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attr_extras

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

Instead of

class InvoiceBuilder
  def initialize(invoice, employee)
    @invoice, @employee = invoice, employee
  end

  private

  attr_reader :invoice, :employee
end

you can just do

class InvoiceBuilder
  pattr_initialize :invoice, :employee
end

This nicely complements Ruby's built-in attr_accessor, attr_reader and attr_writer.

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

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

Usage

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 one hash argument, assigning @bar (optional) and @baz (required).

attr_initialize [:bar, :baz!] defines an initializer that takes one hash argument, assigning @bar (optional) and @baz (required).

attr_private :foo, :bar

Defines private readers for @foo and @bar.

attr_value :foo, :bar

Defines public readers. Does not define writers, as value objects are typically immutable.

Defines object equality: two value objects of the same class with the same values are equal.

pattr_initialize :foo, :bar

Defines both initializer and private readers: shortcut for

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

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

vattr_initialize :foo, :bar

Defines initializer, public readers and value object identity: shortcut for

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

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

method_object :fooable?, :foo

Defines a .fooable? class method that takes arguments (foo) and delegates to an instance method that can access those arguments as private readers.

This is useful for method objects:

class PriceCalculator
  method_object :calculate,
    :order

  def calculate
    total * factor
  end

  private

  def total
    order.items.map(&:price).inject(:+)
  end

  def factor
    1 + rand
  end
end

class Order
  def price
    PriceCalculator.calculate(self)
  end

  # …
end

Shortcut for

attr_initialize :foo
attr_private :foo

def self.fooable?(foo)
  new(foo).fooable?
end

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

You don't have to specify readers if you don't want them: method_object :fooable? is also valid.

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.

attr_query :foo?, :bar?

Defines query methods like foo?, which is true if (and only if) foo is truthy.

attr_implement :foo, :bar

Defines methods foo and bar that raise e.g. "Implement a 'foo' method", suitable for abstract base classes.

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.

Why not use Struct?

See: "Struct inheritance is overused"

Why not use private; attr_reader :foo?

Instead of attr_private :foo, you could do private; attr_reader :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.

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 then with:

rake

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

RUBYOPT=-w rake

Contributors

License

Copyright (c) 2012-2014 Barsoom AB

MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.