restructured extended executor

REXX

REXX (REstructured eXtended eXecutor) is an interpreted programming language which was developed at IBM. It is a structured high-level programming language which was designed to be both easy to learn and easy to read. Both commercial and open source interpreters for REXX are available on a wide range of computing platforms, and compilers are available for IBM mainframes.

Features

REXX has the following characteristics and features:

REXX has just twenty-three, largely self-evident, instructions (e.g., call, parse, and select) 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 syntax looks similar to PL/I, but has fewer notations; this makes it harder to parse (by program) but easier to use.

History

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.

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, VM/GCS, MVS TSO/E, AS/400, VSE/ESA, AIX, CICS/ESA, 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. Other versions have also been developed for Atari, Amiga, Unix (many variants), Solaris, DEC, Windows, Windows CE, PocketPC, MS-DOS, Palm OS, QNX, OS/2, Linux, BeOS, EPOC32, AtheOS, OpenVMS, OpenEdition, 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.

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 Linux. BREXX is well-known for WinCE and PocketPC platforms.

In 1996 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.

Since the mid-1990s, two newer variants of REXX have appeared:

  • NetRexx — which 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 — which is an object-oriented generally upwards-compatible version of REXX.

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

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 under the Common Public License.

On February 22, 2005, the first public release of Open Object Rexx (ooRexx) was announced.

Spelling

Originally it was just called "Rex", "A Reformed EXecutor"; the extra "X" was added to avoid collisions with other products' names. The expansion of Rexx to the REstructured EXtended EXecutor is believed to be a backronym, lacking word from the folks who added the extra X.

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. By the 1990s, though, that was starting to look a bit dated (fine in monospace, but too big/wide in most other fonts) -- most new things had mixed-case names — so Mike started to use Rexx, or small caps EXX if available (REXX -- but with even smaller small-caps). ANSI also liked that (small caps) which preserves the 'all-caps' history, without it seeming to 'shout' from the page.

These days (2008), in plain text, Mike Cowlishaw seems to prefer Rexx, whereas IBM documents and the majority of the web use REXX. The ANSI standard uses the form preferred by the standardization committee, which has small capitals for the final three letters: REXX.

According to Mike, this all-caps style is used on the cover pages of "The REXX Language" TRL. "The NetRexx Language" written by Mike Cowlishaw uses lowercase exx.

Syntax

Looping

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

Traditional forms:

   do until [condition]
     [instructions]
   end

   do while [condition]
     [instructions]
   end

With an index variable:

   do i = x [to y ][by z]
     [instructions]
   end

The step increment (z above) may be omitted and defaults to 1. The upper limit (y above) can also be omitted, which makes the loop continue forever. You can also loop forever without an index variable with this:

   do forever
     [instructions]
   end

A program can break out of the current loop with the leave instruction (which is the normal way to exit a "forever" loop), or can short-circuit it with the iterate instruction. The do while and do until forms are equivalent to:

  do forever
    if [condition] then leave
    [instructions]
  end
and:
  do forever
    [instructions]
    if [condition] then leave
  end

Conditionals

Testing conditions with IF

   if [condition] then
     do
       [instructions]
     end
   else
     do
       [instructions]
     end

For single instructions, DO and END can also be omitted:

   if [condition] then
     [instruction]
   else
     [instruction]

Testing for multiple conditions

SELECT is REXX's CASE structure, like many other constructs derived from PL/I:

   select
     when [condition] then
         [instruction]
     when [condition] then
       do
         [instructions]
       end
     otherwise
       [instructions] or NOP
   end

NOP indicates no instruction is to be executed.

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:

 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      */

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.

 do i = 1 to 10
   stem.i = 10 - i
 end

Afterwards the following variables with the following values exist: stem.1 = 9, stem.2 = 8, stem.3 = 7...

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

 i = 'Monday'
 stem.i = 2

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

 stem. = 'Unknown'
 stem.1 = 'USA'
 stem.44 = 'UK'
 stem.33 = 'France'

After these assignments the term stem.3 would produce 'Unknown'.

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

 drop stem.

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

By convention (and not as part of the language) the compound stem.0 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

It is also possible to have multiple elements in the tail of a compound variable. 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 REXX compound variables are found in many other languages (associative arrays in AWK, hashes in Perl, Hashtables in Java, etc). 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.

 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

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

 add_word: procedure expose dictionary.
   parse arg w .
   dictionary.w = dictionary.w + 1
   if dictionary.w = 1 /* assume dictionary. = 0 */
     then do
       n = dictionary.0+1
       dictionary.n = w
       dictionary.0 = n
     end
   return
and later
 do i = 1 to dictionary.0
   w = dictionary.i
   say i w dictionary.w
 end
However, REXX provides no safety net here, so if one of your words happens to be a whole number less than dictionary.0 the above technique will fail mysteriously.

Recent implementations of REXX, include 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, list, etc.

 do i over stem.
   say i '-->' stem.i
 end

Keyword instructions

PARSE

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

   parse [upper] origin template

where origin specifies the source:

  • arg (arguments, at top level tail of command line)
  • linein (standard input, e.g. keyboard)
  • pull (REXX data queue or standard input)
  • source (info on how program was executed)
  • value (an expression) with
  • : the keyword with is required to indicate where the expression ends
  • var (a variable)
  • version (version/release number)

and template can be:

  • list of variables
  • column number delimiters
  • literal delimiters

upper is optional; if you specify it, data will be converted to upper case.

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 the following:

   First name is: John
   Last name is: Smith

Using a delimiter as template:

   myVar = "Smith, John"
   parse var myVar LastName "," FirstName
   say "First name is:" firstName
   say "Last name is:"  lastName

also displays the following:

   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 the following:

   Area code is: 202
   Subscriber number is: 123-1234

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

INTERPRET

The INTERPRET instruction is very powerful and one of the two reasons why writing REXX compilers isn't trivial, the other reason being REXX's decimal arbitrary precision arithmetic:

 /* a touch of LISP */
 X = 'square'
 interpret 'say' X || '(4) ; exit'
 SQUARE: return arg(1) * arg(1)

This displays 16 and exits. Because anything in REXX are strings, even rational numbers with exponents, and last but not least complete programs, REXX offers to interpret strings as programs.

This feature was used to implement something like function parameters, e.g. passing SIN, COS, etc. to a procedure SIMPSON to calculate integrals.

Note that 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. Anything else like SIN has to be implemented from scratch or using external libraries. The latter typically don't support arbitrary precision.

Later versions (non-classic) support CALL variable constructs. Together with the built-in function VALUE, CALL allows to avoid many cases of INTERPRET. This is a classic program:

 /* terminated by input "exit" or similar */
 do forever ; interpret linein() ; end

A slightly more sophisticated REXX calculator:

 X = 'input BYE to quit'
 do until X = 'BYE' ; interpret 'say' X ; pull X ; end

PULL is a shorthand for parse upper pull like ARG for parse upper arg.

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 */

SIGNAL

The REXX 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.

This example will run until stopped by the user:

 signal on halt;
 do a = 1
   say a
   do 100000 /* a delay */
   end
 end
 halt:
   say "The program was stopped by the user"
 exit

Virtually all serious REXX programs contain signal on novalue or a similar statement. This disables the "feature", where undefined variables get their own (upper case) name as value. The status of a variable can be checked with the built-in function SYMBOL returning VAR for defined variables.

Function VALUE can be used to get the value of variables without triggering a NOVALUE condition, but its main purpose is to read and set environment variables - similar to POSIX getenv and putenv.

Conditions

ERROR

Positive RC from a system command
FAILURE

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

Abnormal termination (see above)
NOVALUE

An unset variable was referenced (see above)
NOTREADY

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

Invalid program syntax, or some error condition not covered above
LOSTDIGITS

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

When a condition is handled by SIGNAL ON, the SIGL and RC 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 CALL ON construct. That's handy if external functions do not necessarily exist:

  ChangeCodePage: procedure /* protect SIGNAL settings */
    signal on syntax name ChangeCodePage.Trap
    return SysQueryProcessCodePage()
  ChangeCodePage.Trap: return 1004 /* windows-1252 on OS/2 */

See also

References

Books

  • Cowlishaw, Michael. The Rexx Language: A Practical Approach to Programming. Prentice Hall, 1990. ISBN 0-13-780651-5.
  • Cowlishaw, Michael. The NetRexx Language. Prentice Hall, 1997. ISBN 0-13-806332-X.
  • Daney, Charles. Programming in REXX. McGraw-Hill, TX, 1990. ISBN 0-07-015305-1.
  • Deuring, Johannes. REXX Grundlagen für die z/OS Praxis. Germany, 2005. ISBN 3-486-20025-9.
  • Ender, Tom. Object-Oriented Programming With Rexx. John Wiley & Sons, 1997. ISBN 0-471-11844-3.
  • Fosdick, Howard. Rexx Programmer's Reference. Wiley/Wrox, 2005. ISBN 0-7645-7996-7.
  • Gargiulo, Gabriel. REXX with OS/2, TSO, & CMS Features. MVS Training, 1999 (third edition 2004). ISBN 1-892559-03-X.
  • Goldberg, Gabriel and Smith, Philip H. The Rexx Handbook . McGraw-Hill, TX, 1992. ISBN 0-07-023682-8.
  • Goran, Richard K. REXX Reference Summary Handbook. CFS Nevada, Inc.,1997. ISBN 0-9639854-3-4.
  • IBM Redbooks. Implementing Rexx Support in Sdsf. Vervante, 2007. ISBN 0-738-48914-X.
  • Kiesel, Peter C. Rexx: Advanced Techniques for Programmers. McGraw-Hill, TX, 1992. ISBN 0-07-034600-3.
  • O'Hara, Robert P. and Gomberg, David Roos. Modern Programming Using Rexx. Prentice Hall, 1988. ISBN 0-13-597329-5.
  • Rudd, Anthony S. Practical Usage of Rexx. Ellis Horwood Ltd., 1991. ISBN 0-13682-790-X.
  • Schindler, William. Down to Earth Rexx. Perfect Niche Software, 2000. ISBN 0-9677590-0-5.

External links

Classic interpreters

  • Regina: open-source (LGPL) interpreter for Linux, BSD, Windows, etc.
  • REXX/imc: open-source (nonstandard license) interpreter for Unix and Linux systems.
  • BREXX: open-source (GPL) interpreter for DOS, Linux, Windows CE, etc.
  • Reginald: free interpreter for Windows.
  • roo!: freeware interpreter for Windows with object-oriented extensions from Kilowatt Software.
  • r4: freeware interpreter for Windows from Kilowatt Software.
  • REXX for Palm OS: shareware interpreter for Palm OS from Jaxo Inc.
  • Personal REXX: commercial interpreter for Windows, OS/2 and DOS from Quercus Systems.
  • S/REXX: commercial interpreter for UNIX and Windows from Benaroya.
  • uni-REXX: commercial interpreter for UNIX from The Workstation Group Ltd.

Other interpreters

Compilers

Newsgroups

Tutorials

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