Lua_03 - Experiments with Tag-based Inheritance

"Lua_03" implements a simple method for (single) "inheritance" based on "Tag Methods". The basic principle has already been adequately described [1,2,3,4] and shall therefore only briefly be outlined here: every attempt to access a non-existent table element causes the invocation of a related tag method - assuming that such a method has been defined before. This method now looks for a "prototype" of the actual table ("object") and - within that prototype - recursively for the requested element. Only when this search fails as well, the element counts as undiscoverable.

Along with the abovementioned inheritance scheme "Lua_03" contains a number of supporting routines:

• an enhanced type function distinguishes between "plain" tables and those supporting the described inheritance mechanism,

• a generic clone method allows the duplication of objects,

• an inheritsfrom method simplifies the detection of any relationships between objects (with regard to inheritance),

• a new method allows to instantiate new objects from a given protoype. As an additional "feature", this method is installed such that it will be invoked whenever the (principally unexecutable) object itself gets called.

In order to demonstrate these functions and methods, "Lua_03" defines two prototypes which are related to each other: while an "User" object describes a single "user", another "Group" object represents an "user group". Since "User" acts as a prototype for "Group", its methods may also be applied to "Group" objects.

"Lua_03" should be invoked without any command line arguments

  lua Lua_03.lua

and yields the following output:

  Lua_03 - first experiments with tag-based (single) inheritance

  firstUser  = Rozek, Elena
  secondUser = Rozek, Andreas
  Phantom    = (unknown), (unknown)
  UserGroup  = Rozek Family [Rozek, Elena; Rozek, Andreas]

  UserGroup:inheritsfrom(User)? 1
  UserGroup:inheritsfrom(Object)? 1

  type(UserGroup) = object

It is recommended to analyze this output along with the source code - this clarifies the proper behaviour of the abovementioned functions and methods.

Source Code

The source code of this example is available for download:

• Lua_03.lua (ASCII file, 10119 bytes)

As the functions defined within "Lua_03.lua" will also be used in other programming examples, they may be downloaded separately as a Lua script file:

• Lua_03_Lib.lua (ASCII file, 6556 bytes)

References