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REXX

From Academic Kids

REXX (Restructured Extended Executor) is a programming language which was developed at IBM, and several implementations are available under open source licenses. 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.

Contents

Features

REXX has the following characteristics and features:

  • character string basis
  • dynamic data typing (no declarations)
  • no reserved keywords (except in local context)
  • 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
  • straightforward access to system commands and facilities
  • simple error-handling, and built-in tracing and debugger
  • few artificial limitations
  • simplified I/O facilities.

REXX has just twenty-three, largely self-evident, instructions (e.g., call, parse, and select) with minimal punctuation and formatting requirements. It is essentially a 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 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, OS/2, VSE/ESA, AIX, CICS/ESA, and PC-DOS), 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.

Several freeware versions 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 upwards-compatible with ‘classic’ REXX
  • ObjectRexx – which is an object-oriented 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. Symposiums 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 B÷blingen, Germany, in May 2004.

On October 12, 2004, IBM announced their plan to release their ObjectRexx implementation under the Common Public License

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

Syntax

Looping

The DO control structure always begins with a DO and ends with an END.

DO UNTIL:

   do until [condition]
   [instructions]
   end


DO WHILE:

   do while [condition is true]
   [instructions]
   end


Stepping through a variable:

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


Looping forever until exiting with LEAVE:

   do forever
     if [condition] then leave
   end

Looping a fixed number of times

   do i = x to y by z for a
   [instructions]
   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

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

NOP indicates no instruction is to be executed.

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 (command line variable)
  • linein (keyboard)
  • pull (REXX data queue)
  • source (OS/2 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; it 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.

Flexible 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:

 do
   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
 end

Array support

REXX does not have explicit variable declarations, and likewise no arrays. However, a stem structure, similar to an array can be produced easily:

 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...

The drop stem. instruction unsets all ten variables of the form stem.i.

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

SIGNAL: A negative example

The REXX SIGNAL instruction is equivalent to the GOTO in other languages. When correctly used, it can be a very useful instruction, for example to intercept errors or other exceptional cases. But one can also produce illegible, difficult to read code:

 /* A Rexx Program */
   signal define;
 use:
   say a
 signal end;
 define: 
   a = "hello world"
   signal use;
 end:
   exit

Error and exception treatment in REXX

It is possible in REXX to intercept and deal with errors and other exceptions, using the SIGNAL instruction. There are five system conditions: ERROR, FAILURE, HALT, NOVALUE 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;
 a = 1
 do
   a = (a + 1) // 10000
   say a
 end
 halt:
 say "The program was stopped by the user" 
 exit

Conditions

error Positive RC from a system command
failure Negative RC from a system command
halt Abnormal termination
novalue An unset variable was referenced
syntax Invalid REXX program syntax

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.

Under OS/2

REXX is included in the base operating system of OS/2, and is also used as the macro language in many applications. Under OS/2, a REXX program begins with matched comment delimiters, /* */, to indicate to the operating system that it is a REXX program:

   /* sample.cmd */
   say "Hello World"

Instructions between quotes are passed to the OS:

   /* sample.cmd */
   'dir /p /w'

Spelling

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

Originally just "Rex" because the author liked how it sounded, the extra "x" was added to avoid collisions with other products' names.

Literature

  • The Rexx Language: A Practical Approach to Programming, by Michael Cowlishaw, ISBN 0137806515

External links

de:REXX fr:Rexx ru:REXX

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