Takes some boilerplate out of Ruby, lowering the barrier to extracting small focused classes, without the downsides of using Struct.

Provides lower-level methods like attr_private and attr_value that nicely complement Ruby's built-in attr_accessor, attr_reader and attr_writer.

Also higher-level ones like pattr_initialize (or attr_private_initialize) and method_object to really cut down on the boilerplate.

Instead of

class InvoicePolicy
  def initialize(invoice, company:)
    @invoice = invoice
    @company = company
  end
  def payable?
    some_logic(invoice, company)
  end
  private
  attr_reader :invoice, :company
end

you can just do

class InvoiceBuilder
  pattr_initialize :invoice, [:company!]
  def payable?
    some_logic(invoice, company)
  end
end

And instead of

class PayInvoice
  def self.call(invoice, amount)
    new(invoice, amount).call
  end
  def initialize(invoice, amount)
    @invoice = invoice
    @amount = amount
  end
  def call
    PaymentGateway.charge(invoice.id, amount_in_cents)
  end
  private
  def amount_in_cents
    amount * 100
  end
  attr_reader :invoice, :amount
end

you can just do

class PayInvoice
  method_object :invoice, :amount
  def call
    PaymentGateway.charge(invoice.id, amount_in_cents)
  end
  private
  def amount_in_cents
    amount * 100
  end
end

Supports positional arguments as well as optional and required keyword arguments.

Also provides conveniences for creating value objects, method objects, query methods and abstract methods.

Usage

• attr_initialize
• attr_private
• attr_value
• pattr_initialize / attr_private_initialize
• vattr_initialize / attr_value_initialize
• rattr_initialize / attr_reader_initialize
• aattr_initialize / attr_accessor_initialize
• static_facade
• method_object
• attr_implement
• cattr_implement
• attr_query
• attr_id_query

attr_initialize

attr_initialize :foo, :bar defines an initializer that takes two arguments and assigns @foo and @bar.

attr_initialize :foo, [:bar, :baz!] defines an initializer that takes one regular argument, assigning @foo, and two keyword arguments, assigning @bar (optional) and @baz (required).

attr_initialize [:bar, :baz!] defines an initializer that takes two keyword arguments, assigning @bar (optional) and @baz (required).

If you pass unknown keyword arguments, you will get an ArgumentError. If you don't pass required arguments and don't define default value for them, you will get a KeyError.

attr_initialize can also accept a block which will be invoked after initialization. This is useful for e.g. initializing private data as necessary.

Default values

Keyword arguments can have default values:

attr_initialize [:bar, baz: "default value"] defines an initializer that takes two keyword arguments, assigning @bar (optional) and @baz (optional with default value "default value").

Note that default values are evaluated when the class is loaded and not on every instantition. So attr_initialize [time: Time.now] might not do what you expect.

You can always use regular Ruby methods to achieve this:

class Foo
  attr_initialize [:time]
  private
  def time
    @time || Time.now
  end
end

Or just use a regular initializer with default values.

attr_private

attr_private :foo, :bar defines private readers for @foo and @bar.

attr_value

attr_value :foo, :bar defines public readers for @foo and @bar and also defines object equality: two value objects of the same class with the same values will be considered equal (with == and eql?, in Sets, as Hash keys etc).

It does not define writers, because value objects are typically immutable.

pattr_initialize

attr_private_initialize

pattr_initialize :foo, :bar defines both initializer and private readers. Shortcut for:

attr_initialize :foo, :bar
attr_private :foo, :bar

pattr_initialize is aliased as attr_private_initialize if you prefer a longer but clearer name.

Example:

class Item
  pattr_initialize :name, :price
  def price_with_vat
    price * 1.25
  end
end
Item.new("Pug", 100).price_with_vat  # => 125.0

The attr_initialize notation for keyword arguments is also supported: pattr_initialize :foo, [:bar, :baz!]

vattr_initialize

attr_value_initialize

vattr_initialize :foo, :bar defines initializer, public readers and value object identity. Shortcut for:

attr_initialize :foo, :bar
attr_value :foo, :bar

vattr_initialize is aliased as attr_value_initialize if you prefer a longer but clearer name.

Example:

class Country
  vattr_initialize :code
end
Country.new("SE") == Country.new("SE")  # => true
Country.new("SE").code  # => "SE"

The attr_initialize notation for keyword arguments is also supported: vattr_initialize :foo, [:bar, :baz!]

rattr_initialize

attr_reader_initialize

rattr_initialize :foo, :bar defines both initializer and public readers. Shortcut for:

attr_initialize :foo, :bar
attr_reader :foo, :bar

rattr_initialize is aliased as attr_reader_initialize if you prefer a longer but clearer name.

Example:

class PublishBook
  rattr_initialize :book_name, :publisher_backend
  def call
    publisher_backend.publish book_name
  end
end
service = PublishBook.new("A Novel", publisher)
service.book_name  # => "A Novel"

The attr_initialize notation for keyword arguments is also supported: rattr_initialize :foo, [:bar, :baz!]

aattr_initialize

attr_accessor_initialize

aattr_initialize :foo, :bar defines an initializer, public readers, and public writers. It's a shortcut for:

attr_initialize :foo, :bar
attr_accessor :foo, :bar

aattr_initialize is aliased as attr_accessor_initialize, if you prefer a longer but clearer name.

Example:

class Client
  aattr_initialize :username, :access_token
end
client = Client.new("barsoom", "SECRET")
client.username # => "barsoom"
client.access_token = "NEW_SECRET"
client.access_token # => "NEW_SECRET"

The attr_initialize notation for keyword arguments and blocks is also supported.

static_facade

static_facade :allow?, :user defines an .allow? class method that delegates to an instance method by the same name, having first provided user as a private reader.

This is handy when a class-method API makes sense but you still want the refactorability of instance methods.

Example:

class PublishingPolicy
  static_facade :allow?, :user
  def allow?
    user.admin? && complicated_extracted_method
  end
  private
  def complicated_extracted_method
    # …
  end
end
PublishingPolicy.allow?(user)

static_facade :allow?, :user is a shortcut for

pattr_initialize :user
def self.allow?(user)
  new(user).allow?
end

The attr_initialize notation for keyword arguments is also supported: static_facade :allow?, :user, [:user_agent, :ip!]

You don't have to specify arguments/readers if you don't want them: just static_facade :tuesday? is also valid.

You can specify multiple method names as long as they can share the same initializer arguments: static_facade [:allow?, :deny?], :user, [:user_agent, :ip!]

Any block given to the class method will be passed on to the instance method.

"Static façade" is the least bad name for this pattern we've come up with. Suggestions are welcome.

method_object

NOTE: v4.0.0 made a breaking change! static_facade does exactly what method_object used to do; the new method_object no longer accepts a method name argument.

method_object :foo defines a .call class method that delegates to an instance method by the same name, having first provided foo as a private reader.

This is a special case of static_facade for when you want a Method Object, and the class name itself will communicate the action it performs.

Example:

class CalculatePrice
  method_object :order
  def call
    total * factor
  end
  private
  def total
    order.items.map(&:price).inject(:+)
  end
  def factor
    1 + rand
  end
end
class Order
  def price
    CalculatePrice.call(self)
  end
end

You could even do CalculatePrice.(self) if you like, since we're using the call convention.

method_object :foo is a shortcut for

static_facade :call, :foo

which is a shortcut for

pattr_initialize :foo
def self.call(foo)
  new(foo).call
end

The attr_initialize notation for keyword arguments is also supported: method_object :foo, [:bar, :baz!]

You don't have to specify arguments/readers if you don't want them: just method_object is also valid.

Any block given to the class method will be passed on to the instance method.

attr_implement

attr_implement :foo, :bar defines nullary (0-argument) methods foo and bar that raise e.g. "Implement a 'foo()' method".

attr_implement :foo, [:name, :age] will define a binary (2-argument) method foo that raises "Implement a 'foo(name, age)' method".

This is suitable for abstract methods in base classes, e.g. when using the template method pattern.

It's more or less a shortcut for

def my_method
  raise "Implement me in a subclass!"
end

though it is shorter, more declarative, gives you a clear message and handles edge cases you might not have thought about (see tests).

Note that you can also use this with modules, to effectively mix in interfaces:

module Bookable
  attr_implement :book, [:bookable]
  attr_implement :booked?
end
class Invoice
  include Bookable
end
class Payment
  include Bookable
end

cattr_implement

Like attr_implement but for class methods.

Example:

class TransportOrder
  cattr_implement :must_be_tracked?
end

attr_query

attr_query :foo?, :bar? defines query methods like foo?, which is true if (and only if) foo is truthy.

attr_id_query

attr_id_query :foo?, :bar? defines query methods like foo?, which is true if (and only if) foo_id is truthy. Goes well with Active Record.

Explicit mode

By default, attr_extras will add methods to every class and module.

This is not ideal if you're using attr_extras in a library: those who depend on your library will get those methods as well.

It's also not obvious where the methods come from. You can be more explicit about it, and restrict where the methods are added, like this:

require "attr_extras/explicit"
class MyLib
  extend AttrExtras.mixin
  pattr_initialize :now_this_class_can_use_attr_extras
end

Crucially, you need to require "attr_extras/explicit" instead of require "attr_extras". Some frameworks, like Ruby on Rails, may automatically require everything in your Gemfile. You can avoid that with gem "attr_extras", require: "attr_extras/explicit".

In explicit mode, you need to call extend AttrExtras.mixin in every class or module that wants the attr_extras methods.

Philosophy

Findability is a core value. Hence the long name attr_initialize, so you see it when scanning for the initializer; and the enforced questionmarks with attr_id_query :foo?, so you can search for that method.

Q & A

Why not use Struct instead of pattr_initialize?

See: "Struct inheritance is overused"

Why not use private; attr_reader :foo instead of attr_private :foo?

Other than being more to type, declaring attr_reader after private will actually give you a warning (deserved or not) if you run Ruby with warnings turned on.

If you don't want the dependency on attr_extras, you can get rid of the warnings with attr_reader :foo; private :foo. Or just define a regular private method.

Can I use attr_extras in BasicObjects?

No, sorry. It depends on various methods that BasicObjects don't have. Use a regular Object or make do without attr_extras.

Installation

Add this line to your application's Gemfile:

gem "attr_extras"

And then execute:

bundle

Or install it yourself as:

gem install attr_extras

Running the tests

Run them with:

rake

Or to see warnings (try not to have any):

RUBYOPT=-w rake

You can run an individual test using the m gem:

m spec/attr_extras/attr_initialize_spec.rb:48

The tests are intentionally split into two test suites for reasons described in Rakefile.

License

MIT