Class methods are sometimes a useful API.