Proof of concept for a version of https://github.com/barsoom/attr_extras that uses block arguments to get lazy-loaded defaults.

attr_extras_using_blocks_proof_of_concept.rb

module AttrExtras
  def pattr_initialize(&block)
    arg_names = block.parameters.map(&:last)

    arg_names.each do
      attr_reader _1
      private _1
    end

    define_method(:__attr_extras_init_block, &block)

    define_method(:initialize) do |*args, **kwargs|
      # Procs won't enforce the presence of positional arguments. So we need a lambda: https://stackoverflow.com/a/2946734/6962
      lambda = method(:__attr_extras_init_block).to_proc

      block_binding = instance_exec(*args, **kwargs, &lambda)
      raise "Initializer block must return `binding`!" unless block_binding.is_a?(Binding)

      arg_names.each do
        value = block_binding.local_variable_get(_1)
        instance_variable_set("@#{_1}", value)
      end
    end
  end
end

class MyClass
  extend AttrExtras

  # You sadly need to return a `binding` from here. Haven't found a way around it.
  #
  # The `@value_set_from_block = …` is here to show that we can put any additional initializer logic in the block, and even use the args.
  pattr_initialize { |a, b = "default b", c:, time: Time.now| @value_set_from_block = b.reverse; binding }

  attr_reader :value_set_from_block

  def show_values
    pp({ a: a, b: b, c: c, time: time, value_set_from_block: value_set_from_block })
  end
end


MyClass.new("my a", c: "my c").show_values

# This sleep shows that the default Time.now is evaluated on each initialization and not once when the class is defined.
sleep 1
MyClass.new("my a", "my b", c: "my c").show_values

# Complains about missing required arguments, both positional and keyword.
MyClass.new().show_values

An inadvisable way around having to return the binding: https://stackoverflow.com/q/72062159/6962