Object REXX

Object REXX

Paradigm	Multi-paradigm: object-oriented (class-based), procedural (imperative), structured
Designed by	Simon C. Nash (IBM)
First appeared	1988; 36 years ago (1988)
Stable release	5.0.0 / 10 May 2023; 18 months ago (10 May 2023)
Preview release	5.1.0
Implementation language	C++
Platform	IA-32, x86-64, ARM, s390x
OS	Cross-platform: Linux, Microsoft Windows, macOS, Solaris, OpenIndiana, AIX, FreeBSD, OS/2
License	CPL 1.0, GPLv2
Filename extensions	.rxs, .rex, .rexx, .cls
Website	www.rexxla.org
Major implementations
Object REXX (until 2004) ooRexx (since 2005)
Influenced by
Rexx, Smalltalk
Influenced
NetRexx

Object REXX is a high-level, general-purpose, interpreted, object-oriented (class-based) programming language.

It is a follow-on and a significant extension of the Rexx programming language (often called "Classic Rexx"). Object REXX retains all the features and syntax of "Classic Rexx" while adding full object-oriented programming (OOP) capabilities. Although Object REXX does not implement all aspects of the "Information Technology – Programming Language REXX" ANSI X3.274-1996^[1] standard, it offers many new features of its own.

Following its "Classic Rexx" influence, Object REXX is designed to be easy to learn, use, and maintain. Several object-oriented implementations of REXX are available, including Open Object Rexx ("ooRexx") and Rexx Object Oriented ("roo!").^[2]: 311

In 1988, the "Oryx" project at IBM, under the technical direction of Simon C. Nash, experimented with merging "Classic Rexx" with the object model of Smalltalk.^[3][4] The motivation behind the project was to transfer the advantages of OOP to "Classic Rexx" while remaining compatible and thus transferring the usability of "Classic Rexx" to OOP.^[5] Early on, the projects focused on OOP aspect such as treating everything as an object, object-based encapsulation and message passing, object-based concurrency, classes and inheritance.^[6]

This initial work later led under the direction of Rick McGuire to the first prototype of Object REXX, which was presented in 1992. In 1994, IBM announced that Object REXX would replace "Classic Rexx" as the standard REXX interpreter in the next version of OS/2.^[4] In 1996, Object REXX^[a] was released as part of the OS/2 Warp 4 operating system.^[7] In 1997, versions for Windows 95, Windows NT and Linux followed.^[8][9] In 1999 an Object REXX version for AIX V4.1.5 or higher was released.^[10] In 2000, versions for  zLinux and Sun/Solaris followed. For Windows 2000, Windows Me, Windows 98 and Windows NT 4.0, the last major update for Object REXX was released in 2001.^[11]

On 12 October 2004, IBM announced the discontinuation of Object REXX and transferred the source code and licensing rights^[b] to the non-profit Special Interest Group (SIG), the Rexx Language Association (RexxLA). In 2005, the RexxLA released ooRexx as a new incarnation of Object REXX as free and open-source software under the Common Public License.^[12] This first version of ooRexx 3.0.0 has been heavily refactored compared to the original IBM source code in order to increase readability. Later, the ooRexx kernel was rewritten in pure C++, and a new architecture and native interface were designed and implemented under the technical direction of Rick McGuire. This work enabled the RexxLA to release ooRexx 4.0.0 with support for 64-bit in 2009. To this day, the RexxLA continues to develop, support and maintain ooRexx as well as "Classic Rexx" and NetRexx. Furthermore, the RexxLA organizes annual symposia.^[13]

IBM's original Object REXX interpreter continues to be available in OS/2-derived operating systems, such as ArcaOS and eComStation.

The following table contains noteworthy features and changes of major Object REXX and ooRexx interpreter versions. All ooRexx releases and the necessary documentation are available on Sourceforge.^[14][15] For Arch Linux based distributions the current and the development version are available as Arch User Repository. For ooRexx 5.0.0 an unofficial port to OS/2 is available. Since release 5.0.0, portable versions of the interpreter are available that allow it to be used without installation. For Intellij IDEA, the ooRexxPlugin adds support for syntax highlighting, syntax checking, code completion and documentation support modeled after Javadoc using annotations.^[16]

ooRexx is designed to retain all the features of "Classic Rexx". Its conforms^[c] to the ANSI standard for the Rexx language (X3.274-1996, “Programming Language Rexx”^[1]), for interoperability across platforms with other conforming implementations. Thus "Classic Rexx" programs typically run under ooRexx without any changes. This makes it easy to transport both program code and developer knowledge from "Classic Rexx" to ooRexx. To this, ooRexx adds all the features of OOP, such as subclassing, polymorphism, and data encapsulation. Further features include multiple inheritance via mixin classes.

The code that IBM released to open source in 2004 did not include the classes for IBM System Object Model (SOM) and Workplace Shell (WPS) support.^[31] The OS/2 version of IBM Object REXX includes classes to support SOM ^[32] and WPS.^[33] These are also included OS/2's initial follow-on product, eComStation, and also in its current descendant, ArcaOS.^[34]

ooRexx follows the design philosophy of "Classic Rexx" to create a "human-centered" programming language that is easy to learn, code, remember and maintain. This is achieved, in part, by keeping the language small and following the principle of least astonishment.^[2][35] ooRexx 5.0.0, for instance, contains only thirty keyword instructions.^[36] In addition, it is a dynamic programming language that offers flexibility and allows to focus on development rather than language constraints. There is only one data type, the character string, which does not have to be declared, which reduces the effort for programmers compared to strictly typed languages.^[37]

Similar to "Classic Rexx", there are assignment instructions, keyword instructions and command instructions. Unlike many other languages, no keywords are reserved, so there is no need to memorize a long list of reserved words.^[38] In cases where the string is neither an assignment nor a keyword instruction, it is considered a command instruction, which causes the interpreter to pass it to the operating system for execution and set a variable RC for the return code. In addition, the ADDRESS instruction allows commands to be redirected to specific environments such as Bourne Shell, Bash, Z-Shell, Command Prompt and others, as well as intercepting errors.

a = "hello world"               /* assignment instruction       */ do i = 1 to 2                   /* keyword instruction "DO"     */     say "round #" i":" a        /* keyword instruction "SAY"    */ end                             /* keyword instruction "END"    */ "echo Hello World"              /* command to operating system  */ say "RC:" rc                    /* return code                  */ 

As "Classic Rexx", ooRexx has a free-form syntax where the positioning of the program code is irrelevant. Before execution, the interpreter merges multiple unquoted blanks into one, while a character string enclosed in quotation marks (single or double) is not changed. Concatenation can be requested explicitly with two vertical bars (||), or implicitly by separating terms with spaces or by abutting terms. Since a free-form language requires fewer syntactic rules to be considered, it is assumed that it eases the learning effort by reducing the intrinsic cognitive load.^[39]

say   "Hello World!"                   /* output: Hello World!      */ say "   This"     'is'     "REXX!"     /* output:    This is REXX!  */ say "Good""bye" || '!'                 /* output: Goodbye!          */ 

As "Classic Rexx", ooRexx is a case-insensitive programming language. The interpreter capitalizes all characters outside quotation marks before executing them. Because the cases do not need to be differentiated, fewer additional details need to be learned and frustrating syntax errors are avoided.^[39]

a = "This is REXX!" Say A                                 /* output: This is REXX!  */ SAY a                                 /* output: This is REXX!  */ 

Similar to the messaging paradigm implemented by Alan Kay in Smalltalk, everything in ooRexx is an object that can be communicated with by sending messages naming a method. The notion of sending messages to objects as if they were living beings helps beginners to learn OOP concepts.^[38]

In contrast to Smalltalk, there is an explicit message operator, the tilde (~), where the receiving object is placed to the left of it and the result of the operation is returned. Like Smalltallk, ooRexx messages can be cascaded if two tildes (~~) are used instead of one, returning the object that received the method rather than the result produced. The default behavior of most methods can be changed by specifying an option, which can be either spelled out or abbreviated and is not case-sensitive. This enables a literal understanding and reduces the learning effort for beginners.^[39] For example the strip method by default removes leading and trailing blanks. This behavior can be changed, for example, by specifying "leading" or "l" as an option. While functions are nested in "Classic Rexx", messages can be chained in ooRexx, which improves the readability of a statement.^[38]

a = "   I am ooRexx!" say a                                 /* output:    I am ooRexx! */ say a~Strip("Leading")                /* output: I am ooRexx!    */ say a~strip("l")~reverse              /* output: !xxeRoo ma I    */ 

The messaging paradigm allows the simultaneous execution of multiple methods on a single object, so that concurrency is possible. Several objects can be active at the same time and exchange messages with each other for communication and synchronization.^[36]

In addition to the three instruction types of "Classic Rexx", ooRexx adds directive instruction that are not executable, with the first directive marking the end of the executable part of the program.^[36] To make directive instructions readily recognizable, they are introduced with two consecutive colons, such as ::CLASS classname, which causes the interpreter to define a new class with the name "classname".

d = .dog~new                                            /* create and assign a dog */ d~bark                                                  /* let the dog bark        */   ::class dog                                             /* class directive         */ ::method bark                                           /* method directive        */     say "wuff!"                                         /* output: wuff!           */ 

There are four different types of classes: Object classes, which act as a factory for creating objects, mixin classes, which add methods to other classes, abstract classes, which provide definitions for methods, and metaclasses, which can be used to create other classes.^[36]

In addition to the built-in classes, ooRexx 5.0.0 provides 82 built-in functions, including character manipulation, conversion, and information functions, many of which call methods of the String class.^[36] The built-in dynamic link library RexxUtil offers 29 cross-platform, 25 Windows-specific and four Unix-specific functions for manipulating system files, directories, classes and objects.^[36]

Using the ::requires directive, external functions can be integrated on a per program basis. This directive is placed after the main code section and allows access either to public classes and routines of a Rexx program or, by specifying the LIBRARY option, to external libraries. Such libraries are usually organized around domain-specific functions.^[36]

The Rexx extension library provides a class for reading and writing comma-separated values (CSV) files, for creating and processing JavaScript Object Notation (JSON) data and a sub-command environment that partially emulates a TSO/CMS environment.^[40] The RxSock library enables to incorporate TCP/IP protocols,^[41] while the RxFtp library^[42] specifically provides access to the file transfer protocol. The RxMath library offers advanced mathematical functions such as square root calculation, exponential function, logarithm, sine, cosine, tangent, arc sine and power calculation.^[43]

Say rxcalcsin(1)                /* output: 0.0174524064 */  ::requires 'rxmath' LIBRARY 

For Windows, ooRexx includes the OODialog framework allowing to produce Windows dialogs and therefore graphical user interfaces.^[44] In addition to OLE support allows the Windows extension to interact with the Windows program manager, the system event log, the clipboard and the registry as well as to query, edit and interact with windows, menus or submenus.^[45]

For POSIX-compatible operating systems, the nCurses class library enables the writing of text-based user interfaces using the ncurses programming library.^[46] The RxUnixSys library provides functions on most Unix systems for interacting with processes and threads, users and user groups, files and file systems and other.^[47]

The Windows extension includes a Windows Script Host (WSH) Scripting Engine that can be used to perform general automation tasks. It also includes Object Linking and Embedding/ActiveX (OLE) support allowing to interact with Windows programs such as the Microsoft Office via the OLEObject.^[45] ooSQLite provides an interface to SQLite, an in-process library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine.^[48] There is also an external library that implements a bidirectional Java bridge, based on the Bean Scripting Framework, called BSF4ooRexx, which allows Java classes to be used in ooRexx programs.^[38]

• Rexx Language Association
• Open Object Rexx 5.0.0 documentation
• Open Object Rexx on Sourceforge
• Open Object Rexx on Rosetta Code