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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: Monads in Perl (Send + More = Money)
1 direct reply — Read more / Contribute
by LanX
on Dec 02, 2020 at 13:10
    One of the attendants at online meeting yesterday pointed to these blogposts of MJ Dominus somehow complaining that Perl is too "clumsy" to reimplement a Haskell solution based on monads


    Basically you have a nice linearized syntax implementing nested loops to find the solution for a number riddle

    Let's say for concreteness that we would like to solve this cryptarithm puzzle:
    S E N D + M O R E ----------- M O N E Y
    This means that we want to map the letters S, E, N, D, M, O, R, Y to distinct digits 0 through 9 to produce a five-digit and two four-digit numerals which, when added in the indicated way, produce the indicated sum.

    the proposed Perl code was actually the semantic translation of some Python code and was indeed clumsy (NB all those curlies at the end)

    Now here my solution reusing some work I've done in the past with List Comprehensions

    use strict; use warnings; use Data::Dump qw/pp dd/; =pod =cut # --- List comprehension sub from (&$;$) { my ($c_block, undef, $c_tail) = @_; my $var = \$_[1]; sub { for ( &$c_block ) { $$var = $_; $c_tail->() } } } sub when (&$){ # guard my ($c_block, $c_tail) = @_; sub { $c_tail->() if &$c_block } } # --- rem() Helper function to return digits 0..9 except @_ my %digits; @digits{0..9}=(); sub rem { # set difference my %h = %digits; delete @h{@_}; keys %h; } my ($send,$s,$e,$n,$d); my ($more,$m,$o,$r); my ($money,$y); my $do = # send from { rem 0 } $s => from { rem $s } $e => from { rem $s, $e } $n => from { rem $s,$e,$n } $d => # more from { rem 0,$s,$e,$n,$d } $m => from { rem $s,$e,$n,$d,$m } $o => from { rem $s,$e,$n,$d,$m,$o } $r => # money from { rem $s,$e,$n,$d,$m,$o,$r } $y => # guard when { "$s$e$n$d" + "$m$o$r$e" == "$m$o$n$e$y" } # output sub { pp [ "$s$e$n$d" , "$m$o$r$e" , "$m$o$n$e$y" ] } ; &$do;

    Compilation started at Wed Dec 2 19:11:14 C:/Perl_524/bin\perl.exe -w d:/tmp/pm/ [9567, 1085, 10652] Compilation finished at Wed Dec 2 19:11:18

    NB: It could be done better and faster, I've just coded this POC from scratch within an hour.

    Please note that making it "lazy" is not really a problem, left for the interested reader.

    So Monads in Perl are not that complicated ... or what am I missing? ;-)

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery

A readline + -e oddity: readline opens files even with -e --- need "readline STDIN" to read from console
4 direct replies — Read more / Contribute
by cxw
on Nov 30, 2020 at 10:32

    (This is my first meditation. I am posting in hopes it will help someone down the line! I can't find another node about this, but that doesn't mean there isn't one :) .)

    I needed to run a program and then wait for the user to hit Enter. I tried this:

    $ perl -e 'system(@ARGV); readline' echo foo foo Can't open echo: No such file or directory at -e line 1. Can't open foo: No such file or directory at -e line 1.
    Not what I expected!

A short whishlist of Perl5 improvements leaping to Perl7
9 direct replies — Read more / Contribute
by rsFalse
on Nov 24, 2020 at 03:07
    As Perl7 is announced (Announcing Perl 7), and it will be "v5.32 but with different, saner, more modern defaults".
    I'll suggest a list of improvements:
    • Make keyword "my" optional. It MUST be unnecessary to declare all lexical variables as lexical if 'use strict' is ON by default. If 'use strict' is ON then so called 'automine'(Re^4: Opinion: where Perl5 wasn't attractive for me) should be ON by default also. I mean 'my' becomes redundant, isn't it?
Modules design pattern: abstraction vs incarnation (providing not so static data)
4 direct replies — Read more / Contribute
by Discipulus
on Nov 20, 2020 at 07:23
    Hello monks,

    this post is a follow up of Module design for loadable external modules containing data but, after I squeezed my mind to spot the core of the problem and after finding some valid approach, I think it is worth a new Meditation.

    A dark corner where the problem raise

    In the past when I found myself, facing an error or an unexpected behaviour, thinking: "it is perl or it is me?" it was always my fault. This time maybe not. Or maybe this is a problem in the way I imagine new things and perl does not make it easy as usual. What I mean is: modules are intended as an abstraction of behaviours and scripts are generally the incarnation of these behaviours in the real life. A module can be loaded and can export subs or methods. A module can be tested because generally it does nothing but exporting abstract behaviours. Only in few rare cases a module exposes data and if so it is just some bare package variable intended to modify its internal behaviour like $Data::Dumper::Indent and even this simple use can be accomplished in other, nowadays preferred, ways like providing parameters to the constructor and providing accessors for these kind of things.

    What happens if I need a new design pattern? I still need an abstraction, obviously, but I also need a serie of incarnations to be loaded indipendently upon request. These are not plugins that extend the main module with new functionalities: they are different incarnations of a mother abstraction.

    In the Perl::Teacher example I need the abstraction of Perl::Teacher::Lesson but then I need the incarnation of Perl::Teacher::Lesson::first_lesson and Perl::Teacher::Lesson::second_lesson and so on.

    Another project of mine ( my mad and fun Game::Term :) is stucked exactly for the very same reason. Infact I started to code ignoring the above problem and I have designed it to have incarnations (game scenarios in this case) as standalone scripts and this approach lead me to shell out when changing the current scenario, messing the whole thing (shelling out it is always a bad thing and behaves very differently on different platforms like linux and windows).

    A note about data: in the current post and in the previous one with data I intend not static data but a possible longish serie of perl data as others objects, anonymous subroutines, mixed with some (few) more static fragments as texts and questions.

    Abstraction/Incarnation and OO roles

    I was suggested to use a role for this. Honestly I'm not a big fan of OO perl frameworks, or better saying it, I will be a big fan when I will have the need to use all the feautures they provide.
    I understand a role as a trasversal behaviour applicable to different kinds of objects, traversing the simple schema of father-children inheritance. The classic example of the breakble role can be consumed by very different classes of objects like bones, cars, cookies.. But I have many incarnations to one and only one abstraction. So no transversal roles to compose. Well.. I can transform my Perl::Teacher::Lesson into a role, let say Perl::Teacher::Teachable but I cannot see any advantage over a simple inheritance.

    A new design pattern?

    What I imagined is an orchestrator, a super-object (in this case of the Perl::Teacher class, but the same is valid for the Game::Term project) instantiated inside a program shipped within the main distribution.

    This super-object will be able to do many things related to all teaching activities (reading configuration file, interact with the user..) and its Perl::Teacher distribution will include the abstraction of what a course, a lesson, a talk, a question and a test are. But the main activity will be to load a course and its lessons in sequence.

    Real courses are just containers of real lessons and are shipped separetly from the main distribution. Here is the new design pattern I see.

    If you look at solution I provide below, you will see I prefere to use the constructor of the incarnation module to ship the meat to the super object. This pattern is vaguely similar to routes in modern web frameworks: fatty subroutines of behaviours and data. Well... in web programming we were told to separate the logic from the presentation and this sounds sane. I think my case is a bit different because I have mainly perl data (objects of other classes like Perl::Teacher::Test or Perl::Teacher::Question filling a lesson) in my incarnations. There is not static data to serve (as templates of html in the case of web programming) and for this reason I dislike the idea to have external, yaml or json, files containing the data to be served. Infact to use static external files I have to strictly describe their format and I loose the flexibility of a perl module (for example a lesson can provide a special kind of test defining a custom sub or loading another module).

    Here I present some sketch of my approaches, for sake of semplicity not in seprated .pm files but I think you will get an idea.

    Possible approaches

    First option: use the constructor of the incarnation module

    This is my preferred one.
    The abstraction module ( My::Lesson in this example ) defines methods usable by its children. Children (incarnation modules) use the new constructor to fill in the object with all the data it needs. Filling the object is done using methods provided by the abstraction module My::Lesson

    use strict; use warnings; # ABSTRACTION package My::Lesson; sub new{ my $class = shift; return bless { steps => [] }, $class; } sub add_step{ my $self = shift; push @{ $self->{ steps }}, @_; } # INCARNATION package My::Lesson::Example; our @ISA = ( 'My::Lesson' ); sub new{ my $class = shift; my $self = $class->SUPER::new; $self->add_step('one','two'); return $self; } # USAGE # Please note that this is only an example of usage. The real one will + be done by an object of the Perl::Teacher class, like in # # $teacher->current_lesson( My::Lesson::Example->new ) # # or something similar package main; my $lesson = My::Lesson::Example->new;

    Second option: using a bare EXPORT in the incarnation module to provide data

    The abstraction module does not need to provide methods to its children. The abstraction module becomes an incarnation loading a data structure from My::Lesson::Example

    # INCARNATION package My::Lesson::Example; require Exporter; our @ISA = qw( Exporter ); our @EXPORT = qw( @steps ); my @steps = ( 'three', 'four' ); # ABSTRACTION package My::Lesson; use Module::Load 'autoload'; # or CPAN Module::Runtime sub new{ my $class = shift; my %opts = @_; # here must be some logic to load the appropriate incarnation +module. # paying attention to the @steps array (?) # # autoload from Module::Load should be the right tool (not tes +ted) # autoload $opts{lesson}; warn "autoloading $opts{lesson}"; return bless { steps => [ @steps ] }, $class; } # USAGE package main; my $lesson = My::Lesson->new( lesson => 'My::Lesson::Example' );

    Note: first and second approach can be mixed letting the incarnation module authors to use the interface they prefere. Something like this should be enough:

    # ABSTRACTION package My::Lesson; use Module::Load 'autoload'; sub new{ my $class = shift; my %opts = @_; if ( $class->isa( 'My::Lesson' ) ){ return bless { steps => [] }, $class; } elsif ( $class->isa( 'Exporter' ) ){ # autoload $opts{lesson}; warn "autoloading $opts{lesson}"; return bless { steps => [ @steps ] }, $class; } else{ die "incarnation method not recognized for class [$class]! +"; } }

    third option: static data

    File::ShareDir::Install allows you to install read-only data files from a distribution and File::ShareDir will be able to retrieve them. For reasons explained above I dont intend to use this solution for the Perl::Teacher project, but can be a viable solution, for example, to provide static maps to my scenarios in the Game-Term project. Thanks to kcott to show these modules. Anyway I dont see any advice against using __DATA__ in a module (if it is not a enormous amount of data): it will be accessible from within the module itself or from outside specifying the package name as described in Special Literals (taking care to close the filehandle when finished).


    It seems very weird to me that this design problem does not occured before. As always there is the possibility that it is me :)
    I tend to imagine very complex designs with a super-object able to rule and load a plethora of incarnations. In a ideal world my will consists of a mere use Perl::Teacher; my $teacher = Perl::Teacher->new(); $teacher->run;

    The ability to switch from an incarnation to another, at runtime, in Perl is only available through modules. So I need an easy way to provide this mechanism and a clear interface to propose to incarnation modules authors, if any.

    I think my preferred solution, providing the meat in the incarnation module constructor, it is not so nice as interface for eventual authors but is the more perlish one I found at the moment.

    Any comment, suggestion, inspiration will be welcome.

    Thanks for reading.


    There are no rules, there are no thumbs..
    Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.
RFC: Perl Learning Plan
8 direct replies — Read more / Contribute
by Leitz
on Nov 18, 2020 at 18:37

    Learning difficult topics like programming languages, human languages, or the thought processes of the opposite gender can be challenging. Over time, too much challenge can be discouraging and can end the effort before success is achieved.

    In my own learning efforts I have found great success in mapping out the core topics for my next level. Doing so lets me focus on what I need right now and avoid stress about not learning things beyond my current level goal. In Perl, for example, I should really know the syntax and uses of hash references before I dig into object oriented web frameworks. While the idea seems simple, and it is, it does not seem common. For me it has helped in everything from programming languages to things like human languages, shooting sports, writing fiction, and searching for new jobs. Pretty much every aspect of my life has been helped by making a good plan for the next step and ignoring everything else. Except for the "opposite gender" bit, I'm still working on that one...

    Here's my learning plan for becoming a well rounded Perl Apprentice. The goal is to have the basics down solidly so that I can begin my journeymanship. Until these topics are firmly embedded in my head and skills, I do not need to be trying to do Expert Perl stuff.

    Data types and sigils

    • scalar, list, aray, hash, subrouting, references, objects
    • Variable assignment and Scope
    • Context
    • Autovivication

    Control Structures

    • if, for, foreach, until, while, ...

    Pattern Matching

    • Just the basics


    • reading and writing
    • tests and operations
    • Directories


    • Loading and using Standard Library modules
    • Using CPAN


    • Documenting code


    • Writing and using basic tests
    • Debugging


    • Input and output

    Process Management

Thank You from a Newcomer
3 direct replies — Read more / Contribute
by Leudwinus
on Nov 16, 2020 at 21:27

    Wow! I am truly humbled and indebted to all of you who took the time to help me out on my earlier questions. This was my first post here so I was a bit hesitant as I only joined a few weeks ago and wanted to ensure I did my homework before posting. Apologies if I was not able to respond to everyone’s replies but I feel that I must digest everything you’ve provided.

    (I'm posting this here because I was getting embarrassed by how long my other thread was getting.)

    I’m still a novice programmer and even more of an apprentice when it comes to Perl. Reading through the documentation, my head is starting to spin. My question originated because I was trying to come up with my own subroutine to determine permutations of elements in a array and I was getting lost in how to structure the subroutine using recursion (rest assured, questions on that topic are forthcoming!) But it appears to me that I still have more of the fundamentals to learn.

    If I’m honest, some of the shine of Perl is starting to fade but the quality and quantity of help I have received here are motivating me to continue!

My 20th PerlMonks anniversary
4 direct replies — Read more / Contribute
by dmitri
on Nov 14, 2020 at 17:37
    On this date 20 years ago, I created my PerlMonks account. It had served me well. I lurked more than posted. I learned a lot and when I did ask questions, I almost always received an answer. Thank you all. My Perl programming career would have been worse without you.
Greetings to all perlmonks
1 direct reply — Read more / Contribute
by Gado
on Nov 01, 2020 at 10:56
    I some how lost access to my email. This has prevented me from having access to my old account on here. I greet all monk. I use Perl but not always. It's been a language that I love and am passionate about it. But I am not consistent in learning and using it. I hope by being active on here I will learn from others and make new friends, and I hope that will help me make use of the language more often.
Why am I always too late?
1 direct reply — Read more / Contribute
by karlgoethebier
on Oct 16, 2020 at 09:28

    I don’t know and it doesn’t really matter. Some may ask why I ask. Because of by chance I «discovered» this. Very nice. See also FFI::Platypus.

    «The Crux of the Biscuit is the Apostrophe»

    perl -MCrypt::CBC -E 'say Crypt::CBC->new(-key=>'kgb',-cipher=>"Blowfish")->decrypt_hex($ENV{KARL});'Help

When is it time to stop posting to CPAN?
8 direct replies — Read more / Contribute
by perlfan
on Oct 11, 2020 at 06:13
RFC: searching volunteers for wireless library
1 direct reply — Read more / Contribute
by Bpl
on Oct 10, 2020 at 13:42
    Hi monkers!
    some months ago I started the development of Air::Wireless: a pure-perl library which wants to replace wireless-tools(link: )
    At the moment I have not enough time for a serious development of the library, but if someone wants to contribute I'll happy to work on it :)
    Every contributor will be cited in the "contributor" section in every Air::* sections.
    Obviously also Air::Crack ( ) and Air::Reaver ( ) needs more development!
    P.S I forgot to include the link for Air::Wireless
    Edoardo Mantovani, 2020
RFC: RTE::Dump suggestions
2 direct replies — Read more / Contribute
by Bpl
on Oct 10, 2020 at 13:34
    Hi Monkers,

    Recently I have released a new perl library: RTE::Dump

    if anyone wants to test or contribute to the project let me know :P


    Edoardo Mantovani, 2020

Air::Lorcon2 officially out!
2 direct replies — Read more / Contribute
by Bpl
on Oct 04, 2020 at 11:18
    Hi Monkers,
    After some months, finally the first version of Air::Lorcon2 is on github:

    If interested in participating everyone can write an e-mail to:

    Also suggestions, opinions and feedback are well accepted!

    Thanks for every help from this site!

    Edoardo Mantovani, 2020

Thank you!
2 direct replies — Read more / Contribute
by NK7Z
on Oct 03, 2020 at 15:53
    Just a short note to say thank you to the folks that answered someone else's question on why a Perl script failed to run via cron, but ran under the users environment...

    I did a search, found the answer to the other fellows question, (use of relative directories), changed my directories to absolute, and all works now... So... THANK YOU!
2 direct replies — Read more / Contribute
by Tux
on Oct 02, 2020 at 11:43

    There are streaming modules on CPAN that support gzip and Bzip2 and many more based on PerlIO::via, all using syntax like:

    # compress open my $fh, ">:via(XXZ)", "test.xxz"; print $fh $text; # decompress open my $fh, "<:via(XXZ)", "test.xxz"; while (<$fh>) { ... }

    where XXZ is the required PerlIO::via::XXZ (gzip, Bzip2, ...) and .xxz is the extension that goes with it (.gz, .bz2, ...).

    I needed this for xz-(de)compression, but it wasn't there (yet), so I stole from existing modules and whipped up PerlIO::via::xz, which works for what I want.

    BUT, it fails in a scope where $/ = undef;, and I have no tuits to investigate.

    This has not (yet) been put to CPAN, and to be honest, I don't feel like it (yet).

    If anyone thinks this is a good thing to have and wants a kickstart in getting something ready to distribute, please contact me: you're welcome to grab it a use it to make it *your* (first) distribution.

    Enjoy, Have FUN! H.Merijn

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