value

                                 Value

Value is a library for defining immutable value objects in Ruby. A value object is an object whose equality to other objects is determined by its value, not its identity, think dates and amounts of money. A value object should also be immutable, as you don’t want the date “2013-04-22” itself to change but the current date to change from “2013-04-22” to “2013-04-23”. That is, you don’t want entries in a calendar for 2013-04-22 to move to 2013-04-23 simply because the current date changes from 2013-04-22 to 2013-04-23.

A value object consists of one or more attributes stored in instance variables. Value sets up an #initialize method for you that let’s you set these attributes, as, value objects being immutable, this’ll be your only chance to do so. Value also adds equality checks ‹#==› and ‹#eql?› (which are themselves equivalent), a ‹#hash› method, a nice ‹#inspect› method, and a protected attribute reader for each attribute. You may of course add any additional methods that your value object will benefit from.

That’s basically all there’s too it. Let’s now look at using the Value library.

§ Usage

You create value object class by invoking ‹#Value› inside the class
(module) you wish to make into a value object class.  Let’s create a class
that represent points on a plane:

  class Point
    Value :x, :y
  end

A ‹Point› is thus a value object consisting of two sub-values ‹x› and ‹y›
(the coordinates).  Just from invoking ‹#Value›, a ‹Point› object will have
a constructor that takes two arguments to set instance variables ‹@x› and
‹@y›, equality checks ‹#==› and ‹#eql?› (which are the same), a ‹#hash›
method, a nice ‹#inspect› method, and two protected attribute readers ‹#x›
and ‹#y›.  We can thus already creat ‹Point›s:

  origo = Point.new(0, 0)

The default of making the attribute readers protected is often good
practice, but for a ‹Point› it probably makes sense to be able to access
its coordinates:

  class Point
    public(*attributes)
  end

This’ll make all attributes of ‹Point› public.  You can of course choose to
only make certain attributes public:

  class Point
    public :x
  end

Note that this public is standard Ruby functionality.  Adding a method to
‹Point› is of course also possible and very much Rubyish:

  class Point
    def distance(other)
      Math.sqrt((other.x - x)**2 + (other.y - y)**2)
    end
  end

For some value object classes you might want to support optional
attributes.  This is done by providing a default value for the attribute,
like so:

  class Money
    Value :amount, [:currency, :USD]
  end

Here, the ‹currency› attribute will default to ‹:USD›.  You can create
‹Money› via

  dollars = Money.new(2)

but also

  kronor = Money.new(2, :SEK)

All required attributes must come before any optional attributes.

Splat attributes are also supported:

  class List
    Value :'*elements'
  end

  empty = List.new
  suits = List.new(:spades, :hearts, :diamonds, :clubs)

Splat attributes are optional.

Finally, block attributes are also available:

  class Block
    Value :'&block'
  end

  block = Block.new{ |e| e * 2 }

Block attributes are optional.

Comparison beyond ‹#==› is possible by specifingy the ‹:comparable› option
to ‹#Value›, listing one or more attributes that should be included in the
comparison:

  class Vector
    Value :a, :b, :comparable => :a
  end

Note that equality (‹#==› and ‹#eql?›) is always defined based on all
attributes, regardless of arguments to ‹:comparable›.

Here we say that comparisons between ‹Vector›s should be made between the
values of the ‹a› attribute only.  We can also make comparisons between all
attributes of a value object:

  class Vector
    Value :a, :b, :comparable => true
  end

To sum things up, let’s use all possible arguments to ‹#Value› at once:

  class Method
    Value :file, :line, [:name, 'unnamed'], :'*args', :'&block',
          :comparable => [:file, :line]
  end

A ‹Method› consists of file and line information, a possible name, some
arguments, possibly a block, and is comparable on the file and line on
which they appear.

Check out the {full API documentation}¹ for a more explicit description,
should you need it or should you want to extend it.

¹ See http://disu.se/software/value/api/

§ Financing

Currently, most of my time is spent at my day job and in my rather busy
private life.  Please motivate me to spend time on this piece of software
by donating some of your money to this project.  Yeah, I realize that
requesting money to develop software is a bit, well, capitalistic of me.
But please realize that I live in a capitalistic society and I need money
to have other people give me the things that I need to continue living
under the rules of said society.  So, if you feel that this piece of
software has helped you out enough to warrant a reward, please PayPal a
donation to [e-mail]¹.  Thanks!  Your support won’t go unnoticed!

¹ Send a donation: https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=now%40disu%2ese&item_name=Value

§ Reporting Bugs

Please report any bugs that you encounter to the {issue tracker}¹.

¹ See https://github.com/now/value/issues

§ Authors

Nikolai Weibull wrote the code, the tests, the manual pages, and this
README.

§ Licensing

Value is free software: you may redistribute it and/or modify it under the
terms of the {GNU Lesser General Public License, version 3}¹ or later², as
published by the {Free Software Foundation}³.

¹ See http://disu.se/licenses/lgpl-3.0/ ² See http://gnu.org/licenses/ ³ See http://fsf.org/