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Correct attribute memoization for ruby, made easy

branch: master
README.md

AttrMemoizer Build Status Bitdeli Badge

The common ruby idiom for attribute memoization looks like this:

class Example
  def field
    @field ||= some_expensive_task()
  end
end

If some_expensive_task can return a "falsy" value (like nil or false), this doesn't work correctly—the prior memoized value of some_expensive_task will be ignored, and every subsequent call to field will result in another call to some_expensive_task.

AttrMemoizer aims to usurp your misbegotten love of ||=.

Usage

  1. include AttrMemoizer
  2. Call attr_memoizer with the attributes you want memoized
  3. Throw your @something ||= on the ground.
class Example
  include AttrMemoizer
  attr_memoizer :field, :another_field

  def field
    # code that computes field
  end

  def another_field
    # code that computes another_field
  end
end

Calling Example.new.field will call your definition of field, memoize the result for subsequent calls to an ivar called @field, and return that value.

Note that caching method results does not span instances:

class TimeHolder
  include AttrMemoizer
  attr_memoizer :a_time

  def a_time
    Time.now
  end
end

t = TimeHolder.new
t.a_time
#> 2013-01-01 00:00:00 -0800
sleep 1
t.a_time
#> 2013-01-01 00:00:00 -0800 # < this is the memoized value

# But with a new instance, we get a new value:
TimeHolder.new.a_time
#> 2013-01-01 20:26:41 -0800

Gotcha

To keep the metaprogramming demons at bay, we're using alias_method to rename the method—if you rename the attribute or look for consumers of the attribute, you at least has a chance of finding the attribute and their consumers, as opposed to how deferred_attribute worked, which made you call a method that only existed after the attr_memoizer method ran.

The problem with using alias_method at the time that attr_memoizer is called, is that the method may not be defined in the class yet. To get around this issue, we implement the class-level method_added hook, and set up the memoization after the method is defined.

If you are also using method_added, remember to call super at the end of your implementation. See the test cases for examples, and proof that this nonsense all works.

Installation

Add this line to your application's Gemfile:

gem 'attr_memoizer'

And then execute:

$ bundle

Changelog

0.0.1

  • There were previous names for this library. We won't speak of them again.
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