Sunday, December 09, 2012

Error Wrangling

Last week the OTN SQL and PL/SQL Forum hosted of those threads which generate heat and insight without coming to a firm conclusion: this one was titled WHEN OTHERS is a bug. Eventually Rahul, the OP, complained that he was as confused as ever. The problem is, his question asked for a proof of Tom Kyte's opinion that, well, that WHEN OTHERS is a bug. We can't proof an opinion, even an opinion from a well-respected source like Tom. All we can do is weigh in with our own opinions on the topic.

One of the most interesting things in the thread was Steve Cosner's observations on designing code "to be called by non-database, and ... non-Oracle software (Example: Oracle Forms, or a web process)". He uses WHEN OTHERS to produce a return code. Now return codes are often cited as an example of bad practice. Return codes disguise the error and the allow the calling program to proceed as though nothing had gone wrong in the called unit. Exceptions, on the other hand, cannot be ignored.

But Steve goes on to make a crucial point: we must make allowances for how our code will be called. For instance, one time I was writing PL/SQL packages for an E-Business Suite application and I was doing my usual thing, coding procedures which raised exceptions on failure. This was fine for the low level routines which were only used in other PL/SQL routines. But the procedures which ran as concurrent jobs had to finish cleanly, no matter what happened inside; each passed a return code to the concurrent job manager, and logged the details. Similarly the procedures used by Forms passed back an outcome message, prefixed with "Success" or"Error" as appropriate.

This rankled at first but it is undoubtedly true that exceptions make demands on the calling program. Although the demands are not as high in PL/SQL as in say C++ or Java, raising exceptions still changes how the calling function must be coded. We have to work with the grain of the existing code base when introducing new functionality. (Interestingly, Google has a fiat against exceptions in its C++ code base, and developed its Go language to support return codes as the default error handling mechanism. Find out more.)

The point is APIs are a contract. On its side the called program can enforce rules about how it is called - number and validity of input parameters, return values. But it cannot impose rules about what the calling program does with the outcome. So there's no point in exposing a function externally if its behaviour is unacceptable to the program which wants to call it. When the calling program wants to use return codes there's little point in raising exceptions instead. Sure the coder writing the calling program can ignore the value in the return code, but that is why we need code reviews.

So, is WHEN OTHERS a bug? The answer is, as so often, it depends.

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Sunday, December 02, 2012

GOTOs, considered

Extreme programming is old hat now, safe even. The world is ready for something new, something tougher, something that'll... break through. You know? . And here is what the world's been waiting for: Transgressive Programming.

The Transgressive Manifesto is quite short:

It's okay to use GOTO.
The single underlying principle is that we value willful controversy over mindless conformity.

I do have a serious point here. Even programmers who haven't read the original article (because they can't spell Dijkstra and so can't find it through Google) know that GOTOs are "considered harmful". But as Marshall and Webber point out, "the problem lies ... in the loss of knowledge and experience. If something is forbidden for long enough, it becomes difficult to resurrect the knowledge of how to use it."

How many Oracle developers even realise PL/SQL supports GOTO? It does, of course. Why wouldn't it? PL/SQL is a proper programming language.

The standard objection is that there is no role for GOTO because PL/SQL has loops, procedures, CASE, etc. But sometimes we need to explicitly transfer control. In recent times I have have across these examples:

  • a loop which raised a user-defined exception to skip to the END LOOP; point when the data was in certain ststes, thus avoiding large chunk of processing. A GOTO would have have been cleaner, because it is poor practice to represent normal business states as exceptions.
  • a huge function with more than a dozen separate RETURN statements. GOTOs directing flow to a single RETURN call would have been really helpful, because I needed to log the returned value.
  • a condition which set a counter variable to a large number so as to short-circuit a loop. Here a GOTO would simply have been more honest.
These examples are all explicit control transfers: they cause exactly the sort of random paths through the code which Dijkstra inveighed against. But the coders didn't honour the principle underlying his fatwa, they just lacked the moxie to invoke the dread statement. Instead they kludged. I'm not saying that using a GOTO would have redeemed a function with 800 LOC ; clearly there'e a lot more refactoring to be done there. But it would have been better.

Here is a situation I have come across a few times. The spec is to implement a series of searches of increasing coarseness, depending on which arguments are passed; the users want the most focused set of records available, so once a specific search gets some hits we don't need to run the more general searches.

Nested IF statements provide one way to do this:

    result_set := sieve_1(p1=>var1, p2=>var2, p3=>var4, p4=>var5);

    if result_set.count() = 0
    then
        result_set := sieve_2(p1=>var2, p2=>var3, p3=>var4);

        if result_set.count() = 0
        then
            result_set := sieve_3(p1=>var3, p2=>var5);

            if result_set.count() = 0
            then
                ....
            end if;
        end if;
    end if;      
    return result_set;     
Obviously as the number of distinct searches increases the nested indentation drives the code towards the right-hand side of the page. Here is an alternative implementation which breaks the taboo and does away with the tabs.
    result_set := sieve_1(p1=>var1, p2=>var2, p3=>var4, p4=>var5);
    if result_set.count() > 0
    then
        goto return_point;
    end if;
    
    result_set := sieve_2(p1=>var2, p2=>var3, p3=>var4);
    if result_set.count() > 0
    then
        goto return_point;
    end if;

    result_set := sieve_3(p1=>var3, p2=>var5);
    if result_set.count() > 0
    then
        goto return_point;
    end if;
    ...
    << return_point >>
    return result_set;     
I think the second version has a clearer expression of intent. Did we find any records? Yes we did, job's a good'un, let's crack on.

tl;dr
GOTO: not as evil as triggers.

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