Rexx ( restructured extended executor) is a high-level programming language developed at IBM by Mike Cowlishaw.[ Both proprietary and open source Rexx interpreters exist for a wide range of computing platforms, and exist for IBM mainframe computers.]
Rexx is supported in a variety of environments. It is the primary scripting language in some including OS/2, MVS, VM, AmigaOS and is used for macros in some software including SPF/PC, KEDIT, THE and ZOC. With an engine installed, Rexx can be used for scripting and macros in programs that use a Windows Scripting Host ActiveX scripting engine (such as VBScript or JScript). 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,
A script is associated with a Rexx interpreter at runtime in various ways based on context. In mainframe computing, a Rexx script or command is sometimes referred to as an EXEC since that is the name of the file type used for similar CMS CMS EXEC,
Name
Originally, the language was called REX, short for Reformed Executor, but an extra "X" was added to avoid confusion with other products. The name was originally all uppercase because that was the only way to represent it in mainframe code at the time. Both editions of Mike Cowlishaw's first book on the language use all-caps, REXX, although the cover graphic uses mixed case. His book on NetRexx uses mixed case but all caps in the cover graphic with large and small caps, . An expansion that matches the abbreviation, REstructured eXtended eXecutor, was used for the system product in 1984.
Attributes
Objective and subjective attributes of Rexx include:
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Simple syntax
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Ability to route commands to multiple environments
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Ability to support functions, procedures and commands associated with a specific invoking environment.
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Built-in stack with the ability to interoperate with the host stack if there is one
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Small instruction set
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Free-form syntax; indentation is optional but can help readability
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Case-insensitive tokens, including variable names
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Character string basis
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Dynamic data typing; no declarations
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No reserved word, except in local context
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No include file facility
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Arbitrary-precision arithmetic
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Decimal arithmetic, floating-point
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Rich selection of built-in functions, especially string and word processing
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Automatic storage management
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Crash protection
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Content addressable data structures
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Associative array
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Straightforward access to system commands and facilities
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Simple error-handling, and built-in tracing and debugger
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Few artificial limitations
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Simplified I/O facilities
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Unconventional operators
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Only partly supports Unix style command line parameters, except specific implementations
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Provides no basic terminal control as part of the language, except specific implementations
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Provides no generic way to include functions and subroutines from external libraries, except specific implementations
Some claim that Rexx is a relatively simple language. With only 23 instructions (such as call, parse, and select), it has a relatively small instruction set. Rexx has limited punctuation and formatting requirements. Rexx has only one data type, the character string. Some claim that such simplicities make Rexx relatively easy to debug.
Some claim that Rexx code looks similar to PL/I code, but has fewer notations. With fewer notations, it tends to be is harder to parse via a translator, but is easier to write. Simplifying coding was intentional as noted by the Rexx design goal of the principle of least astonishment.
History
pre1990
On his own time, Mike Cowlishaw developed the language and an interpreter for it in assembly language between 20 March 1979 and mid-1982 with the intent to replace the languages EXEC and EXEC 2.
Rexx was first described in public at the SHARE 56 conference in Houston, Texas, in 1981,
Over the years IBM included Rexx in almost all of its operating systems (VM/CMS, MVS, IBM OS/400, VSE/ESA, MUSIC/SP, IBM 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[ The first Rexx compiler appeared in 1987, written for CMS by Lundin and Woodruff.]
ARexx, a Rexx interpreter for Amiga, was included with AmigaOS 2 onwards and was popular for scripting and application control. Many Amiga applications have an "ARexx port" which allows control of the application via a Rexx script. Notably, a Rexx script can switch between Rexx ports to control multiple 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.
In 1992, the two most widely used open-source ports appeared: Ian Collier's REXX/imc for Unix and Anders Christensen's Regina
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:
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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.
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Object REXX: an object-oriented generally upwards-compatible version of Rexx.
In 1996 the 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.
In January 2018 the TIOBE index listed Rexx at position 30.
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
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RexxUtil a package of file and directory functions, windowed I/O, and functions to access system services such as WAIT and POST is available for most Rexx environments.
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Rexx/Tk a toolkit for graphics to be used in Rexx programmes in the same fashion as Tcl/Tk is widely available.
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RexxEd an integrated development environment (IDE) for Rexx was developed for Windows.
[
]
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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 in DOS as well.
Host environment
A Rexx host environment is a named interface for sending commands to an, e.g., application, operating system, subsystem. The Rexx interpreter initially has a list of known environments; the first in the list is known as the default environment. A Rexx script use the ADDRESS statement to change the default environment and may also use it to send a single command to a specific environment without changing the default environment.
Syntax
Instruction types
Rexx has three instruction types
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Assignments – Single clause of the form ''symbol''=''expression'', assigns a value to a variable; e.g. count=count+1, would add 1 to value in variable "count".
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Keyword instructions – The keyword is the first word of the instruction; e.g. '''say''' "Message", would print the word 'Message'.
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Commands – an expression that the interpreter evaluates and passes as a command to the default environment; e.g. "sleep 10" would cause a Unix-like host environment to produce a ten seconds delay.
Address instruction
The address instruction has three roles:
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An address instruction with no operand interchanges the first two host environments; normally this reinstate the previous default environment.
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An address instruction with only an environment moves it to the beginning of the list, making it the default environment.
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An address instruction with an environment and an expression passes the value of the expression as a command to the specified environment.
The ANSI standard added options for redirecting the input and output of commands.
Do groups
The language provides do groups for two purposes:
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To treat a group of instructions within an if or select statement as a unit for purposes of flow control.
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For For loop, similar to many other languages. A single do group may optionally contain repetitor phrases, conditional phrases, or both, with termination whenever any of them is satisfied.
A do group begins with do and ends with end. A single group may serve both purposes. In the related ooRexx and NetRexx, there is both a do and a loop keyword, with almost identical semantics; they differ in that a simple do is equivalent to do 1 while a simple loop is equivalent to loop forever.
An iteration of a do group may be terminated with an iterate statement and the entire group may be terminated with a leave statement.
Simple do
Although it is valid anywhere, a simple do is specifically useful inside conditional statements:
if foo=bar
then
do
i=1;j=3
end
else
do
i=2;j=4
end
Conditional loop
The language supports testing a condition either before (do while) or after (do until) executing a group of code via syntax:
'''do while''' [''condition'']
[''instructions'']
'''end'''
'''do until''' [''condition'']
[''instructions'']
'''end'''
Simple repetitive loop
The language 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:
'''do''' ''expression''
[''instructions'']
'''end'''
Controlled Repetitive Loops
A loop can increment a variable and stop when a limit is reached.
'''do''' ''index''=''start'' ['''to''' ''limit''] ['''by''' ''increment''] ['''for''' ''count'']
[''instructions'']
'''end'''
The increment value is 1 if the by clause is omitted. The loop continues forever if the limit to clause is omitted, unless terminated earlier by another clause or by a leave statement.
Unconditional loop
The language supports an unconditional loop via forever that continues until the loop is terminated or the program is terminated.
'''do forever'''
[''instructions'']
'''end'''
Combined loop
Repetitive elements and conditional phrases can be combined in the same loop;[ e.g.:do i=1 while i<=3; say i; end
]'''do''' ''index''=''start'' ['''to''' ''limit''] ['''by''' increment] ['''for''' ''count''] ['''while''' ''condition'']
[''instructions'']
'''end'''
'''do''' ''expression'' ['''until''' ''condition'']
[''instructions'']
'''end'''
Conditional
The language provides for conditional execution via if, then and else for a group delimited by do and end.
'''if''' [''condition''] '''then'''
'''do'''
[''instructions'']
'''end'''
'''else'''
'''do'''
[''instructions'']
'''end'''
For a single instruction, do and end can be omitted.
'''if''' [''condition''] '''then'''
[''instruction'']
'''else'''
[''instruction'']
Multiple condition branching
The language provides Switch statement via select which derives from the SELECT; form of the PL/I SELECT statement
. Like similar constructs in other dynamic languages, Rexx's SELECT (''expression''); clauses specify full conditions not equality tests of a single value for the statement as some languages do. In that, they are more like cascading when code than like the C or Java if-then-else statement.
'''select'''
'''when''' [''condition''] '''then'''
[''instruction'' or '''nop''']
'''when''' [''condition''] '''then'''
'''do'''
[''instructions'' or '''nop''']
'''end'''
'''otherwise'''
[''instructions'' or '''nop''']
'''end'''
The switch instruction is required if no action is associated with a nop condition.
The when clause is optional. If omitted and no otherwise conditions are met, then the when condition is raised.
Variable
Typing system
Variables 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:
say hello /* => HELLO */
hello = 25
say hello /* => 25 */
hello = "say 5 + 3"
say hello /* => say 5 + 3 */
interpret hello /* => 8 */
drop hello
say hello /* => HELLO */
Evaluation
If no syntax condition handler is configured, then an undefined variable evaluates to its name, in upper case. The built-in function novalue returns "VAR" for a defined variable and does not trigger SYMBOL even if not defined. The novalue function gets the value of a variable without triggering a VALUE condition, but its main purpose is to read and set environment variables, similar to POSIX novalue and getenv.
Compound variable
The language provides the compound variable construct which supports adding fields (called tails) to a variable (called a stem in this context) to support data structures such as lists, arrays, n-dimensional arrays, sparse or dense arrays, balanced or unbalanced trees and records.
The language does not provide special support for numeric array indexing like many other languages do. Instead, a compound variable with numeric tails produce a similar effect.
The following code defines variables putenv...
do i = 1 to 10
stem.i = 10 - i
end
Unlike a typical array, a tail (index) need not identify (be named) an integer value. For example, the following code is valid:
i = 'Monday'
stem.i = 2
A default value can be assigned to a stem via stem.1 = 9, stem.2 = 8, stem.3 = 7 but no tail.
stem. = 'Unknown'
stem.1 = 'USA'
stem.44 = 'UK'
stem.33 = 'France'
In this case ., for example, evaluates to the default value, stem.3.
The whole stem (including any default value) can be erased with the 'Unknown' statement.
drop stem.
By convention (not part of the language) the compound drop 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:
add_word: procedure expose dictionary.
parse arg w
n = dictionary.0 + 1
dictionary.n = w
dictionary.0 = n
return
A stem can have multiple tails. For example:
m = 'July'
d = 15
y = 2005
day.y.m.d = 'Friday'
Multiple numerical tail elements can be used to provide the effect of a multi-dimensional array.
Features similar to the compound variable are found in other languages including associative arrays in AWK, hash table in Perl and in Java, dynamic objects in JavaScript. Most of these languages provide a mechanism to iterate over the keys (tails) of such a construct, but this is lacking in classic Rexx. Instead, it is necessary to store additional information. For example, the following procedure might be used to count each occurrence of a word.
add_word: procedure expose count. word_list
parse arg w .
count.w = count.w + 1 /* assume count. has been set to 0 */
if count.w = 1 then word_list = word_list w
return
and then later:
do i = 1 to words(word_list)
w = word(word_list,i)
say w count.w
end
More recent Rexx variants, including Object REXX and ooRexx, provide a construct to iterate over the tails of a stem.
do i over stem.
say i '-->' stem.i
end
Parse
The stem.0 instruction provides string-handling via syntax:
parse
If
'''parse''' ['''upper'''] ''origin'' [''template'']
is included then the input is converted to upper case before parsing.
origin describes the input as one of the following:
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upper arguments, at top level tail of command line
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arg standard input, e.g. keyboard
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linein Rexx data queue or standard input
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pull info on how program was executed
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source expression value with indicates the end of the expression
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with a variable
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var version/release number
template can be a combination of variables, literal delimiters, and column number delimiters.
Examples
Using a list of variables as template:
myVar = "John Smith"
parse var myVar firstName lastName
say "First name is:" firstName
say "Last name is:" lastName
displays:
First name is: John
Last name is: Smith
Using column number delimiters:
myVar = "(202) 123-1234"
parse var MyVar 2 AreaCode 5 7 SubNumber
say "Area code is:" AreaCode
say "Subscriber number is:" SubNumber
displays:
Area code is: 202
Subscriber number is: 123-1234
Interpret
The version instruction evaluates its argument as a Rexx statement allowing for evaluation of code formatted at runtime. Uses include passing a function as a parameter, arbitrary precision arithmetic, use of the interpret statement with programmatic templates, stemmed arrays, and sparse arrays. The following example displays 16 and exits.
X = 'square'
interpret 'say' X || '(4) ; exit'
SQUARE: return arg(1)**2
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 an otherwise fatal error.
Numeric
say digits() fuzz() form() /* => 9 0 SCIENTIFIC */
say 999999999+1 /* => 1.000000000E+9 */
numeric digits 10 /* only limited by available memory */
say 999999999+1 /* => 1000000000 */
say 0.9999999999=1 /* => 0 (false) */
numeric fuzz 3
say 0.99999999=1 /* => 1 (true) */
say 0.99999999==1 /* => 0 (false) */
say 100*123456789 /* => 1.23456789E+10 */
numeric form engineering
say 100*123456789 /* => 12.34567890E+9 */
say 53 // 7 /* => 4 (rest of division)*/
Calculation of the value √2:
numeric digits 50
n=2; r=1
do forever /* Newton's method */
rr=(n/r+r)/2
if r=rr then leave
r=rr
end
say "SqRt" n "=" r
Output:
Calculation of the value :
numeric digits 50
e=2.5; f=0.5
do n=3
f=f/n
ee=e+f
if e=ee then leave
e=ee
end
say "e =" e
Output:
Error handling
The parse instruction configures the runtime to run custom code to handle a system condition if triggered. Conditions include:
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signal Positive return code from a system command
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error Negative return code from a system command (e.g. command doesn't exist)
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failure Abnormal termination
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halt A variable name was used but the variable is not defined
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novalue Input or output error (e.g. read attempts beyond end of file)
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notready Invalid program syntax, or some other error condition
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syntax Significant digits were lost (ANSI Rexx, not in TRL second edition)
The following fragment prints a message when the user terminates (halts) it:
signal on halt;
do a = 1
say a
do 100000 /* a delay */
end
end
halt:
say "The program was stopped by the user"
exit
Since Rexx version 4, a handler can be named. In the following example, the handler lostdigits is configured to handle a ChangeCodePage.Trap condition.
ChangeCodePage: procedure
signal on syntax name ChangeCodePage.Trap
return SysQueryProcessCodePage()
ChangeCodePage.Trap: return 1004
When a condition is handled (as configured via syntax), the condition can be analyzed via signal on which indicates the last error code and RC which indicates the line number of the code that triggered the condition.
See also
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ISPF
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XEDIT
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Comparison of computer shells
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Comparison of programming languages
Notes
Further reading
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Callaway, Merrill. The ARexx Cookbook: A Tutorial Guide to the ARexx Language on the Commodore Amiga Personal Computer. Whitestone, 1992. .
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Callaway, Merrill. The Rexx Cookbook: A Tutorial Guide to the Rexx Language in OS/2 & Warp on the IBM Personal Computer. Whitestone, 1995. .
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Cowlishaw, Michael. The Rexx Language: A Practical Approach to Programming. Prentice Hall, 1990. .
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Cowlishaw, Michael. The NetRexx Language. Prentice Hall, 1997. .
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Daney, Charles. Programming in REXX. McGraw-Hill, TX, 1990. .
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Ender, Tom. Object-Oriented Programming With Rexx. John Wiley & Sons, 1997. .
-
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Gargiulo, Gabriel. REXX with OS/2, TSO, & CMS Features. MVS Training, 1999 (third edition 2004). .
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Goldberg, Gabriel and Smith, Philip H. The Rexx Handbook . McGraw-Hill, TX, 1992. .
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Goran, Richard K. REXX Reference Summary Handbook. CFS Nevada, Inc.,1997. .
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IBM Redbooks. Implementing Rexx Support in Sdsf. Vervante, 2007. .
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Kiesel, Peter C. Rexx: Advanced Techniques for Programmers. McGraw-Hill, TX, 1992. .
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Marco, Lou ISPF/REXX Development for Experienced Programmers. CBM Books, 1995.
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O'Hara, Robert P. and Gomberg, David Roos. Modern Programming Using Rexx. Prentice Hall, 1988. .
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Rudd, Anthony S. 'Practical Usage of TSO REXX'. CreateSpace, 2012. .
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Schindler, William. Down to Earth Rexx. Perfect Niche Software, 2000. .
External links
