こういうのが欲しいけれど、どうやったらAM::Modelに入りうるだろうか...

activemodel-inspector.rb

module ActiveModelAttributesInspector
  # @return [String]
  def inspect
    values = attributes.map { |key, value| "#{key}: #{value.inspect}" }.join(', ')
    %(<#{self.class.name} #{values}>)
  end

  # @param pp [PP]
  #
  # @return [void]
  def pretty_print(pp)
    pp.object_address_group(self) do
      pp.seplist(attributes.keys, proc { pp.text(',') }) do |attr_name|
        pp.breakable ' '
        pp.group(1) do
          pp.text attr_name.to_s
          pp.text ':'
          pp.breakable
          pp.pp public_send(attr_name)
        end
      end
    end
  end
end

example

class YokuaruForm
  include ActiveModel::Attributes
  attribute :text_field, :string
end

pp YokuaruForm.new

before

=> #<YokuaruForm:0x00007fdc05901ee0
 @attributes=
  #<ActiveModel::AttributeSet:0x00007fdc05901e68
   @attributes=
    {"text_field"=>
      #<ActiveModel::Attribute::WithCastValue:0x00007fdc05901e18
       @name="text_field",
       @original_attribute=nil,
       @type=#<ActiveModel::Type::String:0x00007fdbe3bb3bd8 @limit=nil, @precision=nil, @scale=nil>,
       @value_before_type_cast=nil>}>>

after

=> #<YokuaruForm:0x00007fdbe58e5490 text_field: nil>