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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`](http://thepugautomatic.com/2013/08/struct-inheritance-is-overused/).
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](http://en.wikipedia.org/wiki/Value_object) 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](http://refactoring.com/catalog/replaceMethodWithMethodObject.html):
``` ruby
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
``` ruby
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 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](http://en.wikipedia.org/wiki/Abstract_method#Abstract_methods) in base classes, e.g. when using the [template method pattern](http://en.wikipedia.org/wiki/Template_method_pattern).
## 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"](http://thepugautomatic.com/2013/08/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
* [Henrik Nyh](https://github.com/henrik)
* [Joakim Kolsjö](https://github.com/joakimk)
* [Victor Arias](https://github.com/victorarias)
* [Teo Ljungberg](https://github.com/teoljungberg)
* [Kim Persson](https://github.com/lavinia)
## License
Copyright (c) 2012-2014 [Barsoom AB](http://barsoom.se)
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.