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(tye)Re3: Supersplit

by tye (Sage)
on Jan 04, 2001 at 12:05 UTC ( #49718=note: print w/replies, xml ) Need Help??


in reply to Re (tilly) 4: Supersplit
in thread Supersplit

I prefer to write good error messages and then depend on them. You prefer to use the debugger.

...over writing good error messages? No. And I never said that. That is quite a big jump you've made ): I just said that I prefer using a debugger over adding temporary trace.

But I did figure out some of our disagreement. I'm really only talking about Carp::croak(), which I feel was meant for what I call "precondition failures". In many other languages, the simple preconditions for a function are that you specify the right number of arguments and that they each have the correct type. These are checked at compile time.

Perl doesn't offer such compile-time preconditions most of the time (rarely you can reasonably use prototypes for this) so a Perl programmer who includes lots of error checks is going to generate fatal errors when a function gets the wrong number of arguments, for example.

This type of error is extremely likely to just be a simple coding error in the immediate caller of the function (more on this later). So it makes sense to have Carp::croak() report the error as having come from the immediate caller and including a stack back trace is not likely needed.

From the documentation, this seems to be what Carp::croak() does and what it was intended for. But it appears that the documentation is wrong and Carp::croak() looks at inheritance to decide how far up the call stack to go.

Now, I can see some use for generating warnings that seem to come from the user code and can see using inheritance to figure that out (imperfect but not too unreasonably so)1. But a fatal error is a different matter.

For the module to try to kill the script, there must be a serious problem. These usually amount to precondition failures or assertion failures. Simple precondition failures can be pinned on the immediate caller (modulo wrappers to be discussed shortly) because the source of the error is almost always right there. Complex preconditions should include a stack trace (which can skip the current function context if you like). Assertion failures should also include a stack trace. That is because we may suspect that the user code is at fault but we shouldn't delete useful information about the state of the module even though we don't think that this is the source of the problem. If we know that the user code is at fault, then we should be returning a failure indication (or throwing an exception), but not trying to kill the script.

I have this tiny suspicion that Carp::croak() used to work this way but then had patches applied that didn't include documentation changes. But that is just a tiny hunch.

Now, Perl doesn't have a preprocessor so you often end up with simple wrappers in your module (that don't check their arguments before passing them on) so the simple preconditions might be due to code more than one level up the call stack. Since a module should be tested before it is released, there shouldn't be any precondition failures that are due to code in the same module. So it makes sense to bubble up within the module.

Now, for some huge packages of modules like LWP or CGI, there are lots of modules developed together. Well, CGI never croak()s. LWP croaks a whole 6 times. Either force it to specify that each or all of its calls to Carp::croak() should skip more than one package or have Carp.pm use inheritance along with the fact that all of the package names fall under the same hierarchy. The few big packages of modules shouldn't be setting the behavior for Carp.pm when it is easy (like it is) to find cases that this behavior breaks for.

I didn't look at LWP long enough to see whether any of the croak()s could have originated from user code and passed through a different module/package of LWP (thus requiring an @ISA scan to find the user code and the proper place to report the error). But I did notice that several of the error messages would be pretty confusing if this did happen.

In some ways, I don't think what Carp.pm is doing is horribly wrong. But it is starting to sound to me like you are "fixing" Carp.pm without a design document and that is part of our problem.

So Exporter complaining about %EXPORT_TAGS being set up incorrectly should be throwing a precondition failure which shouldn't (by default) look at (just) inheritance. "Fixing" the problem by lying about your inheritance during a window of time is what sounds like "playing games" to me.

What you keep calling "playing cute games" is simply initializing variables close to when you declare them and before you might use them. This is often easy in Perl. But for "static" variables, doing so currently often requires the very simple step of putting the initialization into a BEGIN block. Nothing fancy nor tricky nor in the slightest way "cute". (:

And in this case in particular you will guaranteed get wrong behaviour from both old and current versions of Perl

Nope. Never have and probably never will. I'd only get wrong behavior if I'd also made stupid mistakes about setting up %EXPORT_TAGS or such. And then I'd have to do that where I have multiple packages inheriting from each other. Yep, I pretty much guarentee that I won't ever run into that. Even if I did, I'd find those during module testing where the error being reported in the wrong place is pretty minor.

Not initializing statics at compile time has bitten me several times, for different reasons each time. The only specific cause that I recall is circular module dependancy which isn't something you run into often but then that only accounts for one of my bites.

I've seen this advocated by several people, so I'm sure they've been bitten by it too. I'm not telling you to trust their decision about whether or not to do this, I'm telling you that several people have been bitten by it enough that they decided to change. You have one case of the reverse that I know I don't care about. I have one specific case that I suspect you don't care about. But there are quite a few cases out there on my side.

Plus, your case really seems to me to be a case of two wrongs making a right. Carp.pm gets the inheritance tree wrong which prevents its other bug from firing. I consider it evidence that you should initialize statics in BEGIN blocks, because look at all that can happen during that window!

Well you know my opinion about the stupid misuses of $Carp::CarpLevel in the past. First of all when proper coding depends on programmers always getting things right, that is guaranteed to go wrong often.

Um, so if programmers have to get things right then sometimes they won't and then things will go wrong. Is that a tautology? Yeah, $Carp::CarpLevel was a terrible design choice. But having a per-module equivalent of $Carp::CarpLevel as well as a perl-call equivalent would be useful and fairly easy to do. And if it designed well and documented, then programmers will very often get it right. Somehow you think that you setting the one true behavior, that you will get it right more often than programmers will? Only if the programmers are given a horrid, undocumented way to do it. Don't let how bad $Carp::CarpLevel was prevent you from designing and implementing a good solution.

Now, you can certainly choose to let this one obscure bug prevent you from coding defensively when it comes to uninitialized static variables; but I think that would be a mistake. :)

1 I'm not a big fan of warnings from modules. Warnings from a module can be quite useful to the script writer (aka module user) but are usually delivered to the script user (non-programmer). The script writer can trap them with $SIG{__WARN__}, for example, but that will also catch warnings that are meant for the script user. So if I want to send a programmer warning from a module, I'll provide a delivery mechanism other than warn.

        - tye (but my friends call me "Tye")

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Re (tilly) 5: Supersplit
by tilly (Archbishop) on Jan 07, 2001 at 23:30 UTC
    Sorry about taking so long to get back on this. The discussion is rather long and energy is limited.

    First of all CGI is a poor example. It is used in a large number of environments and there is no well-defined way to report an error. That makes reporting errors hard. (Indeed the lack of a universal way to report errors in CGI scripts is one of the reasons why the web is such a painful application platform.)

    LWP I would need to look at in more detail. But note that with older versions of Carp the reliance on inheritance was seriously buggy. So even if someone wanted to use the kind of behaviour I defined it was unusable. But the beginning hack that direction goes back quite a few years. I don't know what was originally intended, but it has incorporated (admittedly only half-way) those ideas since at least 5.003.

    But I do have a big disagreement with you, which probably has a lot to do with the different kinds of things we do. I do a lot of batch-processing. There you almost always want to do all of your work in some temporary area, and then when it is all done roll it into production areas. In essence you open a "transaction", do your work, and then "commit" it.

    In that environment you want virtually every error to be fatal. If anything went wrong in processing, don't mess up your production data. Instead die a horrible flaming (but informative) death and leave your current state lying in place so that in the morning a test case can be quickly constructed without redoing an hour or two of processing.

    Therefore I issue few non-fatal warnings. Instead I terminate with prejudice.

    Now your example of a BEGIN issue with circular requires bugs me. If you start using circular inheritance in Perl you will get a lot of problems very, very fast. That is a Don't do that. You are asking for trouble. Therefore rather than trying to figure out how to dodge a few bullets I would suggest rethinking your design to not want to be doing something so inherently fraught with risk.

    Without seeing specific examples of what else is supposed to be wrong, I don't know whether to discount them as well. But considering that I have never been bitten, and I am not seeing examples which would bite me, I will continue to be unconvinced on that issue.

    YMMV. (And in fact apparently does.)

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