Beefy Boxes and Bandwidth Generously Provided by pair Networks
P is for Practical


( #480=superdoc: print w/replies, xml ) Need Help??

If you've discovered something amazing about Perl that you just need to share with everyone, this is the right place.

This section is also used for non-question discussions about Perl, and for any discussions that are not specifically programming related. For example, if you want to share or discuss opinions on hacker culture, the job market, or Perl 6 development, this is the place. (Note, however, that discussions about the PerlMonks web site belong in PerlMonks Discussion.)

Meditations is sometimes used as a sounding-board — a place to post initial drafts of perl tutorials, code modules, book reviews, articles, quizzes, etc. — so that the author can benefit from the collective insight of the monks before publishing the finished item to its proper place (be it Tutorials, Cool Uses for Perl, Reviews, or whatever). If you do this, it is generally considered appropriate to prefix your node title with "RFC:" (for "request for comments").

User Meditations
RFC: pragma pragmatic
3 direct replies — Read more / Contribute
by shmem
on Aug 09, 2017 at 03:55

    We all (that's a rough rounded estimate) know about the most important pragma modules - strict and warnings - and maybe also how to turn them on or off, or how to turn on/off a subset of the facilities provided by them. In comparison, few of us know how a pragma really works. I don't either - to know *really* well, I'd have to wade the perl source code, which I won't, for now.

    This I know from the docs, if I haven't misread them: the variable $^H is used for hints to the compiler, e.g. how to behave in respect to refs, subs and vars. The hash %^H is used for hints from the compiler to the interpreter, i.e. perl's runtime. The current state of %^H at compile time is stored in the optree (another obscure thing most perl programmers don't care about, but that's what's getting run), so its state can be retrieved at runtime via (caller($level))[10].

    We could use that to make the scope of imported functions from some arbitrary package purely lexical, even if that package isn't pragma aware. Here's a shot at this:

    Package pragmatic:

    Usage example:

    package Foo; use 5.10.0; require Exporter; @Foo::ISA = qw(Exporter); our @EXPORT = qw(foo); sub foo { say "Foo::foo at line ".(caller)[2] } 1;
    #!/usr/bin/perl use 5.10.0; sub foo { say "main::foo at line ".(caller)[2] } use pragmatic Foo; foo; # line 5 no pragmatic Foo; foo; # line 7 use pragmatic Foo; foo; # line 9 no pragmatic; foo; # line 11 use pragmatic Foo; foo; # line 13 __END__ Foo::foo at line 5 main::foo at line 7 Foo::foo at line 9 main::foo at line 11 Foo::foo at line 13

    A module imported via pragmatic isn't imported into the caller's namespace, but into the package pragmatic::import. Each symbol from that package is inspected, and after inspection, deleted. If the CODE slot is defined, its content is stored in the hash %symbols for this caller and this package. If a symbol of that name is found in the caller, its CODE slot content is saved to the hash %masked. The caller is given a reference to the undefined subroutine of that name in package pragmatic. At runtime, this triggers AUTOLOAD, where the appropriate subroutine is called, depending on whether the pragma is in effect or not.

    This works for exported subroutines only. If a package exports other glob types, they are passed into the caller's namespace, and they aren't managed by pragmatic. I wish there was a way to govern those too.

    To be done yet:

    • a forget(PACKAGE) method which finishes the pragmatic management and stores the stashed subroutines permanently in the caller's namespace
    • ... (fill the blank)

    What do you think? Bad idea? useful? Big delirium? Comments welcome.

    edit: fixed some bugs

    perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'
Which internal DSL are there in Perl? (Domain Specific Languages - Part 1)
7 direct replies — Read more / Contribute
by LanX
on Aug 02, 2017 at 22:08
    Taking some definitions from Wikipedia and Martin Fowlers Book: (emphasizes added)

    DSLs are small languages, focused on a particular aspect of a software system. You can't build a whole program with a DSL, but you often use multiple DSLs in a system mainly written in a general purpose language.

    DSLs come in two main forms: external and internal

    An external DSL is a language that's parsed independently of the host general purpose language: good examples include regular expressions and CSS

    Internal DSLs are a particular form of API in a host general purpose language, often referred to as a fluent interface.

    They are represented within the syntax of a general-purpose language. Itís a stylized use of that language for a domain-specific purpose.

    People find DSLs valuable because a well designed DSL can be much easier to program with than a traditional library.

    This improves programmer productivity , which is always valuable. In particular it may also improve communication with domain experts , which is an important tool for tackling one of the hardest problems in software development.

    Now there are some more frequently used examples in Perl which come to mind like SQL and Template languages like in Template::Toolkit2

    [% FOREACH file=files %] [% IF ( matches ='dd/mm/(\d+)') ) %] <a href='#'>[% matches.0 %]</a> [% END %] [% END %]

    Both qualify as are for sure as DSLs.

    They are sub-languages which can be used by "domain experts" without any knowledge of Perl.

    And they are external, because neither SQL nor TT commands are implemented as Perl functions or follow Perl syntax.

    • SQL appears as embedded string inside Perl code is only parsed after leaving Perl thru DBI
    • And TT may be parsed with Perl but is not parsed as Perl.
    So far for external DSLs , but what are good examples for internal DSLs in Perl?

    Well designed traditional modules and classes are sometimes close to represent a sub-language but they still come with a lot of boilerplate.

    For instance an expert in the domain of TCL/TK may (probably?) not care much if the "host" language is Perl or Python or Ruby as long as he can call the commonly known methods. (see for comparison of snippets in various languages).

    But the necessity to constantly repeat an $obj-> or PACK::AGE:: on the left hand side of a command doesn't make it a completely "fluent" approach.

    (On a side note: avoiding to repeat this LHS for method calls seems to be the principal mechanism in Ruby for transforming a simple class to what Rubyists like to call a "DSL", a bunch of function calls inside a block are somehow interpreted as method calls on the same object, hence very similar to the with(obj) {} mechanism in JavaScript - please correct me if not)

    Now, what are examples for internal DSLs in Perl?

    The only widely known one which comes to my mind is CGI::HTML::Functions which used to be part of CGI

    For example, here's one way to make an ordered list:
    print ul( li({-type=>'disc'},['Sneezy','Doc','Sleepy','Happy']) );
    This example will result in HTML output that looks like this:
    <ul> <li type="disc">Sneezy</li> <li type="disc">Doc</li> <li type="disc">Sleepy</li> <li type="disc">Happy</li> </ul>

    Nice but IMHO it could be even more fluent.

    But are there others?

    There is for instance Rex which I suppose was inspired by Ruby's Puppet

    use Rex -feature => ['1.0']; desc "Get Uptime"; task "uptime", sub { say run "uptime"; }

    And I think I could theoretically implement most Assembler languages as Perl code just with some minor changes. I just need to implement ASM-commands as uppercase subs, quote literals and adjust the comment symbol.


    CPX '3' ;# Year starts in March to bypass BCS 'MARCH' ;# leap year problem DEY ;# If Jan or Feb, decrement year MARCH: EOR '$7F' ;# Invert A so carry works right ...
    (original example)

    But there must be more examples of internal DSLs in Perl ...

    Could you please help me to find and classify them?

    Cheers Rolf
    (addicted to the Perl Programming Language and ☆☆☆☆ :)
    Je suis Charlie!

RFC: Confirming module being tested
No replies — Read more | Post response
by RonW
on Jul 31, 2017 at 19:33

    In #p5p finds your lack of failing tests disturbing, there was discussion about Perl no longer including the current directory in @INC and how this would affect modules that load other files. In the referenced post,, the author, Mr Fredric, seems concerned that testing environments, such as Test::More may not be working correctly.

    From that, it occurred to me that it might be wise for tests to confirm that the module being tested is the one the test expects.

    Today, the noderep ticker "reminded" me about this concern, so I wrote a first draft of a version confirmation test:

    use 5.006; use strict; use warnings FATAL => 'all'; use Parse::CPAN::Meta; use File::Spec::Functions qw(catfile); use Test::More tests => 2; BEGIN { diag( "Testing My::Module" ); use_ok( 'My::Module' ); } my $loaded_ver = $My::Module::VERSION; my @metafiles = qw( META.json META.yaml ); my $md; for (@metafiles) { my $mf = catfile('..', $_); next unless (-f $mf); $md = Parse::CPAN::Meta->load_file($mf); next unless (defined $md); if ($md->{version} == $loaded_ver) { pass('version check'); } else { BAIL_OUT('Expected ver ' . $md->{version} . ", got ver $loaded +_ver"); } } BAIL_OUT('Missing META.json and META.yaml') unless (defined $md);

    Edit: Corrected typo in title.

    Update: Use catfile()

I love Perl and I love Mojolicious
1 direct reply — Read more / Contribute
by coetry
on Jul 31, 2017 at 00:53
    I haven't done any web development in perl before today and I started with Mojolicious. I've worked with Rails apps before and that's where I learn about models, views, controllers, routes, etc. I understood what happens when a user sends a request from the client and how that request gets handled and served back by the browser. The Ruby ecosystem has a ton of gems (libraries) specifically tailored towards Rails and its definitely a huge full featured framework sort of like how Perl is such a full featured language. I've also actually played with other ruby web frameworks like Sinatra. I'm able to say with my experience with other languages and frameworks that web development with Perl and Mojolicious is very fun. I'm new to Perl in general but have fallen in love with its free-form syntax, plethora of operators, expressiveness, clever community, and of course the documentation is unparalleled to anything I've seen elsewhere. The fact that Mojolicious makes it so easy to expose our crafty Perl hacks from our terminals to the web in such an organized, clean, simple, and fun manner truly makes me smile. I do know that Mojolicious isn't the only option out there to do this. I've heard great thinks about Dancer2 and other options as well. I'm not sure what I'll end up building, but maybe I'll share snippets of that journey in due time!
Perl Security Testing
4 direct replies — Read more / Contribute
by zentara
on Jul 24, 2017 at 10:02
    Hi, the Test Driven Development, for software and for pancakes node interested me, and I went off on a tangent from talexb's original meditation. So I post a new meditation, with my reply as a starter.

    Original reply: ##########################

    I'm a total amateur compared to you fellows, but I do find when I write my code, for the first draft, I almost always print out arrays and variables after everytime I use them. I almost always get things wrong the first time thru, so my method is very helpful to me.

    It's my guess is that the reason TDD failed is that the Test that you didn't account for, is the one that causes the bug, ( if any).

    What is more worring to me is the security vulnerabilities which Perl5 is susceptible to.

    For instance, could a normal or guest user on your machine, with access to Perl scripts, cause a buffer-overflow of some sort, and gain root access? I'm sure the NSA would pay for that information. :-)

    How safe is Perl out there in the wild? Are systems being hacked thru Perl? As far as know, Perl has been very safe in my limited use. I guess security is the number one test.

    So what do you experts feel, know, and or are hiding concerning Perl's security, assuming the scripts are written and run correctly? Was there ever a real buffer overflow exploit? etc

    Should I worry about other users on my linux box getting root escalation if I let them login?

    I'm not really a human, but I play one on earth. ..... an animated JAPH
Test Driven Development, for software and for pancakes
3 direct replies — Read more / Contribute
by talexb
on Jul 23, 2017 at 11:16

    It's Sunday morning in Toronto, so that means pancakes for breakfast.

    I use the same recipe from the Joy of Cooking that I've used for the last fifteen years or so, which starts with me heating up my two cast iron pans to a 4.5 setting. After making the batter, I make just one pancake in the middle of each pan, and check that they cook at the right speed to confirm I have the correct pan temperature. And I taste the finished pancake, to make sure I haven't messed up the recipe. (Yes, it's possible to forget a crucial ingredient when following a well-worn recipe -- it's amazing what forgetting salt does do a dish.)

    I make pancakes four to a pan, so they're 'silver dollar' pancakes, not more than three inches across. I monitor the colour and the speed at which they're cooking as I go, adjusting the heat if necessary. Finished pancakes go into a covered dish to stay warm.

    I believe I'm a careful cook, and I'm also a careful developer. That means test early, and test often. You want no surprises when it's time to dish up the goods to your family (pancakes) or your customers (software). Dreaming up new dishes or new applications it's important to be creative, to try new approaches. Cooking or writing software, you have to be methodical, thoughtful, and meticulous.

    I consider myself very lucky to have gone to a really good university, but what I studied (Systems Design Engineering) bears only a tangential relationship to the work I do now. It was actually really good training at reading/comprehension, time management, mental gymnastics, and getting the job done. As long as another program had the same goals, I think pretty much any degree or diploma would have done. An Engineering degree from Waterloo just helped me get my foot in the door that much easier.

    Conclusion? Do your development with tests, right from the start. You'll thank yourself, again and again. And it makes for good pancakes. ;)

    Alex / talexb / Toronto

    Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

Perl, JavaScript and Strandbeests
2 direct replies — Read more / Contribute
by cavac
on Jul 20, 2017 at 08:19

    A little over a year ago, i have been playing with AI::Genetic, Javascript::V8 and Strandbeest evolution for a Linuxdays Demo. I wrote this originally as a distributed genetic evolution demo, complete with managment web interface, but for the sake of clarity, here is the single-user command-line version of it.

    (I might do another meditation later that show more of the distributed stuff, but my Raspberry Pi cluster is still in a packing crate in the attic... i hope... after moving house in a chaotioc fashion).

    Before i begin, you should note that the Javascript::V8 version on CPAN has a memory leak. You might try my own, non-official version at It is in a mercurial repo, so you should be able to do this:

    hg clone cd JavaScript-V8 perl Makefile.PL make test install distclean

    The magic of Strandbeests

    I was always fascinated my Strandbeests, the huge moving "animals" created by Theo Jansen. Although mostly made out of PVC tubes, they exhibit a really strange, lifelike movement.

    The key part of this creatures of the length relationship between their eleven leg parts that lets them walk with very little effort. As Theo put it: "But even for the computer the number of possible ratios between 11 rods was immense. Suppose every rod can have 10 different lengths, then there are 10,000,000,000,000 possible curves. If the computer were to go through all these possibilities systematically, it would be kept busy for 100,000 years. I didn't have this much time, which is why I opted for the evolutionary method."

    Computers? Doing evolution? Hell yeah, i needed to learn how thats done!

    Why JavaScript?

    While i'm a competent developer when it comes to PostgreSQL databases, HTML and similar stuff, a lot of the math required for analyzing and evaluating the movement of a Strandbeest leg is completely outside my comfort zone (i.e. i have absolutely no clue how to do it).

    Thankfully, someone on Stackoverflow made a nice Javascript demo, which (for my Linuxdays thing) also solved the HTML visualization problem for me.

    Perl to the rescue

    This is where i came in. Now i had two options. Either convert the JS code to Perl (and keep it compatible with the JS version, so the HTML visu was in sync), or just use Javascript without the drawing functions from within Perl. I chose the latter, because it seemed easier.

    To run the genetic evolution algorithm itself, i chose the AI::Genetic module, since this seemed the easiest interface for what i needed.

    Before we begin, we need to define (which is a simplified copy from the one in my maplat_helpers repo):

    We also need James Coglan's Sylvester Vector and Matrix library for JavaScript, saved as "sylvester.js"

    And last but not least in our list of "support" files, the Strandbeest evaluation function itself, saved in a file with the name "strandbeest.js". This is the one with all the graphical functions removed:

    Now that we have to copied portions of our code, let's write some of our own.

    This is basically our caller for the main module and runs the main loop. First we check for some command line arguments, set up the evolver, and then run in an infinite loop.

    We want to support 4 different arguments:

    • --load Loads previous best 3 fits from fittest.dat and seeds the population with them.
    • --save Every time we find a new best fit, we save the best 3 members of the population to fittest.dat.
    • --population_size=1000 Set how many members does each generation have.
    • --crosspolination_count=10 Set the level of cross-polination during evolution.

    Here is the code (file ""):

    Most of what does, is a pretty straight forward wrapper around AI::Genetic, except the Javascript-Handling:

    use JavaScript::V8; ... my $vectorjs = slurpBinFile('sylvester.js'); my $strandbeestjs = slurpBinFile('strandbeest.js'); my $js = $vectorjs . ' ' . $strandbeestjs; ... my $ok = -1; my $errortype = 'MATH_ERROR'; my %scores; my $ctx = JavaScript::V8::Context->new(); $ctx->bind_function(write => sub { print @_ }); $ctx->bind_function(setFailed => sub { $ok = 0; $errortype = shift @_; }); $ctx->bind_function(setFinished => sub { $ok = 1; %scores = @_; }); $ctx->bind(params => \%params); $ctx->eval($js); my $error = $@; my $total = -1000; # Default: Failed! if(defined($error)) { print("SCRIPT ERROR: $error\n"); } elsif($ok == -1) { print("Script didn't call setFailed() or setFinished()\n"); } ...

    Here is the full code of

    While i'm a bit sparse here with explanations (never was the type who could do the teaching stuff), it should give you a nice overview of both how to run Genetic algorithms in Perl, as well as how to use existing JavaScript code to solve a problem.

    Additional information

Fun with Prototypes
4 direct replies — Read more / Contribute
by haukex
on Jul 19, 2017 at 07:54

    Can you tell me:

    1. How does this get parsed? (e.g. try adding parentheses to show precedence)
    2. Given that the body of each of the four subs is { print Dumper(\@_) }, what is the output?
    $_ = "#x"; my @a = qw/b c/; foo /#(\w+)/, @a; bar /#(\w+)/, @a; quz /#(\w+)/, @a; baz /#(\w+)/, @a;



Error reporting in Perl
6 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 12, 2017 at 20:41
    Use of /c modifier is meaningless without /g at (eval 14) line 1, <__A +NONIO__> line 4.

    Would it not be nice to see the offending string in an error code, taint approved? And if it's not a string... Then it's not Perl.

    I slay me,

Barstool Chatter
3 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 09, 2017 at 21:00
    What will the pundits say Perl can't do next?
    $pundits = shift
Goedel's Proof of God in Perl
2 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 09, 2017 at 08:38
A meditation on the naming of perl6
4 direct replies — Read more / Contribute
by stevieb
on Jul 07, 2017 at 09:38
3 direct replies — Read more / Contribute
by Jambo Hamon
on Jul 06, 2017 at 21:21


    How many meditations does it take to reach ``Knowledge of Self''?

    The following links to the randomly choosing elements from an array node. It is meant to show the reflective nature of $_ and is not an excercise in using or creating a link. Display purposes only. :] +&uact=8&ved=0ahUKEwj0yqChgPbUAhVBeD4KHXnPDJsQFggwMAE&url=http%3A%2F%2 +0from%2520an%2520array&usg=AFQjCNEYUua_3UVPyzYBRG4ruf_ZfYRTXw

    If you squint a bit you can see the GET string looks a lot like Perl code?

    a) It's a string with lot's of punctuation.

    b) It's lexical in nature, meaning it's value is only apparent inside the context of the page "url", coincidence? :]



    I thought one of the purposes of the meditations page was to have meditations.

    I mean that I could try to explain what I was thinking when i though let's paste this link from google as a meditation, but isn't it better to just meditate on it?

    Having fun if you are,

Mahalo - Thank you in Hawaii
1 direct reply — Read more / Contribute
by Jambo Hamon
on Jul 06, 2017 at 19:00
    I just want to say something publicly; Even though it might be taken a million different ways. I want to say thank you to the culture of Perl for engaging me. Culture has many different contexts, and the many different abilities of Perl is what makes it work.
Contemplating a documentation series for my Raspberry Pi work
5 direct replies — Read more / Contribute
by stevieb
on Jul 03, 2017 at 13:53

    So, after over a year of learning, development, implementation and testing, I am very near the point where my Raspberry Pi Perl software is ready for prime-time use by the masses. After a few more tweaks, test additions, documentation updates and one last once-over, I plan on writing a "Howto Pi with Perl" type tutorial, across several articles in a series.

    Trying to put together some sort of sane format, I'm looking to those who have publishing experience or technical writers to give my initial thoughts a once-over and provide me any feedback. Most, if not all portions will include breadboard diagrams of each task, code (of course), pictures if reasonable and in some cases potentially even video clips.

    Initial, broad-view structure:

    • very brief intro to the Pi, its capabilities, and overview of the features/limitations of the software
    • a synopsis of what will be being taught, formats/conventions that will be used
    • basic intro on creating and using a Pi object
    • intro to the most basic part of the Pi, the GPIO
    • examples of creating and using a GPIO pin at the most basic level (INPUT, OUTPUT, HIGH and LOW etc)
    • intro and examples using advanced pin features (interrupts, PWM, pull up/down etc)
    • workshop: using simple pin-based sensors and devices (LEDs, hygrometers, thermo-resistors, distance sensors etc)
    • detailed overview of the communication mechanisms included (I2C, Serial, SPI etc)
    • workshop: detailed overview of using the comms, starting very basic, leading into writing your own device/sensor modules using the software
    • workshop: using an LCD and/or OLED screen to display things on it
    • workshop: intro to and using Analog to Digital converters
    • workshop: intro to and using Digital to Analog converters
    • workshop: intro to and using Digital Potentiometers
    • workshop: intro to and using shift registers (basic through to advanced usage)
    • workshop: using a GPS module
    • workshop: advanced uses: utilizing several ICs and sensors in much larger and more involved projects (probably put together a few of these)
    • workshop: connecting to and using features of an Arduino with the Pi (over varying communication methods, including example Arduino sketches)
    • how this software came about, and some basic information on the underlying parts that allow it to work
    • extensive article on how I ensure the code is fully tested after each commit on a custom hardware test platform
    • possibly some details/photos on some of my real life projects I use at home (my indoor grow room automation system for example)
    • possibly a demo example of writing a web UI with jQuery to automate and provide a GUI of a basic project

    update: I think I'm going to use this as my initial template, so I may periodically update it with feedback and new ideas.../update

    For each element that contains a specific topic, include, where feasible/available a section at the bottom of the doc for advanced users perhaps in <small> or equivalent tags (these are just thoughts, nothing firm):

    • link to datasheet
    • list of issues or possible enhancements
    • link to relevant section in the Pi datasheet
    • link to the specific module that houses the code
    • link to a C/C++ example
    • link to Python equivalent

    Any thoughts on this general first thoughts layout? Anything that I should include that's not already? Anything I should remove? How about any thoughts on the individual elements and the manner in which to present them?

    I think I will create the actual demonstration parts first, then weave in the commentary thereafter. At first thought, this seems like the simplest approach that'll allow me to review all of the actual working parts as I go back through and edit in commentary.



Add your Meditation
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and all is quiet...

    How do I use this? | Other CB clients
    Other Users?
    Others contemplating the Monastery: (11)
    As of 2017-11-20 12:45 GMT
    Find Nodes?
      Voting Booth?
      In order to be able to say "I know Perl", you must have:

      Results (286 votes). Check out past polls.