Demonstrating the "final tagless" approach to the "Expression Problem" in Ruby

Final-Tagless Views.rb

class View
  def self.image(url:)
    Image.new.tap { |img| img.url = url }
  end

  def self.button(title:)
    Button.new.tap { |btn| btn.title = title }
  end

  def self.stack(subviews)
    Stack.new.tap { |stk| stk.subviews = subviews }
  end

  def self.text(text)
    Text.new.tap { |txt| txt.text = text }
  end

  def self.map(location:)
    Map.new.tap { |map| map.location = location }
  end
end

class Image < View
  attr_accessor :url
end

class Button < View
  attr_accessor :title
end

class Stack < View
  attr_accessor :subviews
end

class Text < View
  attr_accessor :text
end

class Map < View
  attr_accessor :location
end

class Logger
  def self.image(url:)
    "An image named #{url.inspect}"
  end

  def self.button(title:)
    "[ #{title} ]"
  end

  def self.stack(subviews)
    indent = -> (str) { str.split("\n").map { |line| "  #{line}" }.join("\n") }

    "A stack containing...\n" +
      subviews.map(&indent).join("\n")
  end

  def self.text(text)
    text.inspect
  end

  def self.map(location:)
    "A map located #{location.inspect}"
  end
end

def full_view(i)
  i.stack([
    i.image(url: 'icon.png'),
    i.map(location: 'here'),
    i.stack([
      i.button(title: 'Like'),
      i.button(title: 'Pass')
    ])
  ])
end

full_view(View) 
# => #<Stack:0x007fc4341d0ce0 @subviews=[#<Image:0x007fc4341d0e98 @url="icon.png">, #<Map:0x007fc4341d0e20 @location="here">, #<Stack:0x007fc4341d0d30 @subviews=[#<Button:0x007fc4341d0dd0 @title="Like">, #<Button:0x007fc4341d0d80 @title="Pass">]>]>

puts full_view(Logger)

# >> A stack containing...
# >>   An image named "icon.png"
# >>   A map located "here"
# >>   A stack containing...
# >>     [ Like ]
# >>     [ Pass ]