Rexx

Rexx (Restructured Extended Executor) is a programming language that can be interpreted or compiled. It was developed at IBM by Mike Cowlishaw. It is a structured, high-level programming language designed for ease of learning and reading. Proprietary and open source Rexx interpreters exist for a wide range of computing platforms; compilers exist for IBM mainframe computers.

Rexx is a full language that can be used as a scripting, macro language, and application development language. It is often used for processing data and text and generating reports; this means that Rexx works well in Common Gateway Interface (CGI) programming and is used for this purpose, like later languages such as Perl. Rexx is the primary scripting language in some operating systems, e.g. OS/2, MVS, VM, AmigaOS, and is also used as an internal macro language in some other software, such as SPF/PC, KEDIT, THE and the ZOC terminal emulator. Additionally, the Rexx language can be used for scripting and macros in any program that uses Windows Scripting Host ActiveX scripting engines languages (e.g. VBScript and JScript) if one of the Rexx engines is installed.

Rexx is supplied with VM/SP Release 3 on up, TSO/E Version 2 on up, OS/2 (1.3 and later, where it is officially named Procedures Language/2), AmigaOS Version 2 on up, PC DOS (7.0 or 2000), ArcaOS, and Windows NT 4.0 (Resource Kit: Regina). REXX scripts for CMS share the filetype EXEC with EXEC and EXEC2, and the first line of the script specifies the interpreter to be used. REXX scripts for MVS may be recognized by the low level qualifier EXEC or may be recognized by context and the first line. REXX scripts for OS/2 share the filename extension .cmd with other scripting languages, and the first line of the script specifies the interpreter to be used. REXX macros for REXX-aware applications use extensions determined by the application. In the late 1980s, Rexx became the common scripting language for IBM Systems Application Architecture, where it was renamed "SAA Procedure Language REXX".

In mainframe programming, a Rexx script or command is sometimes referred to as an EXEC in a nod to the CMS file type used for EXEC, EXEC 2 and REXX scripts on CP/CMS and VM/370 through z/VM.

Features
Rexx has the following characteristics and features:


 * Simple syntax
 * The ability to route commands to multiple environments
 * The ability to support functions, procedures and commands associated with a specific invoking environment.
 * A built-in stack, with the ability to interoperate with the host stack if there is one.


 * Small instruction set containing just two dozen instructions
 * Freeform syntax
 * Case-insensitive tokens, including variable names
 * Character string basis
 * Dynamic data typing, no declarations
 * No reserved keywords, except in local context
 * No include file facilities
 * Arbitrary numerical precision
 * Decimal arithmetic, floating-point
 * A rich selection of built-in functions, especially string and word processing
 * Automatic storage management
 * Crash protection
 * Content addressable data structures
 * Associative arrays
 * Straightforward access to system commands and facilities
 * Simple error-handling, and built-in tracing and debugger
 * Few artificial limitations
 * Simplified I/O facilities
 * Unconventional operators
 * Only partly supports Unix style command line parameters, except specific implementations
 * Provides no basic terminal control as part of the language, except specific implementations
 * Provides no generic way to include functions and subroutines from external libraries, except specific implementations

Rexx has just twenty-three, largely self-evident, instructions (such as,  , and  ) with minimal punctuation and formatting requirements. It is essentially an almost free-form language with only one data-type, the character string; this philosophy means that all data are visible (symbolic) and debugging and tracing are simplified.

Rexx's syntax looks similar to PL/I, but has fewer notations; this makes it harder to parse (by program) but easier to use, except for cases where PL/I habits may lead to surprises. One of the REXX design goals was the principle of least astonishment.

pre–1990
Rexx was designed and first implemented, in assembly language, as an 'own-time' project between 20 March 1979 and mid-1982 by Mike Cowlishaw of IBM, originally as a scripting programming language to replace the languages EXEC and EXEC 2. It was designed to be a macro or scripting language for any system. As such, Rexx is considered a precursor to Tcl and Python. Rexx was also intended by its creator to be a simplified and easier to learn version of the PL/I programming language. However, some differences from PL/I may trip up the unwary.

It was first described in public at the SHARE 56 conference in Houston, Texas, in 1981, where customer reaction, championed by Ted Johnston of SLAC, led to it being shipped as an IBM product in 1982.

Over the years IBM included Rexx in almost all of its operating systems (VM/CMS, MVS TSO/E, IBM OS/400, VSE/ESA, AIX, PC DOS, and OS/2), and has made versions available for Novell NetWare, Windows, Java, and Linux.

The first non-IBM version was written for PC DOS by Charles Daney in 1984/5 and marketed by the Mansfield Software Group (founded by Kevin J. Kearney in 1986). The first compiler version appeared in 1987, written for CMS by Lundin and Woodruff. Other versions have also been developed for Atari, AmigaOS, Unix (many variants), Solaris, DEC, Windows, Windows CE, Pocket PC, DOS, Palm OS, QNX, OS/2, Linux, BeOS, EPOC32/Symbian, AtheOS, OpenVMS, Apple Macintosh, and Mac OS X.

The Amiga version of Rexx, called ARexx, was included with AmigaOS 2 onwards and was popular for scripting as well as application control. Many Amiga applications have an "ARexx port" built into them which allows control of the application from Rexx. One single Rexx script could even switch between different Rexx ports in order to control several running applications.

1990 to present
In 1990, Cathie Dager of SLAC organized the first independent Rexx symposium, which led to the forming of the REXX Language Association. Symposia are held annually.

Several freeware versions of Rexx are available. In 1992, the two most widely used open-source ports appeared: Ian Collier's REXX/imc for Unix and Anders Christensen's Regina (later adopted by Mark Hessling) for Windows and Unix. BREXX is well known for WinCE and Pocket PC platforms, and has been "back-ported" to VM/370 and MVS.

OS/2 has a visual development system from Watcom VX-REXX. Another dialect was VisPro REXX from Hockware.

Portable Rexx by Kilowatt and Personal Rexx by Quercus are two Rexx interpreters designed for DOS and can be run under Windows as well using a command prompt. Since the mid-1990s, two newer variants of Rexx have appeared:


 * NetRexx: compiles to Java byte-code via Java source code; this has no reserved keywords at all, and uses the Java object model, and is therefore not generally upwards-compatible with 'classic' Rexx.
 * Object REXX: an object-oriented generally upwards-compatible version of Rexx.

In 1996 American National Standards Institute (ANSI) published a standard for Rexx: ANSI X3.274–1996 "Information Technology – Programming Language REXX". More than two dozen books on Rexx have been published since 1985.

Rexx marked its 25th anniversary on 20 March 2004, which was celebrated at the REXX Language Association's 15th International REXX Symposium in Böblingen, Germany, in May 2004.

On October 12, 2004, IBM announced their plan to release their Object REXX implementation's sources under the Common Public License. Recent releases of Object REXX contain an ActiveX Windows Scripting Host (WSH) scripting engine implementing this version of the Rexx language.

On February 22, 2005, the first public release of Open Object Rexx (ooRexx) was announced. This product contains a WSH scripting engine which allows for programming of the Windows operating system and applications with Rexx in the same fashion in which Visual Basic and JScript are implemented by the default WSH installation and Perl, Tcl, Python third-party scripting engines.

REXX was listed in the TIOBE index as one of the fifty languages in its top 100 not belonging to the top 50.

In 2019, the 30th Rexx Language Association Symposium marked the 40th anniversary of Rexx. The symposium was held in Hursley, England, where Rexx was first designed and implemented.

Toolkits
The RexxUtil toolkit is a package of functions that is available for most Rexx implementations and most host operating systems. RexxUtil is a package of file and directory functions, windowed I/O, and functions to access system services such as WAIT and POST.

Rexx/Tk, a toolkit for graphics to be used in Rexx programmes in the same fashion as Tcl/Tk is widely available.

A Rexx IDE, RxxxEd, has been developed for Windows. RxSock for network communication as well as other add-ons to and implementations of Regina Rexx have been developed, and a Rexx interpreter for the Windows command line is supplied in most Resource Kits for various versions of Windows and works under all of them as well as DOS.

Spelling and capitalization
Originally the language was called Rex (Reformed Executor); the extra "X" was added to avoid collisions with other products' names. REX was originally all uppercase because the mainframe code was uppercase oriented. The style in those days was to have all-caps names, partly because almost all code was still all-caps then. For the product it became REXX, and both editions of Mike Cowlishaw's book use all-caps. The expansion to REstructured eXtended eXecutor was used for the system product in 1984.

Looping
The loop control structure in Rexx begins with a  and ends with an   but comes in several varieties. NetRexx uses the keyword  instead of   for looping, while ooRexx treats   and   as equivalent when looping.

Conditional loops
Rexx supports a variety of traditional structured-programming loops while testing a condition either before or after  the list of instructions are executed:

Repetitive loops
Like most languages, Rexx can loop while incrementing an index variable and stop when a limit is reached:

The increment may be omitted and defaults to 1. The limit can also be omitted, which makes the loop continue forever.

Rexx permits counted loops, where an expression is computed at the start of the loop and the instructions within the loop are executed that many times:

Rexx can even loop until the program is terminated:

A program can break out of the current loop with the  instruction, which is the normal way to exit a   loop, or can short-circuit it with the   instruction.

Combined loops
Like PL/I, Rexx allows both conditional and repetitive elements to be combined in the same loop:

Conditionals
Testing conditions with :

The  clause is optional.

For single instructions,  and   can also be omitted:

Indentation is optional, but it helps improve the readability.

Testing for multiple conditions
SELECT is Rexx's CASE structure, derived from the SELECT; form of the PL/I SELECT statement. Like some implementations of CASE constructs in other dynamic languages, Rexx's clauses specify full conditions, which need not be related to each other. In that, they are more like cascaded sets of  code than they are like the C or Java  statement.

The instruction performs "no operation", and is used when the programmer wishes to do nothing in a place where one or more instructions would be required.

The clause is optional. If omitted and no WHEN conditions are met, then the SYNTAX condition is raised.

Simple variables
Variables in Rexx are typeless, and initially are evaluated as their names, in upper case. Thus a variable's type can vary with its use in the program:

Compound variables
Unlike many other programming languages, classic Rexx has no direct support for arrays of variables addressed by a numerical index. Instead it provides compound variables. A compound variable consists of a stem followed by a tail. A  (dot) is used to join the stem to the tail. If the tails used are numeric, it is easy to produce the same effect as an array.

Afterwards the following variables with the following values exist: ...

Unlike arrays, the index for a stem variable is not required to have an integer value. For example, the following code is valid:

In Rexx it is also possible to set a default value for a stem.

After these assignments the term  would produce.

The whole stem can also be erased with the DROP statement.

This also has the effect of removing any default value set previously.

By convention (and not as part of the language) the compound  is often used to keep track of how many items are in a stem, for example a procedure to add a word to a list might be coded like this:

It is also possible to have multiple elements in the tail of a compound variable. For example:

Multiple numerical tail elements can be used to provide the effect of a multi-dimensional array.

Features similar to Rexx compound variables are found in many other languages (including associative arrays in AWK, hashes in Perl and Hashtables in Java). Most of these languages provide an instruction to iterate over all the keys (or tails in Rexx terms) of such a construct, but this is lacking in classic Rexx. Instead it is necessary to keep auxiliary lists of tail values as appropriate. For example, in a program to count words the following procedure might be used to record each occurrence of a word.

and then later:

At the cost of some clarity it is possible to combine these techniques into a single stem:

and later:

Rexx provides no safety net here, so if one of the words happens to be a whole number less than  this technique will fail mysteriously.

Recent implementations of Rexx, including IBM's Object REXX and the open source implementations like ooRexx include a new language construct to simplify iteration over the value of a stem, or over another collection object such as an array, table or list.

In sum, compound variables provide a mechanism to create almost any kind of data structure in Rexx. These include lists or simple arrays, n-dimensional arrays, sparse or dense arrays, balanced or unbalanced trees, records, and more.

PARSE
The  instruction is particularly powerful; it combines some useful string-handling functions. Its syntax is:

where origin specifies the source:


 * (arguments, at top level tail of command line)
 * (standard input, e.g. keyboard)
 * (Rexx data queue or standard input)
 * (info on how program was executed)
 * (an expression) : the keyword   is required to indicate where the expression ends
 * (a variable)
 * (version/release number)

and template can be:


 * list of variables
 * column number delimiters
 * literal delimiters

is optional; if specified, data will be converted to upper case before parsing.

Examples
Using a list of variables as template

displays the following:

Using a delimiter as template:

also displays the following:

Using column number delimiters:

displays the following:

A template can use a combination of variables, literal delimiters, and column number delimiters.

INTERPRET
The INTERPRET instruction evaluates its argument and treats its value as a Rexx statement. Sometimes INTERPRET is the clearest way to perform a task, but it is often used where clearer code is possible using, e.g.,.

Other uses of INTERPRET are Rexx's (decimal) arbitrary precision arithmetic (including fuzzy comparisons), use of the PARSE statement with programmatic templates, stemmed arrays, and sparse arrays.

This displays 16 and exits. Because variable contents in Rexx are strings, including rational numbers with exponents and even entire programs, Rexx offers to interpret strings as evaluated expressions.

This feature could be used to pass functions as function parameters, such as passing SIN or COS to a procedure to calculate integrals.

Rexx offers only basic math functions like ABS, DIGITS, MAX, MIN, SIGN, RANDOM, and a complete set of hex plus binary conversions with bit operations. More complex functions like SIN were implemented from scratch or obtained from third party external libraries. Some external libraries, typically those implemented in traditional languages, did not support extended precision.

Later versions (non-classic) support  constructs. Together with the built-in function, CALL can be used in place of many cases of. This is a classic program:

A slightly more sophisticated "Rexx calculator":

is shorthand for. Likewise,  is shorthand for.

The power of the INTERPRET instruction had other uses. The Valour software package relied upon Rexx's interpretive ability to implement an OOP environment. Another use was found in an unreleased Westinghouse product called Time Machine that was able to fully recover following a fatal error.

SIGNAL
The SIGNAL instruction is intended for abnormal changes in the flow of control (see the next section). However, it can be misused and treated like the GOTO statement found in other languages (although it is not strictly equivalent, because it terminates loops and other constructs). This can produce difficult-to-read code.

Error handling and exceptions
It is possible in Rexx to intercept and deal with errors and other exceptions, using the SIGNAL instruction. There are seven system conditions: ERROR, FAILURE, HALT, NOVALUE, NOTREADY, LOSTDIGITS and SYNTAX. Handling of each can be switched on and off in the source code as desired.

The following program will run until terminated by the user:

A  statement intercepts uses of undefined variables, which would otherwise get their own (upper case) name as their value. Regardless of the state of the  condition, the status of a variable can always be checked with the built-in function   returning VAR for defined variables.

The  function can be used to get the value of variables without triggering a   condition, but its main purpose is to read and set environment variables, similar to POSIX   and.

Conditions

 * Positive RC from a system command
 * Positive RC from a system command


 * Negative RC for a system command (e.g. command doesn't exist)
 * Negative RC for a system command (e.g. command doesn't exist)


 * Abnormal termination
 * Abnormal termination


 * An unset variable was referenced
 * An unset variable was referenced


 * Input or output error (e.g. read attempts beyond end of file)
 * Input or output error (e.g. read attempts beyond end of file)


 * Invalid program syntax, or some other error condition
 * Invalid program syntax, or some other error condition


 * Significant digits are lost (ANSI Rexx, not in TRL second edition)
 * Significant digits are lost (ANSI Rexx, not in TRL second edition)

When a condition is handled by, the   and   system variables can be analyzed to understand the situation. RC contains the Rexx error code and SIGL contains the line number where the error arose.

Beginning with Rexx version 4 conditions can get names, and there's also a  construct. That's handy if external functions do not necessarily exist: