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I think we are agreed on the qr// idea. I just wanted to mention that it was possible, not advocate its use.

So now on to the more substantial BEGIN issue.

Having read and thought about what you wrote, the following points summarize my current position fairly well:

  1. I form my own opinions. Telling me what other people have been convinced of is a waste of time. Give me examples and reasons please.
  2. Complaints about poorly written error messages don't go very far with me.
  3. Our differing views about debuggers explain to some extent our disagreements on error messages.
  4. There are valid reasons for having a standard way to have error messages reported from elsewhere. There needs to be some flexibility in what that elsewhere is.
  5. It is a bad idea to make wrappers know a lot about what they are wrapping or vice versa.
  6. It is a bad idea to make the programmer have to be explicit about where exactly the error should be reported from.
  7. Inheritance is the wrong handle to use for deciding where errors get reported. But it is at least a relative of the right concepts.
  8. I still think that having user code play cute games with the internal definitions of what happens during run-time and compilation is a Bad Idea.

More on those points.

  1. I form my own opinions. Telling me what other people have been convinced of is a waste of time. Give me examples and reasons please.

    This is how I have always been. The fact that other people think something doesn't tell me why I should think that. Perhaps when I think about it I will come to a different conclusion. More importantly, knowing why I believe what I do is more important than having some rote rules to learn from.

    For instance Perl 5.6 introduces the idea of 'our'. A lot of people - including many who I know are better at both Perl and programming in general than I am, think that it is a great idea. However that didn't stop me from thinking about it and deciding to respectfully disagree.

    Now perhaps it is a great idea. If so then by forming my own opinion and putting it forth I get to find out something about how I don't understand programming in a useful way. The opportunity for that is more important to me than having a stupid opinion from time to time. And not doing that leads to Cargo Cult thinking.

    So rather than tell me that some example out there convinced some people to have the official documentation give me the examples. Besides which I note that the current version of Perl (ie what I see in 5.7 snapshots) does not recommend using BEGIN blocks. Or suggest base. So not everyone can have been convinced.

  2. Complaints about poorly written error messages don't go very far with me.

    This should be a, "Well, duh" item. In Perl there are many different options for reporting an error. You can die. You can warn. From Carp you can choose to carp, croak, cluck, and confess. The only one of these that I have not used in production code is cluck. Each one makes sense in different places, and what message you should write depends strongly on which one you are using.

    Complaining that people don't know how to write useful error messages sounds to me like a user education issue. If you don't know how to write an informative message then it doesn't matter what function you use, your errors will be uninformative at best and seriously misleading at worst. So learn to write better messages.

    For the record from most to least frequently I use confess, croak, die, warn, carp, and cluck. When I am writing stuff that others will use I move croak and carp up in that list.

  3. Our differing views about debuggers explain to some extent our disagreements on error messages.

    I was trying to figure out why we disagree. And this is the best explanation I came up with.

    As we discussed in Are debuggers good?, I prefer to write good error messages and then depend on them. You prefer to use the debugger. This difference in philosophy affects our view of error messages. If the error message gives a good place to start in the debugger, it is likely useful for you. I want it to give me me much more than that. I want it to finger a piece of code and give me information on why that code in particular is likely to be the cause of the problem.

    As a result I demand far more out of my error messages than you do. So I put (and expect others to put) far more energy into what is reported where and when. And into deciding what is reported how. Which leads to an interesting cycle. Because I put that energy in, I constantly find that errors are pinpointing my exact problem in a useful way, and I do not have the desire to use the debugger. The same happens in reverse for you.

    Or at least that is the best theory I can come up with at the moment.

  4. There are valid reasons for having a standard way to have error messages reported from elsewhere. There needs to be some flexibility in what that elsewhere is.

    There is a world of difference between where an error is found and what the most useful place to report it from is. What both croak and carp are for is having a library report an error from the context of a user. If the library does this then it is up to the library to provide enough context about the error to make the report useful. (The lazy way to provide context is, of course, to just confess to your sins.)

    The reason for this is that whoever is maintaining the user-code should not need to know anything about the internals of the library to figure out the code. And a real-world useful library is unlikely to just belong in one package. For instance look at LWP. So if you encounter a problem (eg you cannot resolve the site you are trying to connect to) that has nothing to do with your library, deciding where to report it by reporting the error as soon as you get out of your current package is likely to be useless.

  5. It is a bad idea to make wrappers know a lot about what they are wrapping or vice versa.

    Very often people want to wrap something else. For instance XS wrappers to allow people in Perl to use C libraries, the DBI wrapper to give a wide variety of DBD drivers a common Perl interface, so on and so forth. This is both a common thing to do and often a good design idea. This should be encouraged, not discouraged.

    When you insist that the wrapper should know every possible thing that can go wrong inside the thing wrapped and handle all of them, you have just made wrapping a much more complex thing to do, and are discouraging wrappers. If I wrap a socket connection with an interface that makes ftp requests transparent, and then someone uses my code, there is no reason on Earth that I should have to do my own tests that the connection can be made or won't break. Not only is there no good way for me to do that, but I am reusing code that (hopefully) already does that for me - why should I reinvent the wheel?

    Conversely the thing wrapped shouldn't need to care that it is wrapped. After all you have an interface into it, not vica versa. If it is written well it should not be making assumptions about how it is being used. If it is making such assumptions then it is preventing itself from being easily reused.

  6. It is a bad idea to make the programmer have to be explicit about where exactly the error should be reported from.

    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. For instance the most common security whole in every year of the last ten has remained the basic buffer overflow. It isn't hard. Don't put more into a data structure than it has room for. But people simply cannot get it right. (But the ever growing popularity of languages like Perl that do dynamic allocation and so prevent buffer overflows may someday change this.)

    Well the one thing I don't want to see messed up on a regular basis is error reporting. OK, you have to ask for errors to be reported. But if you require people to also have to synchronize information correctly, they will get that wrong. Guaranteed. They will do things like hard-code the level. (Which Carp and perldb.pl both do with $Carp::CarpLevel.) Or they will write their own nifty functions to figure out where they think the error should come from. (warnings.pm does this.) And they will get it wrong. Do you want proof? Just look at how regularly the Perl core has messed up on this given the opportunity! And the Perl core is supposedly filled with competent people!

    In particular woe betide the poor joe who wants to wrap a library that for no particular reason chooses to be special and do it differently!

    Software design involves a constant balancing act between what complexity needs to be exposed in your interface, and what interface aspects are going to be the cause of ongoing problems. In this case my strong opinion is that asking for libraries to know enough about their environment and how they are used that they can correctly decide where it is reported from is just begging for a nightmare.

  7. Inheritance is the wrong handle to use for deciding where errors get reported. But it is at least a relative of the right concepts.

    I am not a mind reader. I am not even positive who wrote the original Carp. However I had to think through a lot of the decisions made, and while I have to say that the original was a hack, it was a hack that caught a lot of important ideas.

    The use of inheritance in Carp captures the following insight. It should not matter whether a method was inherited or overridden. That is an implementation detail. The rewrite takes that basic insight and makes its application more consistent. But it is still retained. If it matters to you whether a given method was over-ridden or inherited, then perhaps inheritance is not what you are looking for.

    As you rightly point out, this does not generally hold. Here are several classes of issues:

    • The module (like HTML::ParseTree) expects you to inherit and subclass from it.
    • You have a has-a relationship rather than is-a. (This is very common in wrappers, and we both agree that using has-a is usually cleaner than relying on is-a.)
    • The library (which is complex enough to be spread across several packages) is not object-oriented.
    I claim it as a bug in Carp that it does not have a more flexible declaration mechanism for handling these cases. I don't think that coming up with one is by any means impossible. However when dealing with reasonably mature packages (eg stuff off of CPAN) it will often turn out that if package A inherits from package B then they shouldn't really be reporting errors in each other.

  8. I still think that having user code play cute games with the internal definitions of what happens during run-time and compilation is generally unwarranted, and for this case in particular is a bad idea.

    This is where we started. I am personally a fan of trying to KISS. Have what people are likely to do pretty much by default work well. If you can, make it work right to have people consistently do the simple thing. If you can't then make it a big deal.

    Now I have no idea that if code that doesn't play cute BEGIN games relies on code that does, you can get all sorts of fun games. But my answer to that is to not play cute games in the first place, not to play more.

    And in this case in particular you will guaranteed get wrong behaviour from both old and current versions of Perl (as well as versions likely to be released in the near future) by playing games with BEGIN. So Don't Do That. And if you do do that and get hurt, Don't Complain.

OK, so this is far, far more than I intended to write, but it hopefully is interesting. I don't think that the concept of Carp is broken. I am emphatically unconvinced that BEGIN games are justified. And I have just said far more on that than I should. :-)


In reply to Re (tilly) 4: Supersplit by tilly
in thread Supersplit by jeroenes

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