optional

option types to make nils a thing of the past

Tony Hoare, inventor of the null reference, calls it his "billion-dollar mistake". You will be no stranger to the ubiquitous NoMethodError: undefined method 'foo' for nil:NilClass. But it doesn't have to be this way.

There are, crucially, two distinct types of values (or rather lack of values) that are usually, in Ruby, represented as nil. There are values that should always be present - and here the fact that they are nil actually indicates that an error has occurred somewhere else in your program - and those values that may or may not be set - where each case is valid. For example, a person may or may not be wearing a hat.

class Hat
  def doff
    …
  end
end

gwen    = Person.create(name: "Gwen", hat: Some[:fedora])
charlie = Person.create(name: "Charlie", hat: None)

class Person

  def greet
    puts "hello!"
    hat.do { |h| doff h }
  end

end

gwen.hat.match do |m|
  m.some (:flat_cap) { puts "Hey up!" }
  m.some (:fedora)   { |h| puts "*touches brim of #{h} respectfully*" }
  m.none             { puts "Hello!" }
end