Very simple DSL in Ruby

Question

I want to write a DSL that works like this.

text = Screen.write do
  label text: 'Something'
  label text: 'stupid'
end
puts text => Somethingstupid

I don't know much metaprogramming ( I'm starting to learn it now) and the DSL tutorials online weren't really good. I think I should have something like this:

class Screen
  attr_accessor :content
  def initialize 
    @content = ""
  end
  def self.draw (&block)
    self.instance_eval(&block)
    @content
  end

The problem is I have no idea about the label text: part. This is maybe the most important part becasue I should later add functionality to it like label text: 'border', border: '|' => |border| label text: 'border', border: '|' => |border| and label text: 'UPCASE', style: :downcase => upcase . So how should I tackle this problem. Any ideas and help are welcomed.

Answer 1

There're many different ways in which you can implement something like this. To keep it simple, you can just evaluate the block within a new Screen object and return the result.

class Screen
  attr_reader :texts

  def initialize
    @texts = []
  end

  def label(hash)
    # Validation + check for other keys
    texts << hash[:text]
  end

  def to_s
    texts.join
  end

  def self.write(&block)
    raise "No block given" unless block_given?

    Screen.new.tap do |s|
      s.instance_eval(&block)
    end
  end
end

text = Screen.write do
  label text: 'Something'
  label text: 'stupid'
end

p "Rendered: #{text}"

Answer 2

I see no reason to have any instance methods. Am I missing something?

Code

class Screen
  def self.hwrite(&block) write(false, &block) end
  def self.vwrite(&block) write(true,  &block) end

  private

  def self.write(vlabel, &block)
    @str = ''
    @vlabel = vlabel
    instance_eval &block
  end

  def self.label(h)
    @str << h[:text]
    @str << '\n' if @vlabel
    @str 
  end
end

BasicObject#instance_eval changes self within the block to Screen .

Examples

str = Screen.hwrite do
  label text: 'Something '
  label text: 'is amiss'
end
  #=> "Something is amiss"

str = Screen.vwrite do
  label text: 'Something is rotten in '
  label text: 'the state of Denmark'
end
  #=> "Something is rotten in \\nthe state of Denmark\\n"