Example of a recursive multi-level DSL in Ruby using Docile gem

example_multi_level_dsl.rb

require 'docile'

# Family tree node, mother and father are Person values as well
Person = Struct.new(:name, :mother, :father)

# Recursive dsl in a mutating builder using Docile
def person(&block)
  Docile.dsl_eval(PersonBuilder.new, &block).build
end

# Notice how the builder uses `person` to recurse
class PersonBuilder
  def initialize
    @p = Person.new
  end
  
  def name(v)
    @p.name = v
  end
  
  def mother(&block)
    @p.mother = person(&block) 
  end  
  
  def father(&block)
    @p.father = person(&block) 
  end  

  def build
    @p
  end
end

# Example DSL usage
p = person {
  name 'John Smith'
  mother {
    name 'Mary Smith'
  }
  father {
    name 'Tom Smith'
    mother {
      name 'Jane Smith'
    }
  }
}

puts p

# Output from this example
#
#=> #<struct Person name="John Smith",
#                   mother=#<struct Person name="Mary Smith", mother=nil, father=nil>,
#                   father=#<struct Person name="Tom Smith",
#                                          mother=#<struct Person name="Jane Smith", mother=nil, father=nil>,
#                                          father=nil>>
#