Thursday, October 20, 2005

Perl 6 at EuroOSCON

This is a brief summary of my notes from the Perl 6 and pugs sessions. I have only included the stuff that was new compared to the Perl 6 Appendix in the 'Perl Programming' course. I might have repeated myself.

use strict and use warnings are on by default;

Quotes are only required around a hash key when {} are used.

$hash{'key'} becomes %hash

Barewords are not allowed. Ever. Even for file handles.

fail{…} warn{…} die{…} throw a 'not yet implemented' type warning – the Perl 6 developers must be using that a lot ;-)

Many improvements to interpolation:

"{ executable code like a do block} "

Subroutine calls are allowed: "&mysub(args)"

Interpolate an array: "@array[]"

Interpolate a hash: "%hash{}"

sprintf is probably obsolete, printf definitely is:

print $'%6.2f');

print $"%-20s: %2.6f", "\n");

Control can be made over exactly what is interpolated:

qq:c(0)/ / # don't interpolate

qq:s(1)/ / # only interpolate scalars

qq:s(1):a(1)/ / # interpolate scalars and arrays


Here document syntax is changed:

my $var = q:to /END/ # s(1):a(1):h(1) can be added


Not sure where the semi-colon goes

Ranges have a new feature:

x..^y means from x up to y-1

x^..^y means from x+1 to y-1

To open a file with an automatic chomp::

my $fh = open $filename

This can be overridden.

while (<$fh>) {…}


while = $fh {…}

while (<>){…}


while = $ARGS{…} or while =<> {…}

=<> is known as the 'fish' operator

There is now a 'prompt' verb

Most built-ins that operator on scalars and arrays are now methods:

$var.substr(…) # returns the substring

$var .= $var.substr(…) # changes the substring

No need for Data::Dumper, instead:


And there is much more, Damian could not complete his talk because of time, and it is not downloadable. I'll keep you posted as I play with pugs.

Wednesday, October 19, 2005

EuroOSCON - whatever. Is Perl 6 too complex?

I asked the main man, yer actual Larry Wall, this question. Poor guy - he is a very nice chap - thought he was just signing a book for me. There was no one else around, so I consulted the oracle (I won't use an uppercase O, you'll get the wrong idea).

Q. Will Perl 6 still be suitable for guys who just want a better language that ksh or .bat files?
A. The easy things will still be easy

Q. Will Perl 6 still be suitable for sys.admin's?
A. Hey! I'm still a sys.admin.
Me: No you are not, you are a language demi-god.
A. Shrug. Sys.Admins will still be able to use it.
Q. Are you saying that because that is what I want to hear?
A. Sys.Admins will still be able to use it.

Thank-you very much.
We also discussed training strategies for Perl 6, and even for Perl 5, and I wasn't that hard on him. Larry Wall is a very nice chap (oh, I said that).

EuroOSCON 6 - Perl 6 Update

Well I did type all this in, but the network broke when I sent it, and was lost. You will have to wait.

EuroOSCON 5 - Installation Installation Installation

I saw a demo today of a brilliant web-based Perl debugger called Devel::ebug::HTML. It uses the browser, and appears to have everything you want from a visual debugger.
Trying to install it is a nighmare. There is no README file, so no list of dependancies. I start downloading and installing them by hand and choke when I come to Catalyst. So I switch off my firewall (oooooooh) and run cpan, and nothing installs. Tried ppm - the same. Everything downloads OK, but every single one fails on the make - it should be using nmake on windows.
Doing this manually would take for ever, so I give up.

Other Open Source products seem to suffer from the same thing, and it is about time the community did something about it. OK, that means you and me. Something is being done about it, there are many people working on this. I appreciate the complexity, but I don't think the underlying issue is technical. It is trying to bring order to the bazaar, and I am not sure that is possible without turning it into M&S.

Tuesday, October 18, 2005

EuroOSCON 4 - 'Ere we go, 'Ere we go, 'Ere we go!

Various speakers and sessions attended (more tonight), here is a summary:


Book sales year on year current figures:
C# down 3%
Python up 30%
VB down 1%
Perl up 4%
Ruby up > %1000
Linux(generic) down 4%
Read Hat down 24%
Other Linux/Unix up 2,807%

Now does that tell several stories?


Microsoft dialog boxes: Q. "Would you like tea or coffee?

The other talks were just saying what a wonderful thing Open Source is, and how it is the future. Yeh Yeh Yeh.

Perl 6

Lots of detail, I have to sort out my notes in detail - another post tomorrow.
Damian and Larry's presentation was titled "Perl 6: End-game"
Damian Conway is a trainer, and is giving 2-day Perl 6 courses NOW, and expects to be giving full blown Perl 6 courses next summer.

Have more sessions this evening, tomorrow is quieter.

EuroOSCON 3 - Perl Best Practices (2) - prototypes

Like the true geek I am, this has kept me awake all night.

Damian Conway is dead against prototypes, and says they should all die a horrible death in a pit filled with spikes.
And snakes (I forgot about the snakes).

Prototypes on $@% are rubbish – I agree, but \$\@\% have some advantages.

Take the swap_arrays code in his book (p.196), let's think of some simple user mistookes:

Without prototypes:

swap_arrays(@sheep, @goats); # Missed out the \

Without prototypes, provided you are using strict, you get:

Can't use string ("Blackface") as an ARRAY ref while "strict refs" in use at …

("Blackface" is a sheep, we know about these things where I do live)

BUT the error is given as being in the subroutine, not at the call (where the actual error is). perl also stops right there, it does not detect that the next argument is wrong as well – we need to do another run to find that.

Mostly if we supply the wrong type, use strict 'refs' will save our bacon (or mutton). BUT:

swap_arrays([qw(one two)], [qw(three four)]);

does not give us an error, surprisingly. OK, at the end of the day we should not be calling a subroutine we do not understand, but if a user can, she will. If she thinks that the two lists are flattened (Damian's assumption with prototypes), then she clearly does not understand the thing anyhow.

With prototypes:

swap_arrays(\@sheep, \@goats); # Added a \

With a prototype we get an error for each argument at fault:

Type of arg 1 to main::swap_arrays must be array (not reference constructor)

Type of arg 2 to main::swap_arrays must be array (not reference constructor)

Execution of C:\ aborted due to compilation errors.

This time the error is reported correctly.

Also, with:

swap_arrays([qw(one two)], [qw(three four)]);

and a prototype we get a similar error message, again identified with the correct line number.

And another thing

And I nearly forgot, empty prototypes are the only way I know (other than hand-coding) of preventing any arguments being passed, as in:

sub void_sub () { ,,, }

Calling this subroutine would give an error at the point of the erroneous call.

The Volcano beats Joe

Let's face it though, Damian is right. If you want a standard there cannot be exceptions. The problem with prototypes is that everyone thinks they are like C/C++ prototypes, but most people don't even understand THEM. Take:

size_t myfunc (char string[4]);


int another ();

Neither do what most people think they do, and are misleading. So how do you expect to understand Perl prototypes on the same (false) basis? KISS.

Monday, October 17, 2005

EuroOSCON 2 - Perl Best Practices

Damian Conway's talk on this subject woke me up.
A lot of great ideas and tips, methinks I need an extra slide at the end of each chapter on style, or something like it. Do you think people will mine a six day course? That's the problem, I keep picking up more stuff to put into the course!

At least I found something to take out - subroutine prototypes.
A packed room, with lots of experts, and it appears I was the only person who understood prototypes (except Damian, of course). That is scary, and probably indicates we sould not spend so much time on them in our course. Damian hates them, I just find them 'flaky' personally - but no more flaky than lots of other parts of Perl. Then again, it is Damian's mission to destroy flaky Perl, and we have to start somewhere...

I got a chance for a brief chat with Mr. Conway (he comes from that country which just lost the ashes), but I have more questions for him. Here is a reminder for me:

Why use -t *STDIN instead of STDIN?
Shouldn't IO::Prompt offer the locale specific Y/N?
An idea for a pragma - RegExp::NoCapture - make all parenth. groups non-capturing. Could REs be optimised better?

Some good speakers tonight - Damian Conway (again) and Larry Wall - really looking forward to that.

BTW - am I grateful for downloading FireFox? The W/Lan here does not work with MS Internet Explorer. Apparently it is the same problem as I mentioned with ActiveState (see below), Or so they say.....


This conference is so cool, it is Arctic.
Went to the Database Tutorial this morning - another chapter to re-write! Not that there is anything wrong with the Perl Programming Appendix on DBI, but I just learnt so many cool things, that the chapter will probably double in size.
So what did find out?
First, my Perl knowledge is not so bad ;-) It is always worrying going up against a bunch of enthusiastic geeks, but hey, I don't think I let the side down. That was a confidence boost (even lecturers need that sometimes)
Next: dbi:DBM: is a prototyping database bundled with DBI, looks great for quick training demos
Next: queries can be done against a dbh, but this is not efficient (we don't mention that)
Next: if $sth->disconnect is not done, you can get a memory leak
Next: placeholders are very cool if you know how to use them (I feel lots of example slides coming on)
Next: I had missed dbh attributes before (set in the connect)
Next: Lots of things about RaiseError
Next: $sth metadata hashes are tied, so using these invoke method calls
Next: and other stuff as well

A lot I already knew, like American jokes don't work on a European audience.

Friday, October 14, 2005

Fedora 4 /etc/profile

Found a buglet in this file this week.
The /etc/profile file is the first file executed by a Korn shell session on login, but it has been hijacked by Bash. On Linux the Korn shell is not used much, except by training companies of course. I doubt that /etc/profile is ever tested with ksh, so it is not surprising there is a bug, however it is a shame that the mistake is such a simple one - doesn't give one much confidence.

On sign-on to ksh we get a syntax error in /etc/profile, on the line:

if [ $EUID = 0 ]; then

The EUID shell variable is set by Bash, and is a read-only value giving the effective user-id (if you want to know the difference between effective and real user-ids, come on QAs Unix Programming course). The code is checking for a 'root' user. On Korn shell this variable is not set.

The fix is quite easy, quote the variable:

if [ "$EUID" = 0 ]; then

ALWAYS quote variables in single [. One of the effects of using [[ is that quotes are not necessary around a variable, and if the authors had come on QAs Mastering BASH Shell Scripts course, they would have known that.

Tuesday, October 04, 2005

Sneaky bat files

The ActiveState Perl implementation contains some examples of imbedding Perl inside a Windows .bat file. Of course if we wanted to do that sort of thing in a Unix shell we would probably use a 'here' document, but bat files do not have that level of sophistication.

The structure of these bat files is as follows:

@rem = '--*-Perl-*--

perl -x -S "%0" %*

goto endofperl
@rem ';

#!/usr/local/bin/perl -w
#line 9
... Perl code ...

The "%0" is the name of the current bat file, and %* is a list of the arguments passed into the bat file (like $0 and $* in the Korn shell and Bash).

So we are executing the bat file as if it was a perl script! This is where the –x option to perl comes in. Check perlrun, -x means that the perl code starts at the #! line, and preceding lines are to be ignored. The –S option means that %path% is used to find the perl script. There is a problem with this.

The quotes around "%0" would appear to protect us from imbedded spaces in the filename, but they do not. Paths with imbedded spaces do not work with this method. Strangely, this works if the quotes are omitted. The implication is that most of the ActivePerl bat files do not work with imbedded spaces in path names.

(The pugs Windows installation has a similar problem)

The –S option is not really needed. This is because to run the bat file we need to specify a hierarchic or full pathname anyway (Explorer makes the script's directory its current directory).

The sneaky part is the use of @rem.

The bat file starts with a non-echo'ed remark:

@rem = '--*-Perl-*--
… bat file stuff …
@rem ';

The idea here is that, by a happy coincidence, @rem comment prefix in a bat file is a valid perl command, to assign to an array called @rem. The assignment ends on the final comment, with a close quote (notice that the first @rem has no close quote).
I can only guess that this is in the bat files for historical reasons, since the –x option to perl makes this trickery unnecessary. The goto (whatever that is) is required to leap over the perl code.

The #line directive sets the line number when error reporting, or for the die statement. This means that we can set the line numbers relative to the start of our bat file, rather than the start of the perl code.

Finally, the __END__ label tells perl that this is the end of the script, and ignores any junk that follows.

A link of interest

Will Linux Benefit from Microsoft's SNAFU in Massachusetts? by Tom Adelstein -- Microsoft had everything to lose in Massachusetts and did.