[28105] in Perl-Users-Digest
Perl-Users Digest, Issue: 9469 Volume: 10
daemon@ATHENA.MIT.EDU (Perl-Users Digest)
Fri Jul 14 06:05:36 2006
Date: Fri, 14 Jul 2006 03:05:04 -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 Fri, 14 Jul 2006 Volume: 10 Number: 9469
Today's topics:
About iServicePro david@iservicepro.com
Posting Guidelines for comp.lang.perl.misc ($Revision: tadmc@augustmail.com
problem in loading perl plugin into Eclipse <appumail@gmail.com>
Re: RegEx and AND anno4000@radom.zrz.tu-berlin.de
Re: What is a type error? <gneuner2/@comcast.net>
Re: What is a type error? <marshall.spight@gmail.com>
Re: What is a type error? <marshall.spight@gmail.com>
Re: What is a type error? <jo@durchholz.org>
Re: What is a type error? <rossberg@ps.uni-sb.de>
Re: What is a type error? <rossberg@ps.uni-sb.de>
Re: What is a type error? <jo@durchholz.org>
Digest Administrivia (Last modified: 6 Apr 01) (Perl-Users-Digest Admin)
----------------------------------------------------------------------
Date: 14 Jul 2006 00:48:20 -0700
From: david@iservicepro.com
Subject: About iServicePro
Message-Id: <1152863300.796305.70430@s13g2000cwa.googlegroups.com>
iServicePro is an innovative approach to offering affordable tools for
Java and J2EE developers. in particular, we are a professional team for
applying oneself to develop and integrate eclipse plug-in for user
applications.
Our mission now is to deliver business value and to maximize
developers' productivity by consistently delivering reliable products
and a true end-to-end seamless J2EE development environment.
iServicePro is the eclipse plug-in solution for all your UML, Web,
J2EE, JSP, XML, Struts, JSF, Hibernate and application server
integration needs.
We are committed to the success of your trial. If you need to see a
demonstration of Data Service or need assistance during the trial
process, please do not hesitate to contact us.
www.iservicepro.com
------------------------------
Date: 14 Jul 2006 07:22:14 GMT
From: tadmc@augustmail.com
Subject: Posting Guidelines for comp.lang.perl.misc ($Revision: 1.6 $)
Message-Id: <44b74625$0$57728$ae4e5890@news.nationwide.net>
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: 1.6 $)
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.augustmail.com/~tadmc/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
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_group_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 <tadmc@augustmail.com> and many others on the
comp.lang.perl.misc newsgroup.
------------------------------
Date: Fri, 14 Jul 2006 15:02:10 +0530
From: Murugesh <appumail@gmail.com>
Subject: problem in loading perl plugin into Eclipse
Message-Id: <DOJtg.11$uB2.32@news.oracle.com>
Hi,
Could any tried loading perl plugin from http://e-p-i-c.sourceforge.net/
or, could any one give pointers to Perl plugin for Eclipse.
TIA,
Murugesh
http://appumail.googlepages.com
------------------------------
Date: 14 Jul 2006 09:11:12 GMT
From: anno4000@radom.zrz.tu-berlin.de
Subject: Re: RegEx and AND
Message-Id: <4hp5dgFk8ljU1@news.dfncis.de>
Brian Wakem <no@email.com> wrote in comp.lang.perl.misc:
> tinu wrote:
>
> > Hi All
> >
> > I need to do a simple RegEx expression for and AND. I dont know how to
> > do this.
> > I have the following string:
> > "The tree is taller then the flower".
> >
> > I would like to search for "tree" AND "flower". How Do I do this?
> >
> > Thanks for your help
>
>
> No need to use a regex. index is much more suited and faster.
>
>
> if ( (index($string,'tree') != -1) and (index($string,'flower') != -1) ) {
> ..
> }
It is a good engineering principle to use the simplest tool that does the
job, but other criteria are more important. In this case, readability
takes precedence. The Perl programmer's way of saying "$string contains
'tree' somewhere" is
$string =~ /tree/;
"index($string,'tree') != -1" is not a very intuitive way of saying
the same thing. Boolean "1 + index(...)" is only slightly better.
Speed should only be a secondary concern before a program is up and
running, but the speed difference in index() and m// is marginal
anyway. Indeed, on my machine the regex solution is slightly faster.
Anno
------------------------------
Date: Fri, 14 Jul 2006 03:24:36 -0400
From: George Neuner <gneuner2/@comcast.net>
Subject: Re: What is a type error?
Message-Id: <0ideb2pm854tbmaqf30fnnr7e8jtd9hi0k@4ax.com>
On 13 Jul 2006 08:45:49 -0700, "Marshall" <marshall.spight@gmail.com>
wrote:
>
>On the other hand, there is no problem domain for which pointers
>are a requirement. I agree they are deucedly convenient, though.
>
I would argue that pointers/references _are_ a requirement for I/O. I
know of no workable method for interpreting raw bits as meaningful
data other than to overlay a typed template upon them.
Categorically disallowing address manipulation functionally cripples
the language because an important class of programs (system programs)
cannot be written.
Of course, languages can go overboard the other way too. IMO, C did
not need to provide address arithmetic at the language level,
reinterpretable references and array indexing would have sufficed for
any use. Modula 3's type safe view is an example of getting it right.
It is quite reasonable to say "I don't write _____ so I don't need
[whatever language feature enables writing it]". It is important,
however, to be aware of the limitation and make your choice
deliberately.
George
--
for email reply remove "/" from address
------------------------------
Date: 14 Jul 2006 01:03:16 -0700
From: "Marshall" <marshall.spight@gmail.com>
Subject: Re: What is a type error?
Message-Id: <1152864196.285749.113580@m73g2000cwd.googlegroups.com>
George Neuner wrote:
> On 13 Jul 2006 08:45:49 -0700, "Marshall" <marshall.spight@gmail.com>
> wrote:
> >
> >On the other hand, there is no problem domain for which pointers
> >are a requirement. I agree they are deucedly convenient, though.
> >
>
> I would argue that pointers/references _are_ a requirement for I/O. I
> know of no workable method for interpreting raw bits as meaningful
> data other than to overlay a typed template upon them.
I think I know what you mean. I agree that pointers are necessary
for, e.g., device drivers. So I have to weaken my earlier statement.
> Categorically disallowing address manipulation functionally cripples
> the language because an important class of programs (system programs)
> cannot be written.
That's fair, although I could argue how important systems programming
is these days. (And C/C++ are cock-of-the-walk there anyway.)
> Of course, languages can go overboard the other way too. IMO, C did
> not need to provide address arithmetic at the language level,
> reinterpretable references and array indexing would have sufficed for
> any use. Modula 3's type safe view is an example of getting it right.
>
> It is quite reasonable to say "I don't write _____ so I don't need
> [whatever language feature enables writing it]". It is important,
> however, to be aware of the limitation and make your choice
> deliberately.
Agreed.
Marshall
------------------------------
Date: 14 Jul 2006 01:34:38 -0700
From: "Marshall" <marshall.spight@gmail.com>
Subject: Re: What is a type error?
Message-Id: <1152866078.567124.135170@i42g2000cwa.googlegroups.com>
Chris Smith wrote:
> Marshall <marshall.spight@gmail.com> wrote:
> > > What you are asking for is some subset of identity, and I've not yet
> > > succeeded in understanding exactly what it is or what its limits are...
> > > except that so far, it seems to have everything to do with pointers or
> > > aliasing.
> >
> > Perhaps it is specifically first-class identity, rather than identity
> > per se.
>
> As in: "the value of one variable can (be/refer to/depend on) the
> identity of another variable"? I can certainly see this as as
> reasonable concept to consider.
Yes, I think it's important. We've become so accustomed, in
modern languages, to considering variables as first-class,
that we cannot see any more that many of the problems
attributed to variables (such as aliasing) are actually
problems only with *first-class* variables. The historical
reality that non-first-class variables are associated with
more antique languages (Fortran/pre-OO) makes us shy
away from even considering removing first-class status
from variables. But I think it's an important point in the
design space (although I certainly respect Andreas'
reluctance; to paraphrase, "first class or bust." :-) )
After all, what are the alternatives? Purely-functional
languages remove themselves from a large class of
problems that I consider important: data management.
I have explored the OO path to its bitter end and am
convinced it is not the way. So what is left? Uniqueness
types and logic programming, I suppose. I enjoy logic
programming but it doesn't seem quite right. But notice:
no pointers there! And it doesn't seem to suffer from the
lack. I really don't know enough about uniqueness types
to evaluate them. But again, I already have my proposed
solution: variables but no first-class variables. (And yes,
I'm acutely aware of how problematic that makes
closures. :-()
> > > I'm not yet convinced that this is any different from a language with
> > > standard pointer aliasing. If I have two tables A and B, and a foreign
> > > key from A into B, then I run into the same problems with enforcing
> > > constraints that I would see with a pointer model... when I update a
> > > relation, I need to potentially check every other relation that contains
> > > a foreign key into it, in order to ensure that its constraints are not
> > > violated by that constraint. That's the same thing that is being
> > > pointed out as a negative consequence of aliasing in other languages.
> >
> > No, that's not the same thing. What you are describing here is
> > not an aliasing issue, but simply the consequences of allowing
> > constraints to mention more than one variable.
>
> > A foreign key constraint is a multi-variable constraint.
> > Specifically, a foreign key from table A, attribute a
> > to table B, attribute b is the constraint:
> >
> > forall a in A, exists b in B such that a = b.
> >
> > Note that two variables, A and B, are referenced in
> > the constraint.
>
> There's confusion here coming from different usages of the word
> variable. Let us talk instead of values, and of the abstract structures
> that gives them meaning.
I don't think that will work. We cannot discuss constraints, nor
aliasing, without bringing variables in to the picture. (Aliasing
may exist without variables but it is a non-problem then.)
> In both cases (invariants in a hypothetical
> imperative language, and in a relational database), the constraints make
> reference to these structures of values (relations, for example, or
> various kinds of data structures), and not to the individual values or
> objects that they contain.
I am not certain what this means.
> In both cases, the problem is not that we
> don't know what structures to check to verify the invariant; rather,
> it's that we have to check ALL of the values in that structure.
But not all values in all structures, as you do in the presence
of aliasing.
> As someone pointed out, this is to be expected in a world of mutable
> things with identity that are globally locatable. It is simple fact
> that if I tell you "I spoke to Barbara's husband", you may need to trace
> down who Barbara's husband is before you could discover that, for
> example, maybe I actually spoke to your boss, or to your nephew's best-
> friend's father. If databases are capable of modeling these kinds of
> relationships (and of course they are), then they are as susceptible to
> "aliasing" -- in a logical sense that avoids mention of pointer -- as
> anyone else.
I don't see that they're the same thing. Similar is some ways, yes,
but not the same.
As I said much earlier, the terminology is problematic, and it may
be that we (or just I) simply lack the precision of terms necessary
for the conversation.
Marshall
------------------------------
Date: Fri, 14 Jul 2006 11:16:52 +0200
From: Joachim Durchholz <jo@durchholz.org>
Subject: Re: What is a type error?
Message-Id: <e97nn9$6g0$1@online.de>
Marshall schrieb:
> What about my example of SQL? Mutation, no pointers, no aliasing.
> Yet: useful.
Sorry, but SQL does have aliasing.
E.g. if you have records that have name="John", surname="Doe", the
statements
SELECT * FROM persons WHERE name = "John"
and
SELECT * FROM persons WHERE name = "Doe"
are aliases of each other.
The alias is actually in the WHERE clause. And this *can* get you into
trouble if you have something that does
UPDATE ... WHERE name = "John"
and
UPDATE ... WHERE surname = "Doe"
e.g. doing something with the Johns, then updating the names of all
Does, and finally restoring the Johns (but not recognizing that changing
the names of all Does might have changed your set of Johns).
Conceptually, this is just the same as having two different access path
to the same memory cell. Or accessing the same global variable through a
call-by-reference parameter and via its global name.
BTW with views, you get not just aliasing but what makes aliasing really
dangerous. Without views, you can simply survey all the queries that you
are working with and lexically compare table and field names to see
whether there's aliasing. With views, the names that you see in a
lexical scope are not enough to determine aliasing.
E.g. if you use a view that builds upon the set of Johns but aren't
aware of that (possibly due to abstraction barriers), and you change the
name field of all Does, then you're altering the view without a chance
to locally catch the bug. That's just as bad as with any pointer
aliasing problem.
Regards,
Jo
------------------------------
Date: Fri, 14 Jul 2006 11:24:45 +0200
From: Andreas Rossberg <rossberg@ps.uni-sb.de>
Subject: Re: What is a type error?
Message-Id: <e97nst$a45id$4@hades.rz.uni-saarland.de>
Marshall wrote:
>
> After all, what are the alternatives? Purely-functional
> languages remove themselves from a large class of
> problems that I consider important: data management.
Maybe, but I have yet to see how second-class variables are really more
adequate in dealing with it.
And note that even with second-class state you can still have aliasing
issues - you just need mutable arrays and pass around indices. Keys in
databases are a more general form of the same problem.
> I have explored the OO path to its bitter end and am
> convinced it is not the way. So what is left? Uniqueness
> types and logic programming, I suppose. I enjoy logic
> programming but it doesn't seem quite right. But notice:
> no pointers there! And it doesn't seem to suffer from the
> lack.
Uh, aliasing all over the place! Actually, I think that logic
programming, usually based on deep unification, brings by far the worst
incarnation of aliasing issues to the table.
- Andreas
--
Andreas Rossberg, rossberg@ps.uni-sb.de
------------------------------
Date: Fri, 14 Jul 2006 11:10:53 +0200
From: Andreas Rossberg <rossberg@ps.uni-sb.de>
Subject: Re: What is a type error?
Message-Id: <e97n2t$a45id$3@hades.rz.uni-saarland.de>
Darren New wrote:
> Andreas Rossberg wrote:
>
>> Yes, technically you are right. But this makes a pretty weak notion of
>> mutability. All stateful data structures had to stay within their
>> lexical scope, and could never be passed to a function.
>
> Not really. The way Hermes handles this is with destructive assignment.
> Each variable holds a value, and you (almost) cannot have multiple
> variables referring to the same value.
OK, this is interesting. I don't know Hermes, is this sort of like a
dynamically checked equivalent of linear or uniqueness typing?
> If you want to assign Y to X, you use
> X := Y
> after which Y is considered to be uninitialized. If you want X and Y to
> have the same value, you use
> X := copy of Y
> after which X and Y have the same value but are otherwise unrelated, and
> changes to one don't affect the other.
Mh, but if I understand correctly, this seems to require performing a
deep copy - which is well-known to be problematic, and particularly
breaks all kinds of abstractions. Not to mention the issue with
uninitialized variables that I would expect occuring all over the place.
So unless I'm misunderstanding something, this feels like trading one
evil for an even greater one.
- Andreas
------------------------------
Date: Fri, 14 Jul 2006 11:51:58 +0200
From: Joachim Durchholz <jo@durchholz.org>
Subject: Re: What is a type error?
Message-Id: <e97pp2$9p0$1@online.de>
Marshall schrieb:
> void foo() {
> int i = 0;
> int j = 0;
> j = 1;
> i = 2;
> // check value of j here. It is still 1, no matter what you filled
> // in above.
> // The assignment to i cannot be made to affect the value of j.
> }
>
> Those two local primitive variables cannot be made to have the same
> identity.
Sure. To state it more clearly, they cannot be aliases.
> But you can update them, so this is an example of mutability
> without the possibility of identity.
You're being a bit sloppy with terminology here. "Identity" in the
phrase above refers to two entities, in the sense of "i and j cannot be
identical".
Identity is actually a property of a single entity, namely that what
remains constant regardless of what you do with the entity.
Regards,
Jo
------------------------------
Date: 6 Apr 2001 21:33:47 GMT (Last modified)
From: Perl-Users-Request@ruby.oce.orst.edu (Perl-Users-Digest Admin)
Subject: Digest Administrivia (Last modified: 6 Apr 01)
Message-Id: <null>
Administrivia:
#The Perl-Users Digest is a retransmission of the USENET newsgroup
#comp.lang.perl.misc. For subscription or unsubscription requests, send
#the single line:
#
# subscribe perl-users
#or:
# unsubscribe perl-users
#
#to almanac@ruby.oce.orst.edu.
NOTE: due to the current flood of worm email banging on ruby, the smtp
server on ruby has been shut off until further notice.
To submit articles to comp.lang.perl.announce, send your article to
clpa@perl.com.
#To request back copies (available for a week or so), send your request
#to almanac@ruby.oce.orst.edu with the command "send perl-users x.y",
#where x is the volume number and y is the issue number.
#For other requests pertaining to the digest, send mail to
#perl-users-request@ruby.oce.orst.edu. Do not waste your time or mine
#sending perl questions to the -request address, I don't have time to
#answer them even if I did know the answer.
------------------------------
End of Perl-Users Digest V10 Issue 9469
***************************************
