(also posted to ibm-main listserv)
So as you might know, I'm interested/excited about the new "EXIT" statements in Enterprise COBOL. I just had a thought on a good use of EXIT SECTION.
We too often run in to issues where a developer will add a new paragraph for a new process, and they will add it so that positionally it becomes part of a SECTION within the procedure division, and this it is unintentionally performed as the result of a PERFORM of that section.
For example, take this following simple code:
procedure division. main section. mainline. perform one perform two goback. one section. [...do something...] two section. [...do something else...]
Let's say that a new developer wants to add a third process. They add the following to the end of the program:
three. [...do a third thing...]
They then add "perform three" after "perform two" in the "mainline" paragraph. So what happens? The code within paragraph "three" gets executed twice; the first time as part of "perform two", because "three" is part of the section named "two", and then again as a result of the new "perform three".
Now in this example it would probably become obvious during testing that this is occurring, so hopefully (!!) it would not make it to production. But some times the "perform two" may only occur in certain edge cases, and so the unintentional execution of "three" might not occur during the testing phases.
So many will agree that procedure division sections are a bad idea. Well that's a fine opinion, and one I can get behind. But the fact is, they exist in many programs, and to often they have this sort of bug. So what to do?
I had a thought that using an explicit "EXIT SECTION" to the end of all procedure division sections would be a good first start. In the above example there could be a "one-exit" paragraph at the end of "one section" that contains just the "exit section" statement. And a "two-exit" paragraph at the end of "two section" that did the same.
By doing this you have a "defense" against someone doing what happened here. Now even if "three" is unintentionally made part of section "two" we will no longer drop through into it. The EXIT SECTION (unlike the fairly useless legacy EXIT statement, which is a NO-OP) will cause the flow to branch around "three" (and anything following "three") to the "true" end of the section.
Of course there is nothing to enforce this standard. But perhaps there could be! I'm thinking of an RFE to add a compiler option that would require all sections to be terminated by a paragraph consisting of only the EXIT SECTION statement. Could that catch all of these types of bugs? I'm thinking it could. Take the example above, with this new rule in force:
procedure division. main section. mainline. perform one perform two goback. exit-mainline. exit section. one section. [...do something...] exit-one. exit section. two section. [...do something else...] exit-two. exit section.
Now if someone adds something after exit-two it will violate the rule and give a warning, or even an error. The developer would fix it by adding "three section" instead of "three", as well as its own "exit-section" paragraph with the "exit section" statement.
Obviously pretty much every existing COBOL program that uses procedure division sections would violate the rule. But perhaps there could be a simple "migration tool" that could update source code to conform to the rule. A more advanced migration tool might even look for existing "exit-section" paragraphs with just the "EXIT", which is truly how statement procedure division sections currently should be "terminated", and if there is not a SECTION (or end of source) following it, report an error. This assumes all sections are terminated this way, which I'm sure is not the case, but it still seems worthwhile.
Make any sense? I'm sure Bill W will have something to say about it!
Frank
fswarbrick