I spent many hours trying to solve a problem, found a solution, and would like to share it in case others might encounter it.

A checkbox that appears on a form on an HTML page has a "name" attribute. Typically it has a "value" attribute. (If there is no "value" attribute, the default is for the web browser to send the string "on", I think, if the user has checked the checkbox before submitting the form.)

Anyway, if it unchecked, naturally there is no value sent to the script that's receiving the submitted form data. Let's say the HTML in the form states that the name of the checkbox is "member." If the user is a member of a certain civic organization, as explained elsewhere on the web page, the web page asks the user to so advise the website owner by checking the checkbox. If not a member, then the user just leaves the checkbox unchecked.

The interesting part is that the script receiving the form data has no idea whether there was a a checkbox named "member" unless the checkbox had been checked by the user. If it was left unchecked, the script doesn't get a "&member=''" (empty string) or even a "&member" in the submitted query string.

Let's say the script uses the submitted data to add the user to a database. Probably a record in a table in a relational database. The script can be written to assume that the person is not to be recorded as a "member" of the civic organization if there is no value coming in for the "member" parameter. If so, the script inserts a record in the table, puts the empty string (or NULL) into the record's "member" field (here, the field in the table happens to be the same as the name of the form parameter), and life is good.

But let's say the new user is indeed a member of the civic organization, and said so by checking the checkbox. So the script stores a "1" in the member field of the new record.

OK. Next step: provide for the editing of the record. We want to have the script create an editing form that looks like the new-user form. The script pulls the user's record from the database, sees there is a value in the member field, and hence the form is created to include "checked=CHECKED" as an attribute, e.g., <input type="checkbox" name="member" checked=CHECKED> is somewhere in the form.

But it's been six months since the user registered as a new user, and during that time she decided to let her membership in the civic organization lapse. So she wants to use the editing screen to update her record -- to literally uncheck the "member" checkbox. She does so, and submits the form.

The script, because it doesn't receive any information about the member checkbox from the user's web browser (see above), has to ASSUME that a parameter named member was in the form, and that the failure to find any submitted value associated with that parameter, or find the parameter in the first place, means the user is wanting to have her record in the database updated so as to replace "1" with "0" (or the empty string, etc.). OK, no big deal. The code is fairly simple.

But what if there were a different form presented to the user. One that is designed to let her change only her username, for example, and doesn't include the "member" checkbox. The HTML in the form is written to submit the data to the same script. But because the script has been written to assume that no incoming data for "member" means the person is no longer a member, then there's trouble when the script updates her record to change the "1" to "0" in the member field. The user only wanted to change her username.

OK, so we revise the script to make no such assumption. Now, though, how will the script know when to change the "member" field from "1" to "0"? It can't assume that every form has given the user a "member" checkbox. This is the assumption that's been made in every code example that I've seen, though.

The solution seems to be to have the form include a hidden input field that lists the names of all of the checkboxes in the form, so the script can safely assume that they're to be treated as having been submitted and they're to be treated as if the user intended them to be unchecked when the form was submitted. This can be done by printing a <input type=hidden value="member"> line somewhere inside the <form ...> and </form> tags.

I used the CGI.pm module to create a form, used its checkbox() function to print a checkbox inside it, and found that, lo and behold, the HTML includes a line that says something like <input type="hidden" name=".cgiparams" value="member"> just before the </form> tag. I had successfully reinvented the wheel.

CGI.pm is then able to redisplay the form (if, for example, the user supplied invalid data in the form such that the script is written to redisplay the form along with an error message), and the stickiness aspect of its created checkboxes means that a checkbox that initially was displayed as being checked will be correctly redisplayed as being unchecked if the user had in fact unchecked the checkbox before submitting the form. CGI.pm apparently knows that the checkbox it creates upon redisplay of the form is not to be checked this time, because it finds the member parameter in the .cgiparams list that was part of the submitted form and it determines that no value came in for the member parameter (or that the parameter didn't come in at all).

My strategy in generating web forms has been to take advantage of CGI.pm's sticky checkboxes and to use HTML::Template for templates. (I don't want to use CGI.pm to create and display the entire form using the 1995-ish technique of $q->start_html; $q->start_form [etc.]; $q->end_form; $q->end_html; in the script, even if that seems to have been a good and novel idea back in the day.)

So my script creates a sticky checkbox by calling $q->checkbox( -name => "member" ). CGI.pm returns a string of HTML that can be sent over to the template, where there is a <TMPL_VAR MEMBER_CHECKBOX> embedded in its text, waiting to receive the HTML for the checkbox, which might be checked due to CGI.pm's stickiness, or might not be.

Alas, the sticky feature still wasn't working. A "view source" revealed that the magic <input type="hidden" name=".cgiparams" value="member"> wasn't part of the form in the generated web page. But, of course, why should it be? I hadn't sent it over to the template. But where is CGI.pm generating that string? How do I get it?

It turns out that the string containing the hidden list of fields is prepended to </form> when CGI.pm's $q->end_form() is called. But beware (ask me how I know), this is the case only if $q->start_form() has been called before $q->checkbox() was called. it's not enough that the CGI module was instantiated. My template had its own <form action="https://yaddayadda.com/script.cgi"> and its own </form> tag, so I hadn't thought of using needing to call CGI->start_form() or CGI->end_form(). I only needed the creation of the sticky checkboxes.

What wasn't intuitive to me, and might be useful to others, is that I needed to have my script actually call something like my $throwaway_string = $q->start_form(); somewhere before $template->param( MEMBER_CHECKBOX => $q->checkbox(-name => "member") ); and then needed to send over the list of hidden fields by doing something like my $hidden_list_of_fields_and_closing_form_tag = $q->end_form(); -- after the last checkbox or any other form field had been created by using a CGI.pm function -- followed by something like $template->param( HIDDEN_LIST_OF_FIELDS_AND_CLOSING_FORM_TAG => $hidden_list_of_fields_and_closing_form_tag );

I don't need to send over the results of start_form(); I just need to have called it. The sticky checkboxes work even with the <form action=...> that's hard-coded in my template. But apparently I do need to call start_form() in order for CGI.pm to start paying attention and keeping a list of the names of the checkboxes (or other HTML form elements) that it's thereafter creating, so that $q->end_form() knows what to return in the way of a hidden list of fields.

I know that a web application platform might handle this particular edge case behind the scenes, but I'm not sure. I've done some development with Dancer2, but even that fine platform seems to assume that the developer has picked and implemented some Perl module (ideally, one that's more recent and more sophisticated than CGI.pm) to handle the creation of form elements, and to somehow handle this checkbox edge case.

Edited for clarity, probably unsuccessfully due to author's inability to communicate precisely :-)

Here is my understanding of what the theory behind what I am trying to do is, correct me where I am wrong, also please check my questions at the end:

The intersection of N hyperplanes (Ndim) is also known as the solution to "the system of N linear equations(=planes) with N variables". However, I am interested in the case when I have less equations. The result is not a single point in Ndim but a lower-dimensions plane. For example in 3D. The intersection of 3 planes (M=3, N=3), assuming any pair is not parallel, is a single 3D point and the intersection of 2 planes (M=2, N=3) is a line. In Ndim, the solution would be an Mdim hyperplane "living" in Ndim. Below, I call this a "hyperline".

An example of the above problem is in this SO question. The result is the equation of the intersection line in parametric form. Arbitrary values of the parameter t yields a point on this line. Ideally this is what I want to achieve: finding points on the intersection of the planes.

Solving the system of N linear equations with N variables (Ndim) can be done with, for example, Math::Matrix. Given N planes in the form a1x+b1y+c1z+...n1=0 etc., a matrix A is formed with each row being [a1, b1, c1, ..., n1] etc. then: Math::Matrix->new([[a1,b1,c1,...,n1],...])->solve->print;

I have experimented with solve() for the case of M linear equations (Ndim) where M = N-1:

use Math::Matrix;
Math::Matrix->new(
  [1, 2, 1, -1], # ax+by+cz+n=0
  [2, 3, -2, 2]
)->solve->print;
# says
# -7.00000    7.00000 
#  4.00000   -4.00000
[download]

I interpret the result as: the 1st row refers to the 1st dim (aka x) and we have

x -7*t + 7 = 0 => x = 7*t - 7
[download]

y + 4*t -4 = 0 => y = -4*t + 4
[download]

I have also experimented with transforming the planes matrix to the Reduced Row Echelon Form. Which makes it convenient to work out the "hyperline" equation. This functionality is offered by Math::MatrixLUP :

use Math::MatrixLUP;
my @planes3D = (
  [1, 2, 1, -1], # ax+by+cz+n=0
  [2, 3, -2, 2]
);
print Math::MatrixLUP->new(\@planes3D)->rref
# says
# [1, 0, -7, 7],
# [0, 1, 4, -4]
[download]

Which I interpret as: 1st row has 1 on 1st col, that's the 1st dim (aka x) :

x -7 * t + 7 = 0 => x = 7*t-7
[download]

So that works well.

Here is a demo I whipped up:

I have some questions:

• Two planes are parallel when their normal vectors (the coefficients of the dimensions: a, b, c, ... (but not the constant n) are multiples. E.g. nv1 = k * nv2. And this translates to checking if all the ratios of the coefficients : a1/a2 = b1/b2 = c1/c2 = ... are the same. My question is: what happens if any coefficient is zero (or actually both coefficients (e.g. a1 and a2) are zero?
• How can I calculate the intersection when M < (N-1)? I.e. above I am always checking the intersection of M planes in Ndim where M = N-1. But what if there are even less planes? e.g.

  my @planes5D = (
    [1, 2, 1, -2, 3, -1], # ax+by+cz+n=0
    [2, 3, -2, 4, -4, 2],
    [3, -1, -2, -5, 6, 2],
    # only 3 planes (it was successful with 4)
  )
  [download]

  In short, does the parametric equation of the intersecting "hyperline" contain 2 parameters now?
• When I test test_in_beast_space which produces random planes in 666 dimensions, it fails to detect that a point on the intersection "hyperline" lies on all the planes. To do that I substitute the point in each plane equation and expect to have result as zero. However, it's not an exact zero. That's why I am doing a range test to check if it's close to zero. Well the closeness $SMALLNUMBER for 5 dimensions can be as small as 1E-09. But for these many dimensions it can be as low as 1E-02. Is the accuracy lost in summing 666 multiplications that much really?
• I guess Math::Matrix::solve() is safe to be used for MxN matrices (e.g. M equations=planes in N unknowns (dimensions) where M<N)? From solve() of Math::Matrix :

  Solves a equation system given by the matrix. The number of colums mus
  +t be greater than the number of rows. If variables are dependent from
  + each other, the second and all further of the dependent coefficients
  + are 0. This means the method can handle such systems. The method ret
  +urns a matrix containing the solutions in its columns or undef in cas
  +e of error.
  [download]

• The edge cases where the 1st dimension is zero for all planes in 5D and when the 1st+2nd are zero, fail. How can I find the intersection in this case?

Edits: updated the demo to correct the sub are_planes_parallel() as per the comments of hippo and LanX about what happens when plane-equation coefficients are zero.

have fun in beast space! bw bliako

In 1998, a friend gave me a computer. It was in pieces. I knew nothing about nothing. Within a year, I was communicating over dual telephone lines. Within two years of that, I was a sysadmin at an ISP. This was the year I found Perlmonks. I read a book, 'Perl in 21 days' or some such, and found Perl was what I wanted... a way to automate processes.

Within months, I learned a dangerous amount about MySQL, CGI and Perl to allow any invader to break everything. Thankfully, during that time, everyone was out for themselves, and exploitation hadn't yet become a thing.

By 2009, I'd grown a lot in many areas. No where near perfect in the security arena, but I was becoming proficient on how to interact with the open source world, and how to interact with the CPAN. It was this year that I joined Perlmonks as a member, and became a vocal person, not just a listener.

Now, as some of the old timers will attest to, I always claimed "I'm not a programmer". With that said, I have done much work in fields so closely related to programming, that I have to bend and say that yeah, maybe I can classify as a hacker.

Anyone else around who have claimed "I'm not a programmer", or who has been around since the very early days of Perlmonks who would just like to say "I'm still here!!!"?.

-stevieb

I needed to generate random strings from a regular expression and I have found (*) a C++ library (regxstring C++ library by daidodo) which does just that and works fine for my use case. It claims that it supports most Perl 5 regexp syntax. It is quite fast too. So I have ported it into a Perl module, here : String::Random::Regexp::regxstring.

perl -MString::Random::Regexp::regxstring -e 'print @{generate_random_
+strings(q{^\d{3}[-,.][A-V][a-z]{3}\d{2}})};'
[download]

use String::Random::Regexp::regxstring;
my $strings = generate_random_strings('^[a-c]{2}[-,.]\d{3}-[A-Z]{2}$',
+ 10);
print "@$strings\n"
[download]

The XS and C++ code bridging Perl to C++ library are very simple and can serve as an example for doing that for other libraries. Go forth and multiply.

(*) Via this old thread I found Regexp::Genex and there is also String::Random. Neither worked for my use case.

bw, bliako

I was thinking what my options are for controlling a remote farm remotely.

I needed to do basic stuff like run the motor to push the feed to the animals, open a valve to let mains water fill their drinking buckets, check feed and water levels, check temperature. Even count the number of eggs.

The farm has no electricity is off-grid and has no alternative power installed, so it must rely on solar energy. I want to minimise the use of batteries. There is no WIFI, but it is covered by the national 4,5G telephone network.

A smartphone has all the communication modules available and ready: sms, voice, data, bluetooth, even rfid. It also has a good power supply and provided is not loaded with apps, it can last for 24 hours at least for the next sunshine for the solar charger to work. And it allows high-level programming.

Unfortunately a smartphone lacks any connectivity to the world by means of IO ports. And so I am looking for an IO board to the smartphone via its USB or perhaps the bluetooth. The cheaper the better really and simpler too.

I found IOIO-OTG which interfaces android-based smartphone or PC via usb to a lot of IO ports which can read sensors or run motors and actuators.

I really like an IO board+Smartphone solution because I can sms to it commands like "feed animals" or "count eggs". And it can reply back via sms. I can also command it via internet ("data") and perhaps get the odd picture back. There is solar charging available and cheap. And with just a single app running perhaps it can manage 24hrs recharge cycles. From all of the above I love the control-by-sms idea.

My first point of call was the raspberry but i would prefer that there was a plugin module to the pi to do that rather than buying cells+batteries and doing calculations. E.g. https://community.element14.com/products/raspberry-pi/f/forum/53345/power-raspberry-pi-using-solar-panel

I would also like to communicate with the device. The most reliable way I see is via SMS. And the most practical is via 5G internet (data). The PI offers sending SMS but again, it's hands-on and a lot can go wrong.

So, I am looking for recommendations on other IO boards for smartphones (android is just fine), and/or similar solutions for the PI just to be fair to the PI, if people feel that's a better environment.

I have asked hacker news about this here

thanks, bw, bliako

edit:the IOIO-OTG board needs own power supply when plugged into smartphone. Not when plugged into PC

Edit 27/06/2024: I have read that when connecting the board to smartphone (not PC) it must provide its own power supply. It can not be powered from Android. Android then asks you if you want to charge the phone with what it found on its USB port or transfer photos etc. This fits well with the design that the board has its own solar power+battery which then charges the phone as well, and also drives any external motors (TODO: noise from motors into the board). If external power runs out (no sun for days) then, firstly, the board stops and then the phone runs out of its own battery (sending an sms to me when in the critical zone) after a while. When solar power recharges on sun appearing, the board will be able to charge the phone too. Problem I see, how to turn the phone on when power comes back and how to tell it that what is on the USB port (our IO board) should be run on the specified mode (transfer files, charge, whatever) WITHOUT user intervention. Just by its own.

I'll leave out how it took a couple of days of experimenting to figure this out; that wasn't due to the dispatching technique being wrong, but a simple logic error a bit further up the function. If there's any value to mentioning that here, it's to check your assumptions (possibly with printf) when things don't go to expectations.

The obvious way to do this, according to https://perldoc.perl.org/perlcall, is to pass the G_NOARGS flag. However, that is (somewhat obscurely) documented as not being a good idea to use with perl_call_method. The technique I settled on, permalinked at https://github.com/PDLPorters/pdl/blob/cd5ffff1b85ff81effefc2085067e9d71801efd6/Basic/Core/Core.xs#L199-L220:

      SP -= items; PUSHMARK(SP); SPAGAIN; /* these pass this set of ar
+gs on */
      int retvals = perl_call_method("new", G_SCALAR);
      SPAGAIN;
      if (retvals != 1) barf("new returned no values");
      RETVAL = POPs;
[download]

SV *
topdl(klass, arg1, ...)
  SV *klass;
  SV *arg1;
  CODE:
    if (items > 2 ||
      (!SvROK(arg1) && SvTYPE(arg1) < SVt_PVAV) ||
      (SvROK(arg1) && SvTYPE(SvRV(arg1)) == SVt_PVAV)
    ) {
      SP -= items; PUSHMARK(SP); SPAGAIN; /* these pass this set of ar
+gs on */
      int retvals = perl_call_method("new", G_SCALAR);
      SPAGAIN;
      if (retvals != 1) barf("new returned no values");
      RETVAL = POPs;
    } else if (SvROK(arg1) && SvOBJECT(SvRV(arg1))) {
      RETVAL = arg1;
    } else {
      barf("Can not convert a %s to a %s", sv_reftype(arg1, 1), SvPV_n
+olen(klass));
    }
    SvREFCNT_inc(RETVAL);
  OUTPUT:
    RETVAL
[download]

Breaking the code into individual statements, with comments for each:

SP -= items; /* make the local SP point at our ST(0) */
PUSHMARK(SP); /* make the next function's ST(0) be that */
SPAGAIN; /* reset our local SP to the global "top of current stack" */
int retvals = perl_call_method("new", G_SCALAR); /* call method so has
+ right stack frame */
SPAGAIN; /* set our local SP to the global "top of current stack" */
if (retvals != 1) barf("new returned no values"); /* use croak in non-
+PDL code */
RETVAL = POPs; /* set the SV* to the return value */
[download]

Hey monks,

I'm backing up files with chatgpt and perl. I got a pretty good utility program out of it (them, whatever). It's a utility to sort pics and convert .heic to .jpg. I happen to be looking for a particular picture I took in 2021, so this is really helpful. Typical output:

Moved and renamed: /home/fritz/Pictures/6.pics/2022-12-08-220201009.mp
+4 -> /media/hogan/175F-DC61/Organized_Pics/2022/12/2022_12_08_5.mp4
Skipping: Wallpapers (not a file)
Processing file: Screenshot from 2022-10-16 16-58-24.png
File path: /home/fritz/Pictures/Wallpapers/Screenshot from 2022-10-16 
+16-58-24.png
Failed to delete original file /home/fritz/Pictures/Wallpapers/Screens
+hot from 2022-10-16 16-58-24.png: No such file or directory
Moved and renamed: /home/fritz/Pictures/Wallpapers/Screenshot from 202
+2-10-16 16-58-24.png -> /media/hogan/175F-DC61/Organized_Pics/2022/10
+/2022_10_16_13.png
Processing complete! Number of files created: 960
fritz@laptop:~/Documents$
[download]

Source:

#!/usr/bin/perl
use strict;
use warnings;
use File::Find;
use File::Path qw(make_path);
use File::Copy qw(move);
use POSIX qw(strftime);

# Define the source and target directories
my $source_dir = '/home/fritz/Pictures';
my $target_dir = '/media/hogan/175F-DC61/Organized_Pics';

# Check if the source directory exists
unless (-d $source_dir) {
    die "Source directory $source_dir does not exist.\n";
}

# Create the target directory if it doesn't exist
unless (-d $target_dir) {
    make_path($target_dir) or die "Failed to create target directory $
+target_dir: $!\n";
    print "Created target directory: $target_dir\n";
}

# Hash to store file type counts
my %file_types;
my $file_count = 0;

# Subroutine to process each file
sub process_file {
    if (-f $_) {
        my ($ext) = $_ =~ /\.([^.]+)$/;
        $ext = lc $ext if defined $ext;  # Convert to lowercase

        if (defined $ext && $ext =~ /^(jpg|jpeg|png|gif|bmp|tiff|heic|
+mp4)$/) {
            print "Processing file: $_\n";

            my $file_path = $File::Find::name;
            print "File path: $file_path\n";
            
            my $mod_time = (stat($file_path))[9];
            my $date = strftime "%Y_%m_%d", localtime($mod_time);
            my ($year, $month, $day) = split('_', $date);

            my $dest_dir = "$target_dir/$year/$month";
            unless (-d $dest_dir) {
                make_path($dest_dir) or die "Failed to create destinat
+ion directory $dest_dir: $!\n";
                print "Created destination directory: $dest_dir\n";
            }

            my $count = 1;
            my $new_file_name;

            if ($ext eq 'heic') {
                $new_file_name = "${year}_${month}_${day}_$count.jpg";
                while (-e "$dest_dir/$new_file_name") {
                    $count++;
                    $new_file_name = "${year}_${month}_${day}_$count.j
+pg";
                }
                
                my $converted_file_path = "$dest_dir/$new_file_name";
                my $convert_command = "heif-convert $file_path $conver
+ted_file_path";
                system($convert_command) == 0 or die "Failed to conver
+t $file_path: $!\n";
                unlink $file_path or warn "Failed to delete original f
+ile $file_path: $!\n";
                print "Converted and moved: $file_path -> $converted_f
+ile_path\n";
            } else {
                $new_file_name = "${year}_${month}_${day}_$count.$ext"
+;
                while (-e "$dest_dir/$new_file_name") {
                    $count++;
                    $new_file_name = "${year}_${month}_${day}_$count.$
+ext";
                }
                
                move($file_path, "$dest_dir/$new_file_name") or die "F
+ailed to move file $file_path: $!\n";
                unlink $file_path or warn "Failed to delete original f
+ile $file_path: $!\n";
                print "Moved and renamed: $file_path -> $dest_dir/$new
+_file_name\n";
            }

            $file_count++;
        } else {
            print "Skipping file: $_ (not an image)\n";
        }
    } else {
        print "Skipping: $_ (not a file)\n";
    }
}

# Find and process files in the source directory
print "Starting to process files in $source_dir...\n";
find(\&process_file, $source_dir);

print "Processing complete! Number of files created: $file_count\n";
[download]

What bells and whistles might we coach it to add?

Cheers from my own private Idaho,

Stack just monetized all of the data that we, as public advocates of free information, provided.

Is Perlmonks going to do this? If it is, I want all of my content removed immediately. I do not agree to the knowledge I've learned from those before me and I've subsequently shared being sold to anyone without due attribution.

If Perlmonks plans on selling its user data to anyone, I outright refuse to take part, and want my data to be excluded entirely.

-stevieb

void
firstvals_nophys(x)
  pdl *x
  PPCODE:
    if (!(x->state & PDL_ALLOCATED)) barf("firstvals_nophys called on 
+non-ALLOCATED %p", x);
    PDL_Indx i, maxvals = PDLMIN(10, x->nvals);
    EXTEND(SP, maxvals);
    for(i=0; i<maxvals; i++) {
      PDL_Anyval anyval = pdl_get_offs(x, i);
      if (anyval.type < 0) barf("Error getting value, type=%d", anyval
+.type);
      SV *sv = sv_newmortal();
      ANYVAL_TO_SV(sv, anyval);
      PUSHs(sv);
    }
[download]

The solution was simply to do a PUTBACK after the PUSH, which moves the top of the stack above data we care about. The new text of the function with that:

void
firstvals_nophys(x)
  pdl *x
  PPCODE:
    if (!(x->state & PDL_ALLOCATED)) barf("firstvals_nophys called on 
+non-ALLOCATED %p", x);
    PDL_Indx i, maxvals = PDLMIN(10, x->nvals);
    EXTEND(SP, maxvals);
    for(i=0; i<maxvals; i++) {
      PDL_Anyval anyval = pdl_get_offs(x, i);
      if (anyval.type < 0) barf("Error getting value, type=%d", anyval
+.type);
      SV *sv = sv_newmortal();
      ANYVAL_TO_SV(sv, anyval);
      PUSHs(sv);
      PUTBACK;
    }
[download]

I upgraded a machine to the latest Ubuntu and got Perl 5.34.0, which included a problem with the debugger:

  DB<4> v                                                             
+                                                       Undefined subr
+outine &DB::cmd_l called at /usr/share/perl/5.34/perl5db.pl line 6034
+.
 at /usr/share/perl/5.34/perl5db.pl line 6034.                        
+                                                               DB::cm
+d_v("v", "", 56) called at /usr/share/perl/5.34/perl5db.pl line 4798
        DB::cmd_wrapper("v", "", 56) called at /usr/share/perl/5.34/pe
+rl5db.pl line 4311                                             DB::Ob
+j::_handle_cmd_wrapper_commands(DB::Obj=HASH(0x55e85838c150)) called 
+at /usr/share/perl/5.34/perl5db.pl line 32
00                                                                    
+                                                               DB::DB
+ called at report_warnings.pl line 56
Debugged program terminated.  Use q to quit or R to restart,          
+                                                       use o inhibit_
+exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
  DB<4>
[download]

And, of course, the patch worked just fine. What a great community. Thanks for the patch!

PS: Ugh, sorry -- the problem was that the v command that I use a lot (Where am I? Oh, there I am!) crashed the debugger. The patch solves that problem.

Dear Monks and Nuns,

lately I came across an issue with dereferencing array refs. It looks like there is some hidden "lazy deref" when an array ref is used in a foreach loop, compared to the usage as a sub argument. Consider these two subs that do nothing but die. The main difference is the array dereference as an argument to foreach or map.

use experimental 'signatures';

sub map_die ($ar) {
    eval {map {die} @$ar};
}

sub for_die ($ar) {
    eval {
        for (@$ar) {
            die;
        }
    }
}
[download]

use Benchmark 'cmpthese';

my @arr = ((0) x 1e6);

cmpthese(0, {
        map => sub {map_die(\@arr)},
        for => sub {for_die(\@arr)},
    });
__DATA__
         Rate     map     for
map    1257/s      --   -100%
for 1664823/s 132352%      --
[download]

Then I remembered the "lazy generators" from List::Gen and gave it a try. There is some progress, but it cannot come up to foreach.

use List::Gen 'array';

sub gen_die ($ar) {
    eval {
        &array($ar)->map(sub {die});
    }
}

cmpthese(0, {
        map => sub {map_die(\@arr)},
        for => sub {for_die(\@arr)},
        gen => sub {gen_die(\@arr)},
    });
__DATA__
         Rate     map     gen     for
map    1316/s      --    -93%   -100%
gen   18831/s   1330%      --    -99%
for 1662271/s 126174%   8727%      --
[download]

Wouldn't it be nice to have some kind of "explicit lazy dereferencing" in Perl?

Update May 10, 2024: Added the signatures feature as suggested by Danny in Re: The Virtue of Laziness.

brew install pkg-config

brew install libpng
pkg-config --cflags libpng
cpan Imager::File::PNG

brew install jpeg
pkg-config --cflags libjpeg
cpan Imager::File::JPEG

brew install giflib
cpan Imager::File::GIF

GIF: Test code failed: Can't link/include 'gif_lib.h', 'stdio.h', 'errno.h', 'string.h', 'gif'...
! Configure failed for Imager-File-GIF-0.98. See /Users/you/.cpanm/work/1713652269.80239/build.log for details.

cd /Users/you/.cpanm/work/1713652269.80239/Imager-File-GIF-0.98

perl Makefile.PL -v --incpath=/opt/homebrew/include --libpath=/opt/homebrew/var/homebrew/linked/giflib/lib

make
make test
make install

o conf makepl_arg "LIBS=-L/opt/homebrew/var/homebrew/linked/giflib/lib INC=-I/opt/homebrew/include"

sudo /usr/libexec/locate.updatedb

locate giflib
/opt/homebrew/var/homebrew/linked/giflib

locate gif_lib.h        
/opt/homebrew/Cellar/giflib/5.2.1/include/gif_lib.h
/opt/homebrew/include/gif_lib.h

cpan -M https://www.cpan.org -a
[download]

I've just found out the HMRC (the UK's taxation department) uses Perl for at least some of its operations...

The system is troubled today and I received this error revealing the language

Ref: /home/ewf/MODULES/Common/PaymentApiService.pm

Error Code 401 at line 30
[download]

Update: corrected typo in title