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:
More on those points.
- 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
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
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.
- 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.
- 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
Or at least that is the best theory I can come up with
at the moment.
- 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
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.
- 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
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.
- It is a bad idea to make the programmer have to
be explicit about where exactly the error should be
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.
- 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:
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.
- 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
- The library (which is complex enough to be spread
across several packages) is not object-oriented.
- 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
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. :-)