madlib.rb

#!/usr/bin/env ruby

template = <<~HEREDOC
  {{plural noun}} are {{color}},
  {{plural noun}} are {{color}},
  {{food}} is {{adjective}},
  and so are you.
HEREDOC

def prompt(type)
  print type << ': '
  gets.chomp
end

def replace_fields(template)
  template.scan(/{{([^}]+)}}/).flatten.inject(template) do |result, type|
    result.sub "{{#{type}}}", prompt(type)
  end
end

madlib = replace_fields(template)
puts
puts madlib