My suggestion for a lambda-like strict arity alternative to a block

papercraft.rb

module Papercraft
  class Block
    def initialize(&) = singleton_class.define_method(:call, &)
    def apply(...) = self.class.new { call(...) }

    class HTML < Block
      def render(...) = call(...)
      def to_html = render.to_s
    end
  end

  def self.html(&) = Block::HTML.new(&)
end

hello_world = Papercraft.html { |name| "<h1>Hello, #{name}!</h1>" }
#=> #<Papercraft::Block::HTML:0x0...>
hello_world.render('World')
#=> "<h1>Hello, World!</h1>"