class Type
  attr_accessor :bs, :if_eval, :name
  def initialize(name, if_eval, &block)
    self.bs = []
    self.if_eval = method(if_eval)
    self.name =  name

    instance_eval &block
  end

  def b &block
    bs << B.new(block)
  end

  def execute
    bs.each do |b|
      b.block.call
    end
  end
end

class B
  attr_accessor :block
  def initialize(block)
    self.block = block
  end

  def execute
    self.block.call
  end
end

@types = []
def to(name, opts={}, &block)
  @types << Type.new(name, opts[:if], &block)
end

def is_a?(id)
    id == 'a'
end

def is_c?(id)
  id == 'c'
end

to 'a', if: :is_a? do
  b do
    puts 'A1'
  end
  b do
    puts 'A2'
  end
end

to 'c', if: :is_c? do
  b do
    puts 'C1'
  end
  b do
    puts 'C2'
  end
end

['a', 'c', 'c'].each do |id|
  @types.each do |type|
    type.execute if type.if_eval.call(id)
  end
end

# Output:
# A1
# A2
# C1
# C2
# C1
# C2