OO Package for Jim Tcl
======================

Author: Steve Bennett <steveb@workware.net.au>
Date: 1 Nov 2010 09:18:40

OVERVIEW
--------
The pure-Tcl oo package leverages Jim's unique strengths
to provide support for Object Oriented programming.

The oo package can be statically linked with Jim or installed
as a separate Tcl package and loaded with:

  package require oo

DECLARING CLASSES
-----------------
A class is declared with the 'class' proc as follows.

  class myclass ?baseclasses? classvars

This declares a class named 'myclass' with the given dictionary,
'classvars', providing the initial state of all new objects.
It is important to list all class variables in 'classvars', even
if initialised only to the empty string, since the class makes
these variables available in methods and via [myclass vars].

A list of zero or more base classes may also be specified from
which methods and class variables are imported. See INHERITANCE
below for more details.

Declaring a class creates a procedure with the class name along
with some related procedures. For example:

  . class Account {balance 0}
  Account
  . info procs Account*
  {Account get} {Account methods} {Account eval} Account {Account new} {Account destroy}
  {Account vars} {Account classname} {Account classvars} {Account method}

Notice that apart from the main 'Account' procedure, all the remaining procedures (methods)
are prefixed with 'Account' and a space.

PREDEFINED CLASS METHODS
------------------------
Declaring a class pre-defines a number of "class" methods. i.e. those which don't
require an object and simply return or manipulate properties of the class. These are:

  new ?instancevars?::
    Creates and returns new object, optionally overriding the default class variable values.
	Note that the class name is an alias for 'classname new {}' and can be used as a shorthand
	for creating new objects with default values.

  method name arglist body::
    Creates or redefines a method for the class with the given name, argument list and body.

  methods::
    Returns a list of the methods supported by this class, including both class methods
	and instance methods. Also includes base class methods.

  vars::
	Returns a list of the class variables for this class (names
	only). Also includes base class variables.

  classvars::
	Returns a dictionary the class variables, including initial values, for this class.
	Also includes base class variables.

  classname::
    Returns the classname. This can be useful as [$self classname].

Class methods may be invoked either via the class name or via an object of the class.
For example:

  . class Account {balance 0}
  Account
  . Account methods
  classname classvars destroy eval get method methods new vars
  . set a [Account]
  <reference.<Account>.00000000000000000001>
  . $a methods
  classname classvars destroy eval get method methods new vars

PREDEFINED OBJECT METHODS
-------------------------
Declaring a class pre-defines a number of "object" methods. i.e. those which operate
on a specific object.

  destroy::
    Destroys the object. This method may be overridden, but note that it should
	delete the object with {rename $self ""}. This method will also be called
	if the object is reaped during garbage collection.

  get varname::
    Returns the value of the given instance variable.

  eval ?locals? body::
    Makes any given local variables available to the body, along with
	the instance variables, and evaluate the body in that context.
	This can be used for one-off evaluation to avoid declaring a method.

RESERVED METHODS
----------------
The following methods are special

  constructor::
    If this method exists, it is invoked (with no arguments) after an object is created

  unknown methodname ...::
    If an undefined method is invoked, and this method exists, it is called with the methodname
	and the original arguments


CREATING OBJECTS
----------------
An object is created with the 'new' method, or simply by using the classname shortcut.
If the 'new' method is used, the variables for the newly created object (instance variables)
may be initialised. Otherwise they are set to the default values specified when the
class was declared.

For example:

  . class Account {balance 0}
  Account
  . set a [Account]
  <reference.<Account>.00000000000000000001>
  . set b [Account new {balance 1000}]
  <reference.<Account>.00000000000000000002>
  . $a get balance
  0
  . $b get balance
  1000

If the 'constructor' method exists, it is invoked just after the object is created

DECLARING METHODS
-----------------
In addition to the predefined methods, new methods may be declared, or existing
methods redefined with the class method, method.

Declaring a method is very similar to defining a proc, and the arglist
has identical syntax. For example:

  . Account method show {{channel stdout}} { $channel puts "Balance of account is $balance" }
  . $b show
  Balance of account is 1000

All instance variables are available within the method and any
changes to these variables are maintained by the object.

In addition, the $self variables is defined and refers to the current object.
This may be used to invoke further methods on the object. For example:

  . Account method show {} { puts "Balance of account is [$self get balance]" }
  . $b show
  Balance of account is 1000

Notes:
* It is a bad idea to unset an instance variable.
* In general, you should avoid redefining any of the pre-defined methods, except for 'destroy'.
* When accessing the caller's scope with upvar or uplevel, note that there
  are two frame levels between the caller and the method. Thus it is necessary
  to use 'upvar 2' or 'uplevel 2'

INHERITANCE
-----------
For each base class given in a new class declaration, the methods
and variables of those classes are imported into the new class being
defined. Base classes are imported in left to right order, so that if a
method is defined in more than one base class, the later definition
is selected. This applies similarly to class variables.

Within a method, 'super' may be used to explicitly invoke a
base class method on the object. This applies only to the *last*
base class given. For example:

  # Assumes the existence of classes Account and Client
  . Account method debit {amount} { incr balance -$amount }
  . class CreditAccount {Client Account} {type visa}
  CreditAccount
  . CreditAccount method debit {amount} {
    puts "Debit $type card"
	super debit $amount
  }
  . set a [CreditAccount]
  <reference.<Account>.00000000000000000001>
  . $a debit 20
  Debit visa card
  . $a balance
  -20

In the CreditAccount debit method, the call to 'super debit' invokes
the method 'Account debit' since Account is the last base class listed.

OBJECT LIFETIME/GARBAGE COLLECTION
----------------------------------
Objects are implemented as lambdas. That is, they are procedures with state
and are named as references. This means that when an object is no longer
reachable by any name and garbage collection runs, the object will be
discarded and the destructor will be invoked. Note that the garbage collector
can be invoked manually with 'collect' if required.

  . class Account {}
  Account
  . Account method destroy {} { puts dying...; rename $self "" }
  Account destroy
  . proc a {} { set b [Account]; return "" }
  a
  . a
  . collect
  dying...
  1

CLASS METHODS/CLASS STATIC VARIABLES
------------------------------------
All methods defined with 'method' operate on objects (instances).
If a class method is required, it is possible to simply declare one with 'proc'.
The method dispatcher will automatically be able to dispatch to this method.
Using this approach, it is also possible to add class static variables by
defining static variables to the proc. Although strictly these variables
are accessible only to that proc, not the class as a whole.

For example:

  . class Account {}
  Account
  . proc {Account nextid} {} {{id 0}} { incr id }
  Account nextid
  . Account nextid
  1
  . Account nextid
  2
  . set a [Account]
  <reference.<Account>.00000000000000000001>
  . $a nextid
  3
  . $a eval { $self nextid }
  4

HOW METHOD DISPATCH WORKS
-------------------------
All class and object methods are name "classname methodname".

The class method dispatcher is named "classname". When invoked with a methodname,
it simply invokes the method "classname methodname".

The method dispatch is via a two step process. Firstly the object procedure is invoked
with the method name. This procedure then invokes "classname method" which sets up
the appropriate access to the object variables, and then invokes the method body.

EXAMPLES
--------
tree.tcl
~~~~~~~~
The 'tree' package is implemented using the 'oo' package.
See the source code in tree.tcl and a usage example in tests/tree.test

Of particular note is how callbacks and recursive invocation is used in the 'walk' method.

examples/ootest.tcl
~~~~~~~~~~~~~~~~~~~
A comprehensive OO example is provided in examples/ootest.tcl.

It can be run simply as:

  ./jimsh examples/ootest.tcl
