The Values package is my base library for almost everything I do. It provides Values (as opposed to Objects) which are simple, immutable objects. Values can only be created but never modified. This allows for a functional programming style and simplyfies systems, since much less state has to be maintained. Especially I like to see all structure and details (of complex values) at a glance and the ease of creating test values.

The project grew out of the way I model simple objects which is strongly influenced by the functional and dynamic language Lisp. I recognized a pattern and codified support infrastructure for this style. To provide the facilities, I created the class Value as superclass for values. The feature why I implemented this was defaults for instance variables which meant that to needed 2^^(numberOfDefaultVariables) consturctor methods to cover all possibilities. That was far to labour intensive and error prone to do by hand - hence the Values package.

With Values codified, I could write a generic printing method for values which prints them as Smalltalk source which, when evaluated, results in the same value. Values are literal. This is very useful for example and test instances as well as for writing and reading them (files or sockets). But the nicest property of this is the you can see a value with all details at a glance.

Now, I use Values for more than 10 years and I cannot do without it anymore. Most of the classes I define are Values. Objects are used for the “moving parts”, the complex stuff. Since Values are so trivial and simple, I do not need to spend much time with them - they just work very reliable. Instead I can concentrate on complicated objects at the heart of the app. By sourcing out functionalities to Values, the systems become simpler.

A value has instance variables which can only contain values. The order of the variables is relevant and is used extensively. It is recommended to order the variables by importance.

Example: Class Person (subclass of Value)

1. name
2. sex
3. birthday

The variables are set all at once by an initalizing method which has all initial values as parameters. The object becomes immutable after initialization and all instance variables are effectively constants.

Person>>initializeName: aString sex: aBoolean birthday: aDate
  name := aString.
  sex := aBoolean.
  birthday := aDate.
  self beImmutable

The one initializer is called by the constructor on the class side. The constructor returns a fully initialized immutable instance.

Person class>>name: aString sex: aBoolean birthday: aDate
  | inst |
  inst := self new.
  inst initializeName: aString sex: aBoolean birthday: aDate.
  ^inst

The Smalltalk code lives in the Cincom Public Store as bundle Values Project which you can load into your VisualWorks image.

I wrote a (scientific) dry paper about it and presented it at ESUG 2009 in Brest. I think that nobody understood it… . You can buy the paper from the ACM or you can see the draft of the paper with identical content on which the ACM does not have the copyright. The slides of the talk are here.