[32219] in Perl-Users-Digest
Perl-Users Digest, Issue: 3484 Volume: 11
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Tue Aug 30 06:09:25 2011
Date: Tue, 30 Aug 2011 03:09:09 -0700 (PDT)
From: Perl-Users Digest <Perl-Users-Request@ruby.OCE.ORST.EDU>
To: Perl-Users@ruby.OCE.ORST.EDU (Perl-Users Digest)
Perl-Users Digest Tue, 30 Aug 2011 Volume: 11 Number: 3484
Today's topics:
Re: My code works but is clunky and inelegant; help to <uri@StemSystems.com>
Re: My code works but is clunky and inelegant; help to <rweikusat@mssgmbh.com>
Re: My code works but is clunky and inelegant; help to <hjp-usenet2@hjp.at>
Posting Guidelines for comp.lang.perl.misc ($Revision: tadmc@seesig.invalid
Running untrusted code inside a chroot/iptables jail <ignoramus4738@NOSPAM.4738.invalid>
Re: traversing a hash structure <uri@StemSystems.com>
Re: traversing a hash structure sln@netherlands.com
Re: traversing a hash structure <ela@yantai.org>
Why does sort return undef in scalar context ? <willem@toad.stack.nl>
Re: Why does sort return undef in scalar context ? <tadmc@seesig.invalid>
Re: Why does sort return undef in scalar context ? sln@netherlands.com
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: Mon, 29 Aug 2011 12:07:50 -0400
From: "Uri Guttman" <uri@StemSystems.com>
Subject: Re: My code works but is clunky and inelegant; help to improve it?
Message-Id: <87sjok42w9.fsf@quad.sysarch.com>
>>>>> "HL" == Henry Law <news@lawshouse.org> writes:
HL> So either I'd need to disable warnings every time that subroutine is
HL> used (and in other similar circumstances) or else check each time
HL> using some construct like ( notdef() || '' )
a sub can return a value to be used or a boolean but rarely both. your
sub was returning a boolean and as such shouldn't be interpolating it
into a string. testing undef for truth will not ever issue a warning.
uri
--
Uri Guttman -- uri AT perlhunter DOT com --- http://www.perlhunter.com --
------------ Perl Developer Recruiting and Placement Services -------------
----- Perl Code Review, Architecture, Development, Training, Support -------
------------------------------
Date: Mon, 29 Aug 2011 17:53:01 +0100
From: Rainer Weikusat <rweikusat@mssgmbh.com>
Subject: Re: My code works but is clunky and inelegant; help to improve it?
Message-Id: <87d3fob1n6.fsf@sapphire.mobileactivedefense.com>
Tad McClellan <tadmc@seesig.invalid> writes:
[...]
>>>> return '' unless $level;
>>>
>>> If _close_quotes() is ever called in a list context, then this
>>> failure will return a 1-element list.
>>>
>>> It is more usual to return undef in scalar context and the empty
>>> list in list context. You can do that easily with a bare return
>>
>> I'll remember that in future,
>
> You should always be prepared for your subroutines to
> be called in either context.
>
> Or at the least, complain loudly when it is called in an
> un-supported context.
>
> die "somesub() must be used in scalar context\n" if wantarray;
Even if the code is supposed to be used by random third-parties, this
is a questionable practice: It should be entirely sufficient to
document requirements of this kind and expect users of the code to use
it in the way it is supposed to be used when they expect to happen
what is (according to the documentation) supposed to happen instead of
penalizing those who do in order the 'crash' the code of those who
didn't.
>> but in this case I know exactly where the
>> sub is called
>
> Subroutines are meant to be reused. When it is reused someplace
> other than your "exact place", you could return true when
> the correct value is false!
I'd like to submit this as 'Exhibit A' for my older claim that the
idea to use subroutines in order to break down complex operations into
subsequently more specific and less complex subtasks has, by and
large, been ignored using whatever pretext seemed sensible for that
(the usual suspects would be 'But the functional call overhead !!1'
and 'But his hides the details !!2') ...
Subroutines aren't necessarily meant to be reused.
------------------------------
Date: Tue, 30 Aug 2011 00:13:11 +0200
From: "Peter J. Holzer" <hjp-usenet2@hjp.at>
Subject: Re: My code works but is clunky and inelegant; help to improve it?
Message-Id: <slrnj5o3no.jdt.hjp-usenet2@hrunkner.hjp.at>
On 2011-08-29 02:51, Tad McClellan <tadmc@seesig.invalid> wrote:
> Henry Law <news@lawshouse.org> wrote:
>> On 27/08/11 00:28, Tad McClellan wrote:
>>> Henry Law<news@lawshouse.org> wrote:
>
>>> Angle bracket characters are not meta, so should not be backslashed:
>>
>> Laziness,
>
>
> A Laziness that causes folks to work more,
> is a *false* laziness.
While I agree with this statement in general I disagree that it applies
in this case. The regexp syntax was explicitely designed to make it
possible to make sure that a character is used literally without knowing
whether that particular character is meta:
* An alphanumeric character by itself is never meta.
* A non-alphanumeric character preceded by a backslash is never meta.
So if you can't remember whether < is meta, just put a backslash before
it (see perldoc perlre).
An experienced perl programmer knows this and and therefore doesn't have
to work more. The extra backslash is similar to extra parentheses in an
expression - they may not be necessary, but they do no harm and they may
safe both the author and the reader the burden of having to look up the
exact operator precedence.
>>>> return '' unless $level;
>>>
>>> If _close_quotes() is ever called in a list context, then this
>>> failure will return a 1-element list.
>>>
>>> It is more usual to return undef in scalar context and the empty
>>> list in list context. You can do that easily with a bare return
>>
>> I'll remember that in future,
>
>
> You should always be prepared for your subroutines to
> be called in either context.
>
> Or at the least, complain loudly when it is called in an
> un-supported context.
>
> die "somesub() must be used in scalar context\n" if wantarray;
Very bad idea. The subroutine is supposed to return a string which is
supposed to be printed (eventually). It is very likely that the
subroutine will be called like this:
print _close_quotes($current_level, $new_level);
You don't want the sub to die in this case. You do want the result to be
the same as in
$s = _close_quotes($current_level, $new_level);
print $s;
Your change breaks that.
>> but in this case I know exactly where the sub is called
>
> Subroutines are meant to be reused. When it is reused someplace
> other than your "exact place", you could return true when
> the correct value is false!
The sub isn't supposed to return true or false, it is supposed to return
a string. And in this case the correct value is the empty string. It
would be wrong to return undef.
>> and what it's to return, and a zero-length string makes
>> other code, not shown, simpler.
>
>
> Uh, my suggested change also results in a zero-length string.
>
> When undef is used as a string, it acts like the empty string.
Only if "use warnings 'uninitialized'" is not in effect. Otherwise it
emits a warning. Are you now joining forces with Rainer?
hp
------------------------------
Date: Tue, 30 Aug 2011 02:19:53 -0500
From: tadmc@seesig.invalid
Subject: Posting Guidelines for comp.lang.perl.misc ($Revision: 1.9 $)
Message-Id: <wZydnbL9u7mEEsHTnZ2dnUVZ5oidnZ2d@giganews.com>
Outline
Before posting to comp.lang.perl.misc
Must
- Check the Perl Frequently Asked Questions (FAQ)
- Check the other standard Perl docs (*.pod)
Really Really Should
- Lurk for a while before posting
- Search a Usenet archive
If You Like
- Check Other Resources
Posting to comp.lang.perl.misc
Is there a better place to ask your question?
- Question should be about Perl, not about the application area
How to participate (post) in the clpmisc community
- Carefully choose the contents of your Subject header
- Use an effective followup style
- Speak Perl rather than English, when possible
- Ask perl to help you
- Do not re-type Perl code
- Provide enough information
- Do not provide too much information
- Do not post binaries, HTML, or MIME
Social faux pas to avoid
- Asking a Frequently Asked Question
- Asking a question easily answered by a cursory doc search
- Asking for emailed answers
- Beware of saying "doesn't work"
- Sending a "stealth" Cc copy
Be extra cautious when you get upset
- Count to ten before composing a followup when you are upset
- Count to ten after composing and before posting when you are upset
-----------------------------------------------------------------
Posting Guidelines for comp.lang.perl.misc ($Revision: 1.9 $)
This newsgroup, commonly called clpmisc, is a technical newsgroup
intended to be used for discussion of Perl related issues (except job
postings), whether it be comments or questions.
As you would expect, clpmisc discussions are usually very technical in
nature and there are conventions for conduct in technical newsgroups
going somewhat beyond those in non-technical newsgroups.
The article at:
http://www.catb.org/~esr/faqs/smart-questions.html
describes how to get answers from technical people in general.
This article describes things that you should, and should not, do to
increase your chances of getting an answer to your Perl question. It is
available in POD, HTML and plain text formats at:
http://www.rehabitation.com/clpmisc.shtml
For more information about netiquette in general, see the "Netiquette
Guidelines" at:
http://andrew2.andrew.cmu.edu/rfc/rfc1855.html
A note to newsgroup "regulars":
Do not use these guidelines as a "license to flame" or other
meanness. It is possible that a poster is unaware of things
discussed here. Give them the benefit of the doubt, and just
help them learn how to post, rather than assume that they do
know and are being the "bad kind" of Lazy.
A note about technical terms used here:
In this document, we use words like "must" and "should" as
they're used in technical conversation (such as you will
encounter in this newsgroup). When we say that you *must* do
something, we mean that if you don't do that something, then
it's unlikely that you will benefit much from this group.
We're not bossing you around; we're making the point without
lots of words.
Do *NOT* send email to the maintainer of these guidelines. It will be
discarded unread. The guidelines belong to the newsgroup so all
discussion should appear in the newsgroup. I am just the secretary that
writes down the consensus of the group.
Before posting to comp.lang.perl.misc
Must
This section describes things that you *must* do before posting to
clpmisc, in order to maximize your chances of getting meaningful replies
to your inquiry and to avoid getting flamed for being lazy and trying to
have others do your work.
The perl distribution includes documentation that is copied to your hard
drive when you install perl. Also installed is a program for looking
things up in that (and other) documentation named 'perldoc'.
You should either find out where the docs got installed on your system,
or use perldoc to find them for you. Type "perldoc perldoc" to learn how
to use perldoc itself. Type "perldoc perl" to start reading Perl's
standard documentation.
Check the Perl Frequently Asked Questions (FAQ)
Checking the FAQ before posting is required in Big 8 newsgroups in
general, there is nothing clpmisc-specific about this requirement.
You are expected to do this in nearly all newsgroups.
You can use the "-q" switch with perldoc to do a word search of the
questions in the Perl FAQs.
Check the other standard Perl docs (*.pod)
The perl distribution comes with much more documentation than is
available for most other newsgroups, so in clpmisc you should also
see if you can find an answer in the other (non-FAQ) standard docs
before posting.
It is *not* required, or even expected, that you actually *read* all of
Perl's standard docs, only that you spend a few minutes searching them
before posting.
Try doing a word-search in the standard docs for some words/phrases
taken from your problem statement or from your very carefully worded
"Subject:" header.
Really Really Should
This section describes things that you *really should* do before posting
to clpmisc.
Lurk for a while before posting
This is very important and expected in all newsgroups. Lurking means
to monitor a newsgroup for a period to become familiar with local
customs. Each newsgroup has specific customs and rituals. Knowing
these before you participate will help avoid embarrassing social
situations. Consider yourself to be a foreigner at first!
Search a Usenet archive
There are tens of thousands of Perl programmers. It is very likely
that your question has already been asked (and answered). See if you
can find where it has already been answered.
One such searchable archive is:
http://groups.google.com/advanced_search
If You Like
This section describes things that you *can* do before posting to
clpmisc.
Check Other Resources
You may want to check in books or on web sites to see if you can
find the answer to your question.
But you need to consider the source of such information: there are a
lot of very poor Perl books and web sites, and several good ones
too, of course.
Posting to comp.lang.perl.misc
There can be 200 messages in clpmisc in a single day. Nobody is going to
read every article. They must decide somehow which articles they are
going to read, and which they will skip.
Your post is in competition with 199 other posts. You need to "win"
before a person who can help you will even read your question.
These sections describe how you can help keep your article from being
one of the "skipped" ones.
Is there a better place to ask your question?
Question should be about Perl, not about the application area
It can be difficult to separate out where your problem really is,
but you should make a conscious effort to post to the most
applicable newsgroup. That is, after all, where you are the most
likely to find the people who know how to answer your question.
Being able to "partition" a problem is an essential skill for
effectively troubleshooting programming problems. If you don't get
that right, you end up looking for answers in the wrong places.
It should be understood that you may not know that the root of your
problem is not Perl-related (the two most frequent ones are CGI and
Operating System related), so off-topic postings will happen from
time to time. Be gracious when someone helps you find a better place
to ask your question by pointing you to a more applicable newsgroup.
How to participate (post) in the clpmisc community
Carefully choose the contents of your Subject header
You have 40 precious characters of Subject to win out and be one of
the posts that gets read. Don't waste them. Take care while
composing them, they are the key that opens the door to getting an
answer.
Spend them indicating what aspect of Perl others will find if they
should decide to read your article.
Do not spend them indicating "experience level" (guru, newbie...).
Do not spend them pleading (please read, urgent, help!...).
Do not spend them on non-Subjects (Perl question, one-word
Subject...)
For more information on choosing a Subject see "Choosing Good
Subject Lines":
http://www.cpan.org/authors/id/D/DM/DMR/subjects.post
Part of the beauty of newsgroup dynamics, is that you can contribute
to the community with your very first post! If your choice of
Subject leads a fellow Perler to find the thread you are starting,
then even asking a question helps us all.
Use an effective followup style
When composing a followup, quote only enough text to establish the
context for the comments that you will add. Always indicate who
wrote the quoted material. Never quote an entire article. Never
quote a .signature (unless that is what you are commenting on).
Intersperse your comments *following* each section of quoted text to
which they relate. Unappreciated followup styles are referred to as
"top-posting", "Jeopardy" (because the answer comes before the
question), or "TOFU" (Text Over, Fullquote Under).
Reversing the chronology of the dialog makes it much harder to
understand (some folks won't even read it if written in that style).
For more information on quoting style, see:
http://web.presby.edu/~nnqadmin/nnq/nquote.html
Speak Perl rather than English, when possible
Perl is much more precise than natural language. Saying it in Perl
instead will avoid misunderstanding your question or problem.
Do not say: I have variable with "foo\tbar" in it.
Instead say: I have $var = "foo\tbar", or I have $var = 'foo\tbar',
or I have $var = <DATA> (and show the data line).
Ask perl to help you
You can ask perl itself to help you find common programming mistakes
by doing two things: enable warnings (perldoc warnings) and enable
"strict"ures (perldoc strict).
You should not bother the hundreds/thousands of readers of the
newsgroup without first seeing if a machine can help you find your
problem. It is demeaning to be asked to do the work of a machine. It
will annoy the readers of your article.
You can look up any of the messages that perl might issue to find
out what the message means and how to resolve the potential mistake
(perldoc perldiag). If you would like perl to look them up for you,
you can put "use diagnostics;" near the top of your program.
Do not re-type Perl code
Use copy/paste or your editor's "import" function rather than
attempting to type in your code. If you make a typo you will get
followups about your typos instead of about the question you are
trying to get answered.
Provide enough information
If you do the things in this item, you will have an Extremely Good
chance of getting people to try and help you with your problem!
These features are a really big bonus toward your question winning
out over all of the other posts that you are competing with.
First make a short (less than 20-30 lines) and *complete* program
that illustrates the problem you are having. People should be able
to run your program by copy/pasting the code from your article. (You
will find that doing this step very often reveals your problem
directly. Leading to an answer much more quickly and reliably than
posting to Usenet.)
Describe *precisely* the input to your program. Also provide example
input data for your program. If you need to show file input, use the
__DATA__ token (perldata.pod) to provide the file contents inside of
your Perl program.
Show the output (including the verbatim text of any messages) of
your program.
Describe how you want the output to be different from what you are
getting.
If you have no idea at all of how to code up your situation, be sure
to at least describe the 2 things that you *do* know: input and
desired output.
Do not provide too much information
Do not just post your entire program for debugging. Most especially
do not post someone *else's* entire program.
Do not post binaries, HTML, or MIME
clpmisc is a text only newsgroup. If you have images or binaries
that explain your question, put them in a publically accessible
place (like a Web server) and provide a pointer to that location. If
you include code, cut and paste it directly in the message body.
Don't attach anything to the message. Don't post vcards or HTML.
Many people (and even some Usenet servers) will automatically filter
out such messages. Many people will not be able to easily read your
post. Plain text is something everyone can read.
Social faux pas to avoid
The first two below are symptoms of lots of FAQ asking here in clpmisc.
It happens so often that folks will assume that it is happening yet
again. If you have looked but not found, or found but didn't understand
the docs, say so in your article.
Asking a Frequently Asked Question
It should be understood that you may have missed the applicable FAQ
when you checked, which is not a big deal. But if the Frequently
Asked Question is worded similar to your question, folks will assume
that you did not look at all. Don't become indignant at pointers to
the FAQ, particularly if it solves your problem.
Asking a question easily answered by a cursory doc search
If folks think you have not even tried the obvious step of reading
the docs applicable to your problem, they are likely to become
annoyed.
If you are flamed for not checking when you *did* check, then just
shrug it off (and take the answer that you got).
Asking for emailed answers
Emailed answers benefit one person. Posted answers benefit the
entire community. If folks can take the time to answer your
question, then you can take the time to go get the answer in the
same place where you asked the question.
It is OK to ask for a *copy* of the answer to be emailed, but many
will ignore such requests anyway. If you munge your address, you
should never expect (or ask) to get email in response to a Usenet
post.
Ask the question here, get the answer here (maybe).
Beware of saying "doesn't work"
This is a "red flag" phrase. If you find yourself writing that,
pause and see if you can't describe what is not working without
saying "doesn't work". That is, describe how it is not what you
want.
Sending a "stealth" Cc copy
A "stealth Cc" is when you both email and post a reply without
indicating *in the body* that you are doing so.
Be extra cautious when you get upset
Count to ten before composing a followup when you are upset
This is recommended in all Usenet newsgroups. Here in clpmisc, most
flaming sub-threads are not about any feature of Perl at all! They
are most often for what was seen as a breach of netiquette. If you
have lurked for a bit, then you will know what is expected and won't
make such posts in the first place.
But if you get upset, wait a while before writing your followup. I
recommend waiting at least 30 minutes.
Count to ten after composing and before posting when you are upset
After you have written your followup, wait *another* 30 minutes
before committing yourself by posting it. You cannot take it back
once it has been said.
AUTHOR
Tad McClellan and many others on the comp.lang.perl.misc newsgroup.
--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.liamg\100cm.j.dat/"
The above message is a Usenet post.
I don't recall having given anyone permission to use it on a Web site.
------------------------------
Date: Mon, 29 Aug 2011 10:56:28 -0500
From: Ignoramus4738 <ignoramus4738@NOSPAM.4738.invalid>
Subject: Running untrusted code inside a chroot/iptables jail
Message-Id: <LKydnXfq5rwxK8bTnZ2dnUVZ_tCdnZ2d@giganews.com>
I have a system where users can write untrusted code in perl (algebra
calculators for algebra.com).
They used to be able to call one another, which, I thought, was
cool.
I used to use the Safe.pm module to run them. Unfortunately, something
changed in Safe.pm and I can no longer have those untrusted pieces of
code call each other, as Safe.pm refuses to do so and says "require
trapped".
This leads to solvers not working and users leaving those nonworking
pages.
I want to re-architect the whole thing and go away from Safe.pm
entirely.
My plan is to do the following:
1) Run a calculation daemon as user "algebracalc".
2) It would listen on localhost only, to queries from my mod_perl
(CGI) scripts.
3) It would load all required perl modules and open a MySQL connection
with a MySQL user 'algebracalc", which only has a SELECT privilege on
only the one table that it needs (table with source code of said
calculators).
The untrusted code will have access to this MySQL handle, though not
directly through any variables, but it will inherit it and it will be
accessible.
4) Upon receiving an internal connection, the daemon would
a) Fork
b)_Chroot to a "chroot jail". So, the code will be executed in a
chroot jail.
5) Using iptables, user "algebracalc" will NOT be allowed to make any
network connections, to localhost or any other IP address, or to use UDP.
6) Code will have some ulimits set to not abuse my memory or CPU.
Items 1-6 ensure, in my mind, that the unauthorized code will not be
able to abuse my system in any way, such as by sending spams,
accessing unauthorized files, etc.
What have I missed?
Thanks
i
------------------------------
Date: Mon, 29 Aug 2011 12:08:30 -0400
From: "Uri Guttman" <uri@StemSystems.com>
Subject: Re: traversing a hash structure
Message-Id: <87obz842v5.fsf@quad.sysarch.com>
>>>>> "HL" == Henry Law <news@lawshouse.org> writes:
HL> On 29/08/11 05:01, ela wrote:
>> For a market basket analysis, I'd like to traverse a hash structure
>> containing entries like:
>>
>> my %list;
>> $list{"c1"} = "milk%cheese%coca cola";
HL> Just by the way, is this real code? Surely Perl would have tried to
HL> parse %cheese etc as the names of hash variables? Confusing at best.
confusing maybe but hashes don't interpolate in strings.
uri
--
Uri Guttman -- uri AT perlhunter DOT com --- http://www.perlhunter.com --
------------ Perl Developer Recruiting and Placement Services -------------
----- Perl Code Review, Architecture, Development, Training, Support -------
------------------------------
Date: Mon, 29 Aug 2011 12:07:25 -0700
From: sln@netherlands.com
Subject: Re: traversing a hash structure
Message-Id: <ocon57l2lqrau3mvnvrfkj2ghoegg87i3k@4ax.com>
On Mon, 29 Aug 2011 12:01:40 +0800, "ela" <ela@yantai.org> wrote:
>For a market basket analysis, I'd like to traverse a hash structure
>containing entries like:
>
>my %list;
>$list{"c1"} = "milk%cheese%coca cola";
>$list{"c2"} = "coca cola";
>$list{"c3"} = "beef";
>...
>$list{"cn-1"} = "apple%cheese";
>$list{"cn"} = "beef%milk";
>
>so the knowledge derived may look like:
>c1 is associated with c2, .., cn-1, cn
>c3 is associated with cn
>...
>
>i.e. the total number of checking is "n + n-1 + ... + 2 + 1" .
>While this can be easily achieved if the structure is array, the following
>looks clumsy... any better implementation?
>
>while (%list) {
> $assign = 1;
> foreach $key (keys %list) {
> if ($assign == 1) {
> $head = $key;
> $assign = 0;
> next;
> }
> if ($list{$head} =~ /$list{$key}/ or $list{$key} =~
>/$list{$head}/ ) {
> print "$list{$head}\t$list{$key}\t1\n";
> }
> }
> undef $list{$head};
>}
>
Its still being treated as a list in the foreach loop.
In your current implementation, the %list keys never
change so you sit in the while() loop forever, and
$head is assigned the same $key every time, doing the
same operation indefinitely and generating uninitialized
value warnings in the regex.
change this:
> undef $list{$head};
to this:
delete $list{$head};
As a final note, the target of a regular expression
doesen't transparently translate into a regular expression itself.
-sln
------------------------------
Date: Tue, 30 Aug 2011 09:05:13 +0800
From: "ela" <ela@yantai.org>
Subject: Re: traversing a hash structure
Message-Id: <j3hd09$qo0$1@ijustice.itsc.cuhk.edu.hk>
<sln@netherlands.com> wrote in message
news:ocon57l2lqrau3mvnvrfkj2ghoegg87i3k@4ax.com...
> Its still being treated as a list in the foreach loop.
> In your current implementation, the %list keys never
> change so you sit in the while() loop forever, and
> $head is assigned the same $key every time, doing the
> same operation indefinitely and generating uninitialized
> value warnings in the regex.
>
> change this:
>> undef $list{$head};
>
> to this:
> delete $list{$head};
>
> As a final note, the target of a regular expression
> doesen't transparently translate into a regular expression itself.
>
> -sln
Thanks sln. I hope you have already got your favorite job. Thanks again :-)
------------------------------
Date: Mon, 29 Aug 2011 20:36:26 +0000 (UTC)
From: Willem <willem@toad.stack.nl>
Subject: Why does sort return undef in scalar context ?
Message-Id: <slrnj5nu2a.811.willem@toad.stack.nl>
Today I got bitten by a very strange bug/feature:
I wrote a function that returned a sorted list of things.
This function was used in several places, sometimes to use the things in
some way, and in one place just to check if it returned any things.
For example:
...
for my $s (get_sorted_things($foo)) {
do_something_with($s);
}
...
if (get_sorted_things($bar)) {
warn "There were things for $bar!\n";
}
...
sub get_sorted_things
{
my %unique = map { $_->Key => $_ } get_things();
return sort { $a->Property cmp $b->Property } values %unique;
}
And, because somebody decided that sort should always return undef
when called in scalar context, this does not work!
Why was it decided that sort returns undef in scalar context ?
IMO it is much more logical and consistent to have it return the
number of objects in the list that is to be sorted (it doesn't even
need to sort them for that).
SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT
------------------------------
Date: Mon, 29 Aug 2011 17:39:39 -0500
From: Tad McClellan <tadmc@seesig.invalid>
Subject: Re: Why does sort return undef in scalar context ?
Message-Id: <slrnj5o4mv.7ru.tadmc@tadbox.sbcglobal.net>
Willem <willem@toad.stack.nl> wrote:
> Today I got bitten by a very strange bug/feature:
>
> I wrote a function that returned a sorted list of things.
... in list context.
> This function was used in several places, sometimes to use the things in
> some way, and in one place just to check if it returned any things.
So the "one place" is in scalar context.
> For example:
>
> ...
> for my $s (get_sorted_things($foo)) {
Here you pass a $foo argument to get_sorted_things()...
> do_something_with($s);
> }
> ...
> if (get_sorted_things($bar)) {
... and here you pass a $bar argument to get_sorted_things()...
> warn "There were things for $bar!\n";
> }
> ...
> sub get_sorted_things
> {
> my %unique = map { $_->Key => $_ } get_things();
> return sort { $a->Property cmp $b->Property } values %unique;
> }
... but get_sorted_things() does not consult any arguments!
What is the relationship between $foo and $bar and get_things()?
Before I got to the definition of get_sorted_things(),
I had guessed that $bar was an arrayref of things to sort,
if it was then you could just do:
if (@$bar) {
warn "There were things for $bar!\n";
But it isn't. So you can't.
And since I don't know what $bar is, I cannot even attempt to
help you here.
> And, because somebody
Larry Wall.
> decided that sort should always return undef
> when called in scalar context, this does not work!
So fix it!
sub get_sorted_things {
die "get_sorted_things() was called with a useless argument\n" if @_;
my %unique = map { $_->Key => $_ } get_things();
if (wantarry) {
return sort { $a->Property cmp $b->Property } values %unique;
}
else {
return keys %unique;
}
}
> Why was it decided that sort returns undef in scalar context ?
I dunno.
Have to ask Larry, or search for comments in the perl code I guess.
I would want it to return the first element, or the last element,
or the median element, or ... but all of those would O(n log n),
(ie. expensive, there are algorithms for determinig those things that
have a better running time) so then I'd give up and make it
undefined in a scalar context.
I don't know if that was his thinking or not.
> IMO it is much more logical and consistent to have it return the
> number of objects in the list that is to be sorted
If you are writing a call to sort(), then you already know the list
of things that you are going to ask sort() to sort.
Why run it through another function only to find out something
that you already have access to?
> (it doesn't even
> need to sort them for that).
Why would we want to call a function named sort() if we wanted
something that did not even require sorting?
--
Tad McClellan
email: perl -le "print scalar reverse qq/moc.liamg\100cm.j.dat/"
The above message is a Usenet post.
I don't recall having given anyone permission to use it on a Web site.
------------------------------
Date: Mon, 29 Aug 2011 15:44:35 -0700
From: sln@netherlands.com
Subject: Re: Why does sort return undef in scalar context ?
Message-Id: <je5o57t5g4q9kee70et8lnhronbi7ulbfr@4ax.com>
On Mon, 29 Aug 2011 20:36:26 +0000 (UTC), Willem <willem@toad.stack.nl> wrote:
>Today I got bitten by a very strange bug/feature:
>
>I wrote a function that returned a sorted list of things.
>This function was used in several places, sometimes to use the things in
>some way, and in one place just to check if it returned any things.
>
>For example:
>
>...
>for my $s (get_sorted_things($foo)) {
> do_something_with($s);
>}
>...
>if (get_sorted_things($bar)) {
> warn "There were things for $bar!\n";
>}
>...
>sub get_sorted_things
>{
> my %unique = map { $_->Key => $_ } get_things();
> return sort { $a->Property cmp $b->Property } values %unique;
>}
>
>And, because somebody decided that sort should always return undef
>when called in scalar context, this does not work!
>
>Why was it decided that sort returns undef in scalar context ?
>IMO it is much more logical and consistent to have it return the
>number of objects in the list that is to be sorted (it doesn't even
> need to sort them for that).
>
>
I don't know why this is the case, but you should fix it up in the
function that wraps the return of sort().
$ret = get_sort_stuff('aa'=>8, 'bb' => 1 );
print "a_ret = $ret\n\n";
@ret = get_sort_stuff('aa'=>8, 'bb' => 1 );
print "b_ret = @ret\n\n";
$ret = get_sort_stuff();
print "c_ret = $ret\n\n";
@ret = get_sort_stuff();
print "d_ret = @ret\n\n";
if (get_sort_stuff('aa'=>8, 'bb' => 1 )) {
print "conditional > 0\n\n"
}
if (!get_sort_stuff()) {
print "conditional = 0\n"
}
sub get_sort_stuff {
my %unique = @_;
print wantarray ? 'wantarray = 1' : 'wantarray = 0', "\n";
return wantarray ?
sort values %unique :
values %unique ;
}
-sln
------------------------------
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Subject: Digest Administrivia (Last modified: 6 Apr 01)
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