Source: http://www.oorexx.org/

What is Open Object Rexx?

Open Object Rexx (ooRexx) is an Open Source project managed by Rexx Language Association (RexxLA) providing a free implementation of Object Rexx.
ooRexx is distributed under Common Public License (CPL) v1.0. Object Rexx is an enhancement of classic Rexx; a powerful, full-featured programming language which has a human-oriented syntax.

The Open Object Rexx interpreter allows you to write programs procedurally as well as in an object-oriented fashion. Its main benefits include:

* Easy to use and easy to learn
* Upwardly compatible with classic Rexx
* The ability to issue commands to multiple environments
* Offers powerful functions
* Based on English-like commands
* Enhanced with full object orientation
* Designed for object-oriented programming, and also allows Rexx conventional programming
* Provides a standard Rexx API to develop external function libraries written in C

What Rexx Offers

The following aspects of Rexx round out its versatility and function:

* An English-like language: To make Rexx easier to learn and use, many of its instructions are meaningful English words. Unlike some programming languages that use abbreviations and large numbers of punctuation characters, Rexx instructions are common words, such as SAY, PULL, IF...THEN...ELSE, DO...END, and EXIT.
* Fewer rules: Rexx has relatively few rules about format. A single instruction can span many lines, and you can include multiple instructions on a single line. Instructions can begin in any column and you can type them in uppercase, lowercase, or mixed case. You can also skip spaces in a line or entire lines, since there is no line numbering. Keywords are also only reserved in context; ie if you want a variable called date, then it is valid, even though Rexx has a built-in function of the same name. The Rexx interpreter knows which one you are refering to based on its context; just as the programmer also knows.
* Interpreted, not compiled: Rexx is an interpreted language. When a Rexx program executes, its language processor reads each statement from the source file and runs it, one statement at a time. Languages that are not interpreted must first be compiled into object code before being run. This slows down development time.
* Built-in functions and methods: Rexx has a rich set of built-in functions and methods that perform various processing, searching, and comparison operations for text and numbers. These also provide formatting capabilities and arithmetic calculations.
* Typeless variables: Rexx treats all data as objects of different kinds. Variables can hold any kind of object, so you do not need to declare variables as strings or numbers. If you want to do arithmetic operations on a variable, then provided the variable has a recognised numeric value it can be done.
* String handling: Rexx includes powerful functionality for manipulating character strings. This allows programs to read and separate characters, numbers, and mixed input. Rexx performs arithmetic operations on any string that represents a valid number, including those in exponential formats.
* Decimal Arithmetic: Rexx has always based its arithmetic operations on decimal arithmetic, rather than on binary arithmetic that other languages use. Decimal arithmetic is the type of arithmetic humans use and is accurate, unlike binary arithmetic which computers use internally which is inaccurate. Other languages have finally realized the benefits of accurate arithmetic and are starting to include decimal arithmetic capabilities that are based on Rexx's arithmetic. For a complete coverage of decimal arithmetic, see Mike Cowlishaw's Decimal Arithmetic Page.
* Clear error messages and powerful debugging: Rexx displays messages with full and meaningful explanations when a Rexx program encounters an error. In addition, the TRACE instruction provides a powerful debugging tool.

Using Rexx

The Rexx programming language covers several application areas traditionally served by fundamentally different types of programming languages:

* Developing programs of varying complexity: Rexx is a language that provides powerful character and arithmetical abilities in a simple framework. You can write short programs with a minimum of overhead, yet facilities exist to write robust, large programs.
* Tailoring user commands: Command program interpreters are an important component of operating systems. Most operating systems include some form of executive, shell, or batch language. In many cases the language is so embedded in the operating system that it is difficult to use outside its primary environment. Rexx is a language that is designed primarily for generality but also for suitability as a command programming language.
* Creating macros: Many applications are programmable by using macros. In the IT industry, there is a different macro language for almost every type of application, such as editors, assemblers, interactive systems, text processors, spreadsheets, and databases. Because Rexx is a character manipulation language, it can provide the macro language for all those applications. Its more conventional syntax and flexibility make it possible for the same jobs to be done quicker by less experienced people. Rexx has a standard API that makes writing the interface to Rexx portable between different Rexx interpreters.
* Developing prototypes: Interpreter implementations of Rexx can be highly interactive and permit rapid program development. This productivity advantage, together with the ease of interfacing Rexx to system utilities for display and for data input and output, makes the language very suitable for modeling applications and products.

Object-oriented Programming

Object-oriented programmers solve problems by identifying and classifying objects related to the problem. Then they determine what actions or behaviors are required of those objects. Finally, they write the instructions to generate the classes, create the objects, and implement the actions. The main program consists of instructions that send messages to objects.

A billing application, for example, could have an Invoice class and a Receipt class. These two classes could be members of a Forms class. Individual invoices are objects that are instances of the Invoice class.
Other advantages often associated with object-oriented technology are:

* Simplified design through modeling with objects
* Greater code reuse
* Rapid prototyping
* Higher quality of proven components
* Easier and reduced maintenance
* Cost-savings potential
* Increased adaptability and scalability

From Classic Rexx to Open Object Rexx

Open Object Rexx includes features typical of an object-oriented language, such as subclassing, polymorphism, and data encapsulation. It is an extension of the classic Rexx language, which has been expanded to include classes (a base set of classes is supplied), objects, and methods. These extensions do not replace classic Rexx functions or preclude the development or running of classic Rexx programs.

You can program as before, program with objects, or intermix objects with regular Rexx instructions. You decide when to use Rexx's object-oriented features. In general, your current Rexx programs will work without change. But because Open Object Rexx detects more errors at translate time than some other Rexx interpreters, you may have to fix these errors.

Open Object Rexx is fully compatible with earlier versions of IBM REXX Interpreters that were not object-oriented and compatible with other Open Source Rexx interpreters currently available.