What REXX is Not!

Time Sharing Option, Interactive System Productivity Facility and REstructured eXtended eXecutor

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Anuj Dhawan
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What REXX is Not!

Postby Anuj Dhawan » Sun May 12, 2013 11:10 pm

REXX is not CLIST! -- REXX is not TSO! -- REXX is not ISPF! -- REXX is NOT PDF!

REXX is an excellent language for "Rapid Application Development". It is also just fine to use for many scripting type functions on the mainframe that require some sort of "programming". REXX frequently serves as the language behind an ISPF dialog.

Said that and explainging what REXX is not, I believe it'd be nice, on my part, to tell what other things are and then leave on the user to understand it. So here it goes:

CLIST, Command List, pronounced as "C-List", is a procedural programming language for TSO in MVS systems. It originated in OS/360 Release 20 and has assumed a secondary role since the availability of Rexx in TSO/E Version 2. In its basic form, a CLIST program (or "CLIST" for short) can take the form of a simple list of commands to be executed in strict sequence (like a DOS batch file (*.bat) file). However, CLIST also features If-Then-Else logic as well as loop constructs.

CLIST is an interpreted language. That is, the computer must translate a CLIST every time the program is executed. CLISTs therefore tend to be slower than programs written in compiled languages such as COBOL, FORTRAN, or PL/1. CLIST can read/write MVS files and read/write from/to a TSO terminal. It can read parameters from the caller and also features a function to hold global variables and pass them between CLISTs. A CLIST can also call an MVS application program (written in COBOL or PL/I, for example). CLISTs can be run in background (by running JCL which executes the TSO control program (IKJEFT01)). TSO I/O screens and menus using ISPF dialog services can be displayed by CLISTs.

TSO/E is an old-time networking system that constantly monitors all time-sharing system address spaces (TSO users). It has a complex command programming capability that is not for the feint of heart. For the vast majority of TSO/ISPF users, all they need to know is TSO is where you log-on to the system to get to ISPF, which sits on top of TSO and contains the crown jewel -- the editor. Since TSO is a prerequisite to launching ISPF, ISPF is also slavishly dependant upon the press of an AID key (so TSO will see it and take action). For the adventurous coder, ISPF offers a huge set of services that allows you to roll-your-own ISPF dialogs, in ANY language you choose -- COBOL, Assembler, PL/I, C, Pascal, CLIST and oh yeah, REXX.

And PDF is NOT ISPF. PDF is Program Development Facility. ISPF is the product that supports panels and such; PDF is the IBM supplied panels (and what is behind them) such as VIEW, EDIT, BROWSE, etc. Any customized (home grown add ons) at your place of business would not technically be PDF, but would be within ISPF.

Courtsey: I've taken references from many places and I can't remember all of them However, in particular I'd like to Thank Sir Jim Moore.


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