Laufeyjarson writes... » Perl: PBP: 023 Identifiers

This section of the PBP is a long one, and goes into great detail about how to think about naming important parts of your programs.  It covers explicitly modules/namespaces, variables (several types of them), and functions.

I thought, at first glance, it was way too picky and specific, but the more I read it, the more I liked it.

I also discovered that it’s what the cleanest code I’ve worked with does already, either deliberately or through evolution.

The naming for namespaces is described as Noun::Adjective::Adjective…, and that seems reasonable.  I know at work we put an extra thing in front, to put it firmly in the company namespace, then follow those rules.  It makes all our specific stuff start with the work namespace, which can be redundant, but makes it blindingly clear what parts are internal and private, and what parts are general and could be shared.

Naming for functions is also well thought, and goes into quite some detail.  I’m not sure I would have expressed all the detail, but we should all remember that Mr. Conway has also contributed to the Lingua::EN namespace, and understands these issues well.  In this case, I think it served well as he could be clear and correct.

Variable names are also discussed in some detail, with good results.

I admit, I often use short items for nearby, local things, or things that will appear a lot.  I still use loop variables with one letter, especially when they only exist inside a five line long for loop. They are not as clear as they could be, but they try and keep the comprehensibility of the block as a whole high by not requiring more understanding.  That’s probably where I fail the most in these habits.

PAL-Blog: Zoe's Chorpremiere: Die Birkennase

Eine Woche nur Zoe-Posts - darf auch mal sein. Eine Woche zuvor hatte sie gleich zwei Auftritte, letzten Samstag folgte dann der dritte (und vorerst letzte geplante): In wesentlich formellerer Atmosphäre übernamen die Waldgeister den ehrwürdigen Burgwedler Amtshof und nur der Burgwedler Kinderchor schaffte es mit vereinten Kräften Stimmen, sie wieder auszutreiben.

Perl Foundation News: Perl::Lint Progress

During June, Taiki Kawakami continued work on his grant to write Perl::Lint, a static analyzer for Perl 5.

Taiki has implemented more than half the policies and has almost finished work representing complex structures such as block scope.

If you would like to see this work in progress, please take a look at the project's public GitHub repository or attend Taki's talk about Perl::Lint at YAPC::Asia.

If you have any questions or suggestions about this project, please post a comment on this post.

Hacking Thy Fearful Symmetry: Yanick At Perl Conference: Notes of Attendance (YAPC::NA)

It's hard to believe that Orlando was only my third YAPC. And yet, I only attended my first YAPC ever in Asheville in 2011. The next year I went long and had a taste of the ::EU namespace with Frankfurt. For 2013 I had to take a sabbatical for cause of a sprint/summer-long alluring game of strip-thyroid. And that brings us to 2014, and Orlando. This year I don't have any tall tale to tell, but nonetheless, here are a few of my notes and impressions for the event, hoping that they will somehow whet your appetite, and tempt you to come and join in the fun next year.


The show rolled like a well-oiled machine. The venue was a pleasant one, the coffee kept flowing, and good munchies were available during the breaks (honorable mention must go for a vegetarian option being present for the two buffets we had). The real-time streaming was nothing short of spectacular, and the list of talks and keynotes was superb.

Now, some grumbles have been heard about the low level of communication going on for this YAPC. I must say, information was not disseminated as well as it could have been. Both before the conference, and during the events (the BOFs weren't as strongly advertised as they should have been, I'm still uncertain how the t-shirt printing was supposed to work). But when you know how short-handed the organizers were, and how Life really had a good go at driving them insane, it makes you appreciate what they still managed to pull off. With a little luck, next year will see more volunteers, which should help with the communication, and the overall general workload (not to mention mental health) of all concerned.

Game Night

This year was the first time that I experienced YAPC::NA's game night. In a nutshell, the more official dinner party of the days of yore have been replaced by a logistically simpler buffet (this year Mexican-themed, and quite yummy), and a focus on games. The official game was Magic: The Gathering, but a lot of people brought other games, going from classics such a Scrabble, to party favorites like Cards Against Humanity, to more involved wargames like OGRE. The shift from the previous official-ish dinners is to promote more intermingling, and I must say that I like it. You grab some food, you pick a table, and you play. As an ice breaker, it's genius. And it's fun as heck.

As it happens, I landed at Ricardo Signes' table, and has he reported in his own YAPC blog entry, he had brought a few OGRE mini-kits. While the rules were pretty simple for a wargame, it was still quite the challenge to grasp all the details after a quick introduction and a demo game. But, in all honesty? I don't think anybody cared. Metal behemoths rolled on the battlefield. Desperate squadrons of men and balefully underpowered vehicles tried to make a stand. Hilarity, and much pyrotechnic demolition, destruction and discombobulation ensued, and we all had a blast.


Oh my did we have many keynotes this year. Which, mind you, I don't begrudge, because they were so darn good. Mark Keating and Dan Wright were quite informative. Larry was endearing as always (I swear, this man could give a 3-hour seminar on TCL and I would still be charmed by it), Matt Trout was his usual entertaining self. Charlie Stross was, hell, he was FRIGGIN' CHARLIE STROSS, how more cool could it get?

And the dual keynotes of Sawyer and Genehack on the last day were like the two sides of the same coin -- the bold, unbridled enthusiasm of one being met with the wise, weathered experience of the other. They uplifted, they stirred the soul. Ultimately, they begged the question: why are we doing what we do, and how must we grow to stay who we are. Those men are good. It was glorious.


You haven't slept all that much the night before. It's 1:30pm, right after a hefty lunch. The talk is interesting, and the person giving it is a good talker. But your eyelids feel so, so heavy. More coffee is in order as soon as you reach the break, but until then you'll have to soldier o-- oh my, you hope that nobody saw you sharply nod down and wake yourself up.

Sounds familiar? If so, do you want to know something that helps surprisingly well against those attacks of languor? Knitting! It's just involving enough to keep the body awake, but sufficiently automatic to let the brain concentrate on what's happening at the front of the room. I've been lugging my knitting around for a few conferences now, and I must say it works pretty well. I recommend. And might be tempted to give a quick tutorial one of these days. Which brings me to...

Off-Perl Track

This year, the organizers tried a whole track of talks dealing with extracurricular stuff. It went to other languages like Rust to more broadly general life-related things like coffee roasting. Frankly? I love the idea. Not only can it help draw outsiders who would not be that tempted by an all-Perl conference, but it broaden the scope of the knowledge we get to share and be exposed to during the conference to all the shady nuggets that are peripheral to our core activities. I really hope to see more of that during the upcoming years.


Within the premises of the conference, was the presence of three members of the Dancer crew: Sawyer, Mickey and myself. So it goes without saying that we /had/ to have a Dancer BOF. The BOFs were a little bit stealthy this year, so the crowd that showed up wasn't huge, but that's okay -- quality over quantity and all that. In all cases, we talked about the status of things, of when to use Dancer versus the other frameworks, of specific use cases, of plugins, of templates, of horror stories and of things to consider for the future. All in all, it was a pretty nicely filled hour.

And while we're talking about Dancer. If there is one thing that made me squee with delight at the conference, if had to be how much Dancer is used by the community. It's one thing to know that the project you're part of is somehow popular, but it's something else to see listed on the stack of serious systems and part of so many talks. That soooo made my day.

Meatspace Meetings: Massively Meaningful

So, the event was smoothly operated. The talks were informative. The overall atmosphere wholesomely pleasant. But I didn't mention yet why I really enjoy going to YAPC. Oh, it's a little bit for all those reasons, sure, but mostly? It's for something much greater: it's to rekindle with the Tribe.

It's funny. Even though emails, Twitter, bug reports, blog entries, IRC, and dozen of other interfaces ensure that we are interconnected in a diaphanous yet ubiquitous web, even though I personally thrive in written-based media, even for all that, gathering together for a few days once a year does us good. It's where we put a human face on our community, and find back why we belong.

In my experience, this human contact, this very literal handshake between two sides of a transmission, also works wonders to make sparks happen. YAPCs are not where things happen. Rather, and much better, it's where things are started, set in motion. Alliances are forged, potential projects are planned, seeds are thrown in many nooks and crannies.

So yeah, YAPC is mostly, for me, a human experience. A joyful, odd-bally and loving one that would not feel out of place in an old comedy from the Cohen brothers.

And that's why I'll do my best to be there next year.

Perl Foundation News: Maintaining the Perl 5 Core: Report for Month 9 (June 2014)

Dave Mitchell writes:

I spent June mainly processing my p5p mailbox, and looking at various bugs, half of which appeared (initially at least) to be 5.20 regressions.


6:40 [perl #121977] COWification seems expensive in PADMY variables
0:28 [perl #121984] newSVpvf slow in perl 5.20.0
0:28 [perl #122003] uses excessive CPU on OpenBSD
0:07 [perl #122024] PL_sv_no weirdness
1:16 [perl #122077] Bleadperl v5.21.0-274-ga7ab896 breaks SHURIKO/String-Simrank-0.079.tar.gz
1:39 [perl #122099] Perl regression bug since 5.13.11 (masked by CoW since 5.19.1
25:57 process p5p mailbox

36:35 Total (HH::MM)

4.3 weeks
36.6 total hours
8.5 average hours per week

As of 2014/06/30: since the beginning of the grant:

37.1 weeks
528.0 total hours
14.2 average hours per week

There are 272 hours left on the grant

Laufeyjarson writes... » Perl: PBP: 022 Automated Layout

The Best Practice is to get a tool to do the work for you; it’ll be regular, and not spend your time doing it.

My only concern is that Mr. Conway says, “You can take ugly code like this and turn it into something readable.” where the “readable” code isn’t, to my mind, that much better.  Spaces around the parens and an extra blank line do not readable code make.

In general, I agree with this.  It’s funny how rarely I do it, though.  This is because of many old habits which I find hard to change.

I am not much of a customizer.  I’d rather learn to use the tool as it exists than force it to work the way I want it to.  This is true of editors, languages, and environments.  It was a big deal when I changed the desktop picture on my new Mac.  It wasn’t stock; it was a change, and that was unusual.

I don’t do this because I don’t know how, or don’t want to, or don’t think you should do it.

I behave this way because I’m lazy.

Long ago and far away, I was a Windows software engineer.  This was back in the bad old days, when Windows ran on top of MS-DOS, and you had this giant teetering stack of software before you could do your work.

People customized the heck out of those machines!

And every two or three months, Windows would cough up a hairball, and you’d get to reinstall it all.

After a few reinstalls, you just quit caring about some of them.  A few more rebuilds, and you want to do as little as possible to get back to work.

Or, later, I was working in an environment where we rebuilt every machine in the lab via a test script.  Customizations had to be put in the script, which we all shared, and was hard.  It was lots easier just to use the vanilla.

So, my habit is not to customize things.

I don’t customize Visual Studio.

I don’t customize vi/vim.

I don’t get near Emacs with a ten foot pole.  People who love emacs customize it.  (I know enough Emacs to quit!)

I rarely customize the Unix shell – almost no aliases, etc.

That being said, I can use tools to tidy Perl code; makefile targets, scripts, etc.  And those are good things, and should be used.

But they have to be added, and they’re not by default.

I discovered I am using an editor lately with Perltidy built in – Komodo – and I’m using it a lot more.  It’s nice.  But it’s not always there, as I’m not always on a machine with Komodo.

So: Good idea I find hard to do. NOC: 7/18/2014: Scheduled Maintenance -- Moving Day!

More details afterwards, but most services will be unavailable for most of the day on Friday, July 18th 2014.  We're moving!  DNS will still work, but anything running on one of our machines (many webservers and mailservers, etc) will be unavailable.  We'll post updates here.

export TZ=America/Los_Angeles


  • 8:48AM - the first batch of machines shut down and powered off cleanly, which is a good omen. We're in transit to the datacenters at high rates of speed and the unracking will begin when we get there. 
  • 10:02AM 7/19/2014 - apologies for not posting any updates during the day yesterday, we were busy with actually doing the move.  Current state is that we're having trouble getting one of the key vm-hosting machines to boot, and the VMs on it include key servers.  We'll keep working on this this afternoon.
  • 2:00PM 7/19/2014 - the last machine is up and happy again.  everything should be working fine now.

Ricardo Signes: learning some new languages (maybe)

I wanted to make an effort to learn some more languages, old and new, more or less continually as time went on. I started with Forth and Go, and did a tolerable job at getting the basics of each down. I didn't do so well at writing anything of consequence, which isn't too surprising for Forth, but I'm pretty sure I could be writing a lot more Go to get things done. I really do mean to get back to that.

Although I feel like I'm only about 85% done with what I wanted to do with Forth, I'm thinking about what I'll look at next.

On one hand, Pragmatic Press is putting out Seven More Languages in Seven Weeks. The previous book covered Clojure, Haskell, Io, Prolog, Scala, Erlang, and Ruby. I thought it was just okay, but in part it was because I knew the languages well enough to see how the chapters could've been better. The new book is languages that I know, at best, by reputation: Lua, Factor, Elixir, Elm, Julia, MiniKanren, and Idris. Even if the book's only so-so, it's enough to get me going on a few weird-o languages. Also, from Forth to Factor? Woo!

On the subject of useless-but-influential languages, I just ended up with a pile of books on the matter. Stevan Little, Moose author and devotee of all programming languages past and present, is moving to the Netherlands, and couldn't take all his stuff. "You know who'd like a book on Algol-60?" he asked him self. "Rik!"

books from stevan

I'm not sure where I'll start. The Forth book isn't likely, as I've already got some. PostScript might be out for a while, since it's a bit Forthy on its own. Algol is a good possibility, or maybe Eiffel. (I just finished reading Smalltalk Best Practice Patterns, so I may be in the mood for more "here's how you do OO, kid" books.) I'm very interested in SNOBOL4 patterns, although I'll have to see if I can find an implementation I can run.

Probably all of this will have to wait, though. This weekend, OSCON begins, and I'll be taking in whatever I can there, rather than reading any dusty old books.

NEILB: A simple function exporter

I've recently adopted a couple of Exporter modules, and might soon be adopting a third. This has prompted me to look into exporter modules a bit more, and now I'm working on a review of all such modules (list below). While doing this I've been playing around, creating a minimal exporter that does just what I need.

Perl Foundation News: July 2014 Grant Proposals

The Grants Committee has received two grant proposals for the July round. Before the Committee members vote, we would like to solicit feedback from the Perl community on the proposal.

Review the proposal below and please comment there. The Committee members will start the voting process on July 26th and the conclusion will be announced by July 31st.

Perl Foundation News: Grant Proposal: Start ACT - Voyager

We have received the following grant application "Start ACT - Voyager". Please leave feedback in the comments field by July 25th, 2014.

Start ACT - Voyager

  • Name:

    Theo van Hoesel

  • Amount Requested:

    $ 1.000 DBIx::Class

    $ 2.000 Dancer(2) implementation

    $ 1.000 REST api

    $ 2.000 Theme Based templates

    $ 6.000 TOTAL


The Perl Community is a social community that gathers at several places around the world during Conferences and WorkShops and Hackathons. Since more than a decade ACT has been used to provide organisers of these events a central system for registration, payments, talk-submits and scheduling.

However, development has been stalled for a long time and many good initiatives has been surfaced and not yet materialised over the last years.

Start ACT, Voyager, will be a large project that will lay down the fundaments that allow for growth and expansion, based on top of the deep settled original ACT implementation.

Many of the ideas of 'BooK' about ACT2 and the work done by 'Getty' for YACT are inspirational for the roadmap to a new platform for the essential Conference Toolkit.

Benefits to the Perl Community

* we suck at marketing * is a phrase not to be proud of and as a result of that mind set, the main hub for Perl events is not catching up with courante user experience and not providing the means for front-end developers to change anything about it. Gradually, this will be a 'voyage' to upgrade the current ACT to modern standards so new features can be build upon it.

One thing it will provide is a 'Next Generation' REST api. This will allow others to write their own web-applications that will lift the load from the current ACT server tremendously when done by experienced front-enders. Clients are encouraged to cache their retrieved data locally and most of the mobile apps do rely on the presence of such api's.

Eventually in 'the Final Frontier' there will be a complete working theme-based template that allows for responsive web design, including user-profiles and, not less important, a web-base backend to manage 'instances' of conferences. With a few clicks organisers will be able to have a good looking website based on some default themes and layouts. For those that do not want these predefined templates, one can always setup their own server and make calls to the REST-api.


Too many feature-requests and ideas do exist but Start Act - Voyager is merely a road map and along that road the following are essential deliverables:

DBIx::Class Schema of the current ACT Database


A working version of ACT, build with the previously written Schema, which is a replacement of the ACT Core templates and modules.

REST api (the Next Generation)

Mostly responding to GET methods and a few POST and PUTsd

Theme-based Templates (the Final Frontier)

Easy to setup, possibly through a backend to ACT

Project Details

Some of the great ideas will be borrowed from the previous Grant for YACT. However it will not have all the nice bells and whistles that make this project insane big. As said before, it is a platform for growth end extensability and others can build more nice features into it. An ACT Hackathon will be a wonderful opportunity to extend with new features.

One email from 'BooK' contains a whole list of inspiring ideas on what the supposed ACT2 (api based) should be providing and next is a quote from his email:

"Working on an API makes it easier to focus on the data (which is the value of Act) and to leave working on other things for later (the web framework, the conference management tools, etc). I think Act2 should come up with a web application (with better support for conference-specific templates), but should not be tied to it (so that others can do something else with it, like a smartphone app)."

It would be too much to write down all the details, but that quote shows quite clearly which direction StartAct - Voyager should be going.


Since this is quite a huge project, it is hard to say which tiny steps would be needed to get to the end. The milestones are mostly the deliverables as mentioned before.

Project Schedule

Starting on this project will be possible shortly after YAPC::EU. A nice opportunity to have some more discussions with the people that are currently on the mailinglist and IRC. Since the complexity of the project and the number of resources that needs to be accessible through the REST api as well as all the templates that need to be doen (twice), It might easily last up to a year, working one or two days per week and spending some evenings here and there.

Completeness Criteria

The project on itself will be completed when it is possible for an organiser to easily create an instance and choose and modify a predefined theme.

However, each of the for mentioned mile-stones and deliverables need to be completed before actually moving on to the next stage


I'm the oldest 'send-a-newbie', entered into the Perl community back in April 2013. Before that my coding was gibberish but since then I met many many people in the community that showed me how Perl can be really a powerful tool.

Things done in Perl are as simple as an automated 'talks-titler' that generates HD-TV quality title pages from the talks-list in ACT (see the NLPW::2014 video's) to more interesting things like building a OOP wrapper around the ugliest Magento XML-RPC interface.

Currently I'm writing a REST api generator that does for DANCER what does for DBIx::Class: writing all the routes and HTTP-methods into separate modules for each resource found in the database.

Before writing Perl, I have done quite some Objective-C for iOS applications. And long before that, I used to be self-employed web designer.

I'm also one of the organisers of the NLPW::2014 and 2015 and have great personal interest in having a better user experience with ACT based websites, both enduser as maintainer.

It has been my desire to change the user experience of ACT since I first used the web-site... checking my schedule personal schedule on the first day of YAPC::EU::2013 in Kiev... on an iPhone (WARNING: DO NOT TRY THIS AT HOME)

Perl Foundation News: Grant Proposal: Book: Practical Approach to Web Programming with Perl 5

We have received the following grant application "Book: Practical Approach to Web Programming with Perl 5". Please leave feedback in the comments field by July 25th, 2014.

Book: Practical Approach to Web Programming with Perl 5

  • Name:

    Ahmad Bilal

  • Amount Requested:

    USD 1000


This book would teach web programming from ground up through a practical and minimalist approach.

Benefits to the Perl Community

2 years back, when I was first introduced to Perl, I had a horrible time learning it as my first programming language. Most of the resources available were very theoretical, and I could not find practical and simple code. I went through many O'Reilly Media books, to no avail. I realised that most of the books were either outdated, or aimed at programmers already proficient in some other programming language or required strong computer science background.

For example, even searching for "use HTML and Perl together" had produced a lot of vague results back then, even though its so... easy to answer it now, that I progressed further on the path of Perl.

Even modern books like Perl Maven, and Modern Perl: Chromatic, dive into theory first, before coming to practical sample codes. Even though I acknowledge the breath of fresh air they brought to the Perl community.

The extent of things was this, that when I recruited some interns in 2014 spring-summer, for my start-up from top tech-colleges in India, there was only handful of kids who knew Perl. Most of them preferred PHP, Ruby or Python. And the handful few, either knew the Perl as it was in 1990s (due to the syllabus in-force in these colleges) or were specialised in the field of Bio-informatics. I had to develop my own techniques and methods to teach Perl5 to my interns, and thankfully they are now self-proficient. Those techniques and methods gave me a solid base, for the purpose of writing this book.

I aim to write this book, to teach people Perl, by using practical, simple and secure code, ready for first time programmers as well as experienced ones. But since I'm busy with my start-up, I fear that without a grant, I might lose my motivation midway. Thus a grant would help me greatly to cover some basic day-to-day expenses if I'm to put time and effort in writing and researching for this book. Personally, I'm very passionate about this book project.

I would also like to promote, Perl in India through authoring and resultant marketing of this book.


20-40 Scripts, starting from Hello World, Print Submitted Info, Login Script, all the way up to designing Full Fledge Games, and Writing Complex Modules.

Though I have not finalised all the scripts yet, they may go in this pattern:

1. Hello World Script

2. Print Submitted Data (from a HTML Form)

3. Simple Login Script (no encryption, no database)

4. Session Management

5. Advanced Login Script (encryption and database)

6. Account Creation & Password Retrieval System

7. Handling different modern DB like SQLITE, PostgreSQL, MySQL, etc.

8. A Full-Fledge Blog Management

9. A web-based Hangman/Tic-tac-toe game

10. A web-based game of Intermediate Complexity

and so on..

Project Details

I already have a personal blog, where I have written some simple scripts:

You can see on that blog, my personal style, where I explain each and every line (spoon-feeding), besides adding a few simple tips every now and then. In my book, I would aim to improve upon this style to a big extent, reducing inefficiency (using reference codes, additional resources-link, etc) and adding diversity (Board/2D RPG Game programming in Perl) plus some aesthetics like Perl-Meme, Interesting-Facts (about Perl and its community), etc. I have some hidden aces as well, that I would rather reveal in the final version of the book.

Also, sometime back I came across this book called (Perl One-Liners - 130 Programs That Get Things Done). Though I was a bit disappointed with the content of the book, but overall, I liked their approach. It was exactly what I was thinking as well. In my early days, as a perl programmer, I used to constantly come across, a lot of code that was unnecessarily complex, and had no human-value and gave Perl its ugly-complex identity to the un-initiated. I used to think, for god sakes! When are tjey going to write a book that would get REAL things done!!?


I think the deliverables, and their completion would be rather solid inch-stones. The progress can be simply and clearly noticed by completion of each script.

Project Schedule

I can commit a lot of time, since our start-up is also using Perl as our back-end-language. I would begin in August 2014, and ideally complete it before Mid-Dec 2014, but it can change by up to 1-2 months take or leave, as my main concern is simplicity and effectiveness. I would rather take a bit more time, to write a good book, instead of rushing to meet deadlines.

Completeness Criteria

The book would be available in ebook, html, pod, under a public license (would decide on exactly which license, later.. perhaps copy-left).


My name is Ahmad Bilal, I'm an independent developer as well as an entrepreneur based in New Delhi, India. I have always been a unorthodox learner, and a generalist to begin with. I come from a academic family, where my mother, 3 aunts, and many other family members are university professors/school teachers, due to which, from the get-go, I have learned the art of the teaching and writing books from them. My father is in construction business, where he has to manage a lot of unskilled labour, and on a very constant & personal basis engage and train them. So yes.., training is in my blood. Also I'm a firm believer in quality-over-quantity concept, and always excelled in various fields with little or no prior experience as a fast learner due to my practical approach.

In CS, I'm a very good UI Designer with more than 4-5 years of experience (Photoshop, HTML, CSS, etc) mainly wih ,.. besides being familiar with C++ and Python as well.

I look forward to get support for this endeavour from the Perl community :)

Laufeyjarson writes... » Perl: PBP: 021 Lists

Lists need to be formatted to be readable.  The suggestion is to always use parenthesis, indent after a parenthesis, and line things in columns, all with trailing commas.

The book provides clear examples, which I won’t duplicate here.

I have no problem with this suggestion, and do it almost automatically.  I sometimes don’t think to line things up in columns; the differing number of spaces between things bothers me a little, and I find it tedious to do.  If my editor will Perltidy it that way for me, I won’t object, but I won’t think to do it myself.  (The same is true for lining things up in columns anywhere in code.  It’s a waste of my time.)

I found the “do not do these” examples clear, but have worked with people who suggested them; put the comma first means you can always copy/paste the line and not miss it.  It makes more sense for a language where an unused trailing comma is an error, but is still not worth it.  Use commas as list separators, as we’re used to in English – after the item listed.

PAL-Blog: Grashüpfer from outer space

Jedes Jahr läd die Pestalozzi-Stiftung zum Sommerfest. Ein Kindergarten-Fußballtunier, viele Aktivitäten und eine umfangreiches Bühnenprogramm bieten für jeden etwas. Zoe hatte mit Kindergarten und Ballett gleich zwei Auftritte (nächstes Wochenende steht dann der dritte an).

Perlbuzz: Perlbuzz news roundup for 2014-07-14

These links are collected from the Perlbuzz Twitter feed. If you have suggestions for news bits, please mail me at

Dave's Free Press: Journal: Module pre-requisites analyser

Dave's Free Press: Journal: CPANdeps

Dave's Free Press: Journal: Perl isn't dieing

Dave's Free Press: Journal: YAPC::Europe 2007 report: day 3 : The Fun of Running a Public Web Service, and Session Storage

One of my websites, Sudokugarden, recently surged in traffic, from about 30k visitors per month to more than 100k visitors per month. Here's the tale of what that meant for the server side.

As a bit of background, I built the website in 2007, when I knew a lot less about the web and programming. It runs on a host that I share with a few friends; I don't have root access on that machine, though when the admin is available, I can generally ask him to install stuff for me.

Most parts of the websites are built as static HTML files, with Server Side Includes. Parts of those SSIs are Perl CGI scripts. The most popular part though, which allows you to solve Sudoku in the browser and keeps hiscores, is written as a collection of Perl scripts, backed by a mysql database.

When at peak times the site had more than 10k visitors a day, lots of visitors would get a nasty mysql: Cannot connect: Too many open connections error. The admin wasn't available for bumping the connection limit, so I looked for other solutions.

My first action was to check the logs for spammers and crawlers that might hammered the page, and I found and banned some; but the bulk of the traffic looked completely legitimate, and the problem persisted.

Looking at the seven year old code, I realized that most pages didn't actually need a database connection, if only I could remove the session storage from the database. And, in fact, I could. I used CGI::Session, which has pluggable backend. Switching to a file-based session backend was just a matter of changing the connection string and adding a directory for session storage. Luckily the code was clean enough that this only affected a single subroutine. Everything was fine.

For a while.

Then, about a month later, the host ran out of free disk space. Since it is used for other stuff too (like email, and web hosting for other users) it took me a while to make the connection to the file-based session storage. What happened was 3 million session files on a ext3 file system with a block size of 4 kilobyte. A session is only about 400 byte, but since a file uses up a multiple of the block size, the session storage amounted to 12 gigabyte of used-up disk space, which was all that was left on that machine.

Deleting those sessions turned out to be a problem; I could only log in as my own user, which doesn't have write access to the session files (which are owned by www-data, the Apache user). The solution was to upload a CGI script that deleted the session, but of course that wasn't possible at first, because the disk was full. In the end I had to delete several gigabyte of data from my home directory before I could upload anything again. (Processes running as root were still writing to reserved-to-root portions of the file system, which is why I had to delete so much data before I was able to write again).

Even when I was able to upload the deletion script, it took quite some time to actually delete the session files; mostly because the directory was too large, and deleting files on ext3 is slow. When the files were gone, the empty session directory still used up 200MB of disk space, because the directory index doesn't shrink on file deletion.

Clearly a better solution to session storage was needed. But first I investigated where all those sessions came from, and banned a few spamming IPs. I also changed the code to only create sessions when somebody logs in, not give every visitor a session from the start.

My next attempt was to write the sessions to an SQLite database. It uses about 400 bytes per session (plus a fixed overhead for the db file itself), so it uses only a tenth of storage space that the file-based storage used. The SQLite database has no connection limit, though the old-ish version that was installed on the server doesn't seem to have very fine-grained locking either; within a few days I could errors that the session database was locked.

So I added another layer of workaround: creating a separate session database per leading IP octet. So now there are up to 255 separate session database (plus a 256th for all IPv6 addresses; a decision that will have to be revised when IPv6 usage rises). After a few days of operation, it seems that this setup works well enough. But suspicious as I am, I'll continue monitoring both disk usage and errors from Apache.

So, what happens if this solution fails to work out? I can see basically two approaches: move the site to a server that's fully under my control, and use redis or memcached for session storage; or implement sessions with signed cookies that are stored purely on the client side.

Dave's Free Press: Journal: Devel::CheckLib can now check libraries' contents : Rakudo's Abstract Syntax Tree

After or while a compiler parses a program, the compiler usually translates the source code into a tree format called Abstract Syntax Tree, or AST for short.

The optimizer works on this program representation, and then the code generation stage turns it into a format that the platform underneath it can understand. Actually I wanted to write about the optimizer, but noticed that understanding the AST is crucial to understanding the optimizer, so let's talk about the AST first.

The Rakudo Perl 6 Compiler uses an AST format called QAST. QAST nodes derive from the common superclass QAST::Node, which sets up the basic structure of all QAST classes. Each QAST node has a list of child nodes, possibly a hash map for unstructured annotations, an attribute (confusingly) named node for storing the lower-level parse tree (which is used to extract line numbers and context), and a bit of extra infrastructure.

The most important node classes are the following:

A list of statements. Each child of the node is considered a separate statement.
A single operation that usually maps to a primitive operation of the underlying platform, like adding two integers, or calling a routine.
Those hold integer, float ("numeric") and string constants respectively.
Holds a reference to a more complex object (for example a class) which is serialized separately.
A list of statements that introduces a separate lexical scope.
A variable
A node that can evaluate to different child nodes, depending on the context it is compiled it.

To give you a bit of a feel of how those node types interact, I want to give a few examples of Perl 6 examples, and what AST they could produce. (It turns out that Perl 6 is quite a complex language under the hood, and usually produces a more complicated AST than the obvious one; I'll ignore that for now, in order to introduce you to the basics.)

Ops and Constants

The expression 23 + 42 could, in the simplest case, produce this AST:

Here an QAST::Op encodes a primitive operation, an addition of two numbers. The :op argument specifies which operation to use. The child nodes are two constants, both of type QAST::IVal, which hold the operands of the low-level operation add.

Now the low-level add operation is not polymorphic, it always adds two floating-point values, and the result is a floating-point value again. Since the arguments are integers and not floating point values, they are automatically converted to float first. That's not the desired semantics for Perl 6; actually the operator + is implemented as a subroutine of name &infix:<+>, so the real generated code is closer to
    :name('&infix:<+>'),    # name of the subroutine to call,,

Variables and Blocks

Using a variable is as simple as writing'name-of-the-variable')), but it must be declared first. This is done with'name-of-the-variable'), :decl('var'), :scope('lexical')).

But there is a slight caveat: in Perl 6 a variable is always scoped to a block. So while you can't ordinarily mention a variable prior to its declaration, there are indirect ways to achieve that (lookup by name, and eval(), to name just two).

So in Rakudo there is a convention to create QAST::Block nodes with two QAST::Stmts children. The first holds all the declarations, and the second all the actual code. That way all the declaration always come before the rest of the code.

So my $x = 42; say $x compiles to roughly this:'$x'), :decl('var'), :scope('lexical')),

Polymorphism and QAST::Want

Perl 6 distinguishes between native types and reference types. Native types are closer to the machine, and their type name is always lower case in Perl 6.

Integer literals are polymorphic in that they can be either a native int or a "boxed" reference type Int.

To model this in the AST, QAST::Want nodes can contain multiple child nodes. The compile-time context decides which of those is acutally used.

So the integer literal 42 actually produces not just a simple QAST::IVal node but rather this:

(Note that is just a nice notation to indicate a boxed integer object; it doesn't quite work like this in the code that translate Perl 6 source code into ASTs).

The first child of a QAST::Want node is the one used by default, if no other alternative matches. The comes a list where the elements with odd indexes are format specifications (here Ii for integers) and the elements at even-side indexes are the AST to use in that case.

An interesting format specification is 'v' for void context, which is always chosen when the return value from the current expression isn't used at all. In Perl 6 this is used to eagerly evaluate lazy lists that are used in void context, and for several optimizations.

Dave's Free Press: Journal: I Love Github

Dave's Free Press: Journal: Palm Treo call db module

Ocean of Awareness: Evolvable languages

Ideally, if a syntax is useful and clear, and a programmer can easily read it at a glance, you should be able to add it to an existing language. In this post, I will describe a modest incremental change to the Perl syntax.

It's one I like, because that's beside the point, for two reasons. First, it's simply intended as an example of language evolution. Second, regardless of its merits, it is unlikely to happen, because of the way that Perl 5 is parsed. In this post I will demonstrate a way of writing a parser, so that this change, or others, can be made in a straightforward way, and without designing your language into a corner.

When initializing a hash, Perl 5 allows you to use not just commas, but also the so-called "wide comma" (=>). The wide comma is suggestive visually, and it also has some smarts about what a hash key is: The hash key is always converted into a string, so that wide comma knows that in a key-value pair like this:

    key1 => 711,

that key1 is intended as a string.

But what about something like this?

   company name => 'Kamamaya Technology',
   employee 1 => first name => 'Jane',
   employee 1 => last name => 'Doe',
   employee 1 => title => 'President',
   employee 2 => first name => 'John',
   employee 2 => last name => 'Smith',
   employee 3 => first name => 'Clarence',
   employee 3 => last name => 'Darrow',

Here I think the intent is obvious -- to create an employee database in the form of a hash of hashes, allowing spaces in the keys. In Data::Dumper format, the result would look like:

              'employee 2' => {
                                'last name' => '\'Smith\'',
                                'first name' => '\'John\''
              'company name' => '\'Kamamaya Technology\'',
              'employee 3' => {
                                'last name' => '\'Darrow\'',
                                'first name' => '\'Clarence\''
              'employee 1' => {
                                'title' => '\'President\'',
                                'last name' => '\'Doe\'',
                                'first name' => '\'Jane\''

And in fact, that is the output of the script in this Github gist, which parses the previous "extended Perl 5" snippet using a Marpa grammar before passing it on to Perl.

Perl 5 does not allow a syntax like this, and looking at its parsing code will tell you why -- it's already a maintenance nightmare. The extension I've described above could, in theory, be added to Perl 5, but doing so would aggravate an already desperate maintenance situation.

Now, depending on taste, you may be just as happy that you'll never see the extensions I have just outlined in Perl 5. But I don't think it is as easy to be happy about a parsing technology that quickly paints the languages which use it into a corner.

How it works

The code is in a Github gist. For the purposes of the example, I've implemented a toy subset of Perl. But this approach has been shown to scale. There are full Marpa-powered parsers of C, ECMAScript, XPath, and liberal HTML.

Marpa is a general BNF parser, which means that anything you can write in BNF, Marpa can parse. For practical parsing, what matters are those grammars that can be parsed in linear time, and with Marpa that class is vast, including all the classes of grammar currently in practical use. To describe the class of grammars that Marpa parses in linear time, assume that you have either a left or right parser, with infinite lookahead, that uses regular expressions. (A parser like this is called LR-regular.) Assume that this LR-regular parser parses your grammar. In that case, you can be sure that Marpa will parse that grammar in linear time, and without doing the lookahead. (Instead Marpa tracks possibilities in a highly-optimized table.) Marpa also parses many grammars that are not LR-regular in linear time, but just LR-regular is very likely to include any class of grammar that you will be interested in parsing. The LR-regular grammars easily include all those that can be parsed using yacc, recursive descent or regular expressions.

Marpa excels at those special hacks so necessary in recursive descent and other techniques. Marpa allows you to define events that will stop it at symbols or rules, both before and after. While stopped, you can hand processing over to your own custom code. Your custom code can feed your own tokens to the parse for as long as you like. In doing so, it can consult Marpa to determine exactly what symbols and rules have been recognized and which ones are expected. Once finished with custom processing, you can then ask Marpa to pick up again at any point you wish.

The craps game is over

The bottom line is that if you can describe your language extension in BNF, or in BNF plus some hacks, you can rely on Marpa parsing it in reasonable time. Language design has been like shooting crap in a casino that sets you up to win a lot of the first rolls before the laws of probability grind you down. Marpa changes the game.

To learn more

Marpa::R2 is available on CPAN. A list of my Marpa tutorials can be found here. There are new tutorials by Peter Stuifzand and amon. The Ocean of Awareness blog focuses on Marpa, and it has an annotated guide. Marpa has a web page that I maintain and Ron Savage maintains another. For questions, support and discussion, there is the "marpa parser" Google Group.


Comments on this post can be made in Marpa's Google group. : and p6doc


Earlier this year I tried to assess the readiness of the Perl 6 language, compilers, modules, documentation and so on. While I never got around to publish my findings, one thing was painfully obvious: there is a huge gap in the area of documentation.

There are quite a few resources, but none of them comprehensive (most comprehensive are the synopsis, but they are not meant for the end user), and no single location we can point people to.


So, in the spirit of xkcd, I present yet another incomplete documentation project: and p6doc.

The idea is to take the same approach as perldoc for Perl 5: create user-level documentation in Pod format (here the Perl 6 Pod), and make it available both on a website and via a command line tool. The source (documentation, command line tool, HTML generator) lives at The website is

Oh, and the last Rakudo Star release (2012.06) already shipped p6doc.

Status and Plans

Documentation, website and command line tool are all in very early stages of development.

In the future, I want both p6doc SOMETHING and to either document or link to documentation of SOMETHING, be it a built-in variable, an operator, a type name, routine name, phaser, constant or... all the other possible constructs that occur in Perl 6. URLs and command line arguments specific to each type of construct will also be available (/type/SOMETHING URLs already work).

Finally I want some way to get a "full" view of a type, ie providing all methods from superclasses and roles too.

Help Wanted

All of that is going to be a lot of work, though the most work will be to write the documentation. You too can help! You can write new documentation, gather and incorporate already existing documentation with compatible licenses (for example synopsis, perl 6 advent calendar, examples from rosettacode), add more examples, proof-read the documentation or improve the HTML generation or the command line tool.

If you have any questions about contributing, feel free to ask in #perl6. Of course you can also; create pull requests right away :-). : YAPC Europe 2013 Day 2

The second day of YAPC Europe was enjoyable and informative.

I learned about ZeroMQ, which is a bit like sockets on steriods. Interesting stuff. Sadly Design decisions on p2 didn't quite qualify as interesting.

Matt's PSGI archive is a project to rewrite Matt's infamous script archive in modern Perl. Very promising, and a bit entertaining too.

Lunch was very tasty, more so than the usual mass catering. Kudos to the organizers!

After lunch, jnthn talked about concurrency, parallelism and asynchrony in Perl 6. It was a great talk, backed by great work on the compiler and runtime. Jonathans talk are always to be recommended.

I think I didn't screw up my own talk too badly, at least the timing worked fine. I just forgot to show the last slide. No real harm done.

I also enjoyed mst's State of the Velociraptor, which was a summary of what went on in the Perl world in the last year. (Much better than the YAPC::EU 2010 talk with the same title).

The Lightning talks were as enjoyable as those from the previous day. So all fine!

Next up is the river cruise, I hope to blog about that later on. : Stop The Rewrites!

What follows is a rant. If you're not in the mood to read a rant right now, please stop and come back in an hour or two.

The Internet is full of people who know better than you how to manage your open source project, even if they only know some bits and pieces about it. News at 11.

But there is one particular instance of that advice that I hear often applied to Rakudo Perl 6: Stop the rewrites.

To be honest, I can fully understand the sentiment behind that advice. People see that it has taken us several years to get where we are now, and in their opinion, that's too long. And now we shouldn't waste our time with rewrites, but get the darn thing running already!

But Software development simply doesn't work that way. Especially not if your target is moving, as is Perl 6. (Ok, Perl 6 isn't moving that much anymore, but there are still areas we don't understand very well, so our current understanding of Perl 6 is a moving target).

At some point or another, you realize that with your current design, you can only pile workaround on top of workaround, and hope that the whole thing never collapses.

Picture of
a Jenga tower
Image courtesy of sermoa

Those people who spread the good advice to never do any major rewrites again, they never address what you should do when you face such a situation. Build the tower of workarounds even higher, and pray to Cthulhu that you can build it robust enough to support a whole stack of third-party modules?

Curiously this piece of advice occasionally comes from people who otherwise know a thing or two about software development methodology.

I should also add that since the famous "nom" switchover, which admittedly caused lots of fallout, we had three major rewrites of subsystems (longest-token matching of alternative, bounded serialization and qbootstrap), All three of which caused no new test failures, and two of which caused no fallout from the module ecosystem at all. In return, we have much faster startup (factor 3 to 4 faster) and a much more correct regex engine.

Ocean of Awareness: A Marpa-powered C parser

Jean-Damien Durand has just released MarpaX::Languages::C::AST, which parses C language into an abstract syntax tree (AST). MarpaX::Languages::C::AST has been tested against Perl's C source code, as well as Marpa's own C source.

Because it is Marpa-powered, MarpaX::Languages::C::AST works differently from other C parsers. In the past, C parsers have been syntax-driven -- parsing was based on a BNF description of the C grammar. More recently, C parsers have used hand-written recursive descent -- they have been procedurally-driven.

MarpaX::Languages::C::AST uses both approaches. Marpa has the advantage that it makes full knowledge of the state of the parse available to the programmer, so that procedural logic and syntax-driven parsing can reinforce each other. The result is a combined lexer/parser which is very compact and easy to understand. Among the potential applications:

  • Customized "lints". You can write programs to enforce C language standards and restrictions specific to an individual, a company or a project.
  • C interpreters. By taking the AST and adding your own back end, you can create a special-purpose C interpreter or a special-purpose compiler.
  • C variants. Because the code for the parser is compact and easy to modify, it lends itself to language extension and experimentation. For example, you could reasonably implement compilers to try out the proposals submitted to a standards committee.
  • C supersets. Would you like to see some of the syntax from a favorite language available in C? Here's your chance.

The implementation

A few of Jean-Damien's implementation choices are worth noting. A C parser can take one of two strategies: approximate or precise. A compiler has, of course, to be precise. Tools, such as cross-referencers, often decide to be approximate, or sloppy. Sloppiness is easier to implement and has other advantages: A sloppy tool can tolerate missing C flags: what the C flags should be can be one of the things it guesses at.

Of the two strategies, Jean-Damien decided to go with "precise". MarpaX::Languages::C::AST follows the C11 standard, with either GCC or Microsoft extensions. This has the advantage that MarpaX::Languages::C::AST could be used as the front end of a compiler.

Because MarpaX::Languages::C::AST purpose is to take things as far as an AST, and let the user take over, it does not implement those constraints usually implemented in post-processing. One example of a post-syntactic constraint is the one that bans "case" labels outside of switch statements. Perhaps a future version can include a default "first phase" post-processor to enforce the constraints from the standards. As currently implemented, the user can check for and enforce these constraints in any way he likes. This makes it easier for extensions and customizations, which I think of as the major purpose of MarpaX::Languages::C::AST.

The parsing strategy

Those familar with the C parsing and its special issues may be interested in Jean-Damien's approach to them. MarpaX::Languages::C::AST is, with a few exceptions, syntax-driven -- the parser works from Marpa's SLIF, an extended BNF variant. The SLIF-driven logic is sufficient to deal with the if-then-else issue. Marpa handles right recursion in linear time, so that the if-then-else issue could have been dealt with by rewriting the relevant rules. But Jean-Damien wanted to have his BNF follow closely the grammar in the standards, and he decided to use Marpa's rule ranking facility instead.

More complicated is the ambiguity in C between variable names and types, which actually takes C beyond BNF and context-free grammars into context-sensitive territory. Most C parsers deal with this using lexer or post-processing hacks. Marpa allows the parser to do this more elegantly. Marpa knows the parsing context at all times and can comnunicate this to a user's customized code. Marpa also has the ability to use the parsing context to decide when to switch control from the syntax-driven logic to a user's customized procedural logic, and for the syntax-driven logic to take control back when the procedural logic wants to give it back. This allows the variable-name-versus-type ambiguity to be handled by specifically targeted code which knows the full context of the decisions it needs to make. This code can be written more directly, simply and clearly than was possible with previous parsing methods.


Above I mentioned special-purpose compilers. What about production compilers? MarpaX::Languages::C::AST's upper layers are in Perl, so the speed, while acceptable for special-purpose tools, will probably not be adequate for production. Perhaps a future Marpa-powered C parser will rewrite those upper layers in C, and make the race more interesting.

To learn more

Marpa::R2 is available on CPAN. A list of my Marpa tutorials can be found here. There are new tutorials by Peter Stuifzand and amon. The Ocean of Awareness blog focuses on Marpa, and it has an annotated guide. Marpa also has a web page. For questions, support and discussion, there is the "marpa parser" Google Group. Comments on this post can be made there. : The REPL trick

A recent discussion on IRC prompted me to share a small but neat trick with you.

If there are things you want to do quite often in the Rakudo REPL (the interactive "Read-Evaluate-Print Loop"), it makes sense to create a shortcut for them. And creating shortcuts for often-used stuff is what programming languages excel at, so you do it right in Perl module:

use v6;
module REPLHelper;

sub p(Mu \x) is export {
    x.^ *.^name;

I have placed mine in $HOME/.perl6/repl.

And then you make sure it's loaded automatically:

$ alias p6repl="perl6 -I$HOME/.perl6/repl/ -MREPLHelper"
$ p6repl
> p Int
Int Cool Any Mu

Now you have a neat one-letter function which tells you the parents of an object or a type, in method resolution order. And a way to add more shortcuts when you need them. : News in the Rakudo 2012.06 release

Rakudo development continues to progress nicely, and so there are a few changes in this month's release worth explaining.

Longest Token Matching, List Iteration

The largest chunk of development effort went into Longest-Token Matching for alternations in Regexes, about which Jonathan already blogged. Another significant piece was Patrick's refactor of list iteration. You probably won't notice much of that, except that for-loops are now a bit faster (maybe 10%), and laziness works more reliably in a couple of cases.

String to Number Conversion

String to number conversion is now stricter than before. Previously an expression like +"foo" would simply return 0. Now it fails, ie returns an unthrown exception. If you treat that unthrown exception like a normal value, it blows up with a helpful error message, saying that the conversion to a number has failed. If that's not what you want, you can still write +$str // 0.

require With Argument Lists

require now supports argument lists, and that needs a bit more explaining. In Perl 6 routines are by default only looked up in lexical scopes, and lexical scopes are immutable at run time. So, when loading a module at run time, how do you make functions available to the code that loads the module? Well, you determine at compile time which symbols you want to import, and then do the actual importing at run time:

use v6;
require Test <&plan &ok &is>;
#            ^^^^^^^^^^^^^^^ evaluated at compile time,
#                            declares symbols &plan, &ok and &is
#       ^^^                  loaded at run time

Module Load Debugging

Rakudo had some trouble when modules were precompiled, but its dependencies were not. This happens more often than it sounds, because Rakudo checks timestamps of the involved files, and loads the source version if it is newer than the compiled file. Since many file operations (including simple copying) change the time stamp, that could happen very easily.

To make debugging of such errors easier, you can set the RAKUDO_MODULE_DEBUG environment variable to 1 (or any positive number; currently there is only one debugging level, in the future higher numbers might lead to more output).

$ RAKUDO_MODULE_DEBUG=1 ./perl6 -Ilib t/spec/S11-modules/require.t
MODULE_DEBUG: loading blib/Perl6/BOOTSTRAP.pbc
MODULE_DEBUG: done loading blib/Perl6/BOOTSTRAP.pbc
MODULE_DEBUG: loading lib/Test.pir
MODULE_DEBUG: done loading lib/Test.pir
MODULE_DEBUG: loading t/spec/packages/Fancy/
MODULE_DEBUG: done loading t/spec/packages/Fancy/
ok 1 - can load Fancy::Utilities at run time
ok 2 - can call our-sub from required module
MODULE_DEBUG: loading t/spec/packages/
MODULE_DEBUG: loading t/spec/packages/
MODULE_DEBUG: loading t/spec/packages/B/
MODULE_DEBUG: done loading t/spec/packages/B/
MODULE_DEBUG: done loading t/spec/packages/
MODULE_DEBUG: done loading t/spec/packages/
ok 3 - can require with variable name
ok 4 - can call subroutines in a module by name
ok 5 - require with import list

Module Loading Traces in Compile-Time Errors

If module myA loads module myB, and myB dies during compilation, you now get a backtrace which indicates through which path the erroneous module was loaded:

$ ./perl6 -Ilib -e 'use myA'
Placeholder variable $^x may not be used here because the surrounding block
takes no signature
at lib/
  from module myA (lib/
  from -e:1

Improved autovivification

Perl allows you to treat not-yet-existing array and hash elements as arrays or hashes, and automatically creates those elements for you. This is called autovivification.

my %h;
%h<x>.push: 1, 2, 3; # worked in the previous release too
push %h<y>, 4, 5, 6; # newly works in the 2012.06

Ocean of Awareness: Parsing Ada Lovelace

The application

Abstract Syntax Forests (ASF's) are my most recent project. I am adding ASF's to my Marpa parser. Marpa has long supported ambiguous parsing, and allowed users to iterate through, and examine, all the parses of an ambiguous parse. This was enough for most applications.

Even applications which avoid ambiguity benefit from better ways to detect and locate it. And there are applications that require the ability to select among and manipulate very large sets of ambiguous parses. Prominent among these is Natural Language Processing (NLP). This post will introduce an experiment. Marpa in fact seems to have some potential for NLP.

Writing an efficient ASF in not a simple matter. The naive implementation is to generate complete set of fully expanded abstract syntax trees (AST's). This approach consumes resources that can become exponential in the size of the input. Translation: the naive implementation quickly becomes unuseably slow. Marpa optimizes by aggressively identifying identical subtrees of the AST's. Especially in highly ambiguous parses, many subtrees are identical, and this optimization is often a big win.

Ada Lovelace

My primary NLP example at this point is a quote from Ada Lovelace. It is a long sentence, possibly the longest, from her Notes -- 158 words. A disadvantage of this example is that it is not typical of normal NLP. By modern standards it is an unusually long and complex sentence. An advantage of it, and my reason for the choice, is that it stresses the parser.

The "Note A" from which this sentence is taken is one of Ada's notes on a translation of a paper on the work of her mentor and colleague, Charles Babbage. Ada's "Notes" are longer than the original paper, and far more important. In these "Notes" Ada makes the first distinction between a computer and a calculator, and between software and hardware. In their collaboration, Babbage did all of the hardware design, and he wrote most of the actual programs in her paper. But these two revolutionary ideas, and their elaboration, are Ada's.

Why would Babbage ignore obvious implications of his own invention? The answer is that, while these implications are obvious to us, they simply did not fit into the 1843 view of the world. In those days, algebra was leading-edge math. The ability to manipulate equations was considered an extremely advanced form of reason. For Babbage and his contemporaries, that sort of ability to reason certainly suggested the ability to distinguish between good and evil, and this in turn suggested possession of a soul. Ada's "Notes" were written 20 years after Mary Shelly, while visiting Ada's father in Switzerland, wrote the novel Frankenstein. For Ada's contemporaries, announcing that you planned to create a machine that composed music, or did advanced mathematical reasoning, was not very different from announcing that you planned to assemble a human being in your lab.

Ada was the daughter of the poet Byron. For her, pushing boundaries was a family tradition. Babbage was happy to leave these matters to Ada. As Babbage's son put it, his father

considered the Paper by Menabrea, translated with notes by Lady Lovelace, published in volume 3 of Taylor's 'Scientific Memoirs," as quite disposing of the mathematical aspect of the invention. My business now is not with that.

On reading Ada

Ada's notes are worth reading, but the modern reader has to be prepared to face several layers of difficulty:

  • They are in Victorian English. In modern English, a long complex sentence is usually considered a editing failure. In Ada's time, following Greek and Roman examples, a periodic sentence was considered especially appropriate when making an important point. And good literary style and good scientific style were considered one and the same.
  • They are mathematical, and none of math is of the kind currently studied by programmers.
  • Ada has literally no prior literature on software to build on, and has to invent her terminology. It is almost never the modern terminology, and it can be hard to guess how it relates to modern terminology. For example, does Ada forsee objects, methods and classes? Ada speaks of computing both symbolic results and numeric data, and attaching one to the other. She clearly understands that the symbolic results can represent operations. Ada also clearly understands that numeric data can represent not just the numbers themselves, but notes, positions in a loom, or computer operations. So we have arbitrary data, tagged with symbols that can be both names and operations. But are these objects?
  • Finally, she associates mathematics with philosophy. In her day, this was expected. Unfortunately, modern readers now often see that sort of discussion as irrelevant, or even as a sign of inability to come to the point.

Ada's quote

Those who view mathematical science, not merely as a vast body of abstract and immutable truths, whose intrinsic beauty, symmetry and logical completeness, when regarded in their connexion together as a whole, entitle them to a prominent place in the interest of all profound and logical minds, but as possessing a yet deeper interest for the human race, when it is remembered that this science constitutes the language through which alone we can adequately express the great facts of the natural world, and those unceasing changes of mutual relationship which, visibly or invisibly, consciously or unconsciously to our immediate physical perceptions, are interminably going on in the agencies of the creation we live amidst: those who thus think on mathematical truth as the instrument through which the weak mind of man can most effectually read his Creator's works, will regard with especial interest all that can tend to facilitate the translation of its principles into explicit practical forms.

Ada, the bullet point version

Ada's sentence may look like what happens when two pickups carrying out-of-date dictionaries to the landfill run into each other on the way. But there is, in fact, a good deal of structure and meaning in all those words. Let's take it as bullet points:

  • 1. Math is awesome just for being itself.
  • 2. Math describes and predicts the external world.
  • 3. Math is the best way to get at what it is that is really behind existence.
  • 4. If we can do more and better math, that has to be a good thing.

Ada is connecting her new science of software to the history of thought in the West, something which readers of the time would expect her to do. Bullet point 1 alludes to the Greek view of mathematics, especially Plato's. Bullet point 2 alludes to the scientific view, as pioneered by Galileo and Newton. Bullet point 3 alludes to the post-Classical world view, especially the Christian one. But while the language is Christian, Ada's idea is one that Einstein would have had no trouble with. And bullet 4 is the call to action.

When we come to discuss the parse in detail, we'll see that it follows this structure. As an aside, note Ada's mention of "logical completeness" as one of the virtues of math. Gödel came along nearly a century later and showed this vision, which went back to the Greeks, was an illusion. So Ada did not predict everything. On the other hand, Gödel's result was also a complete surprise to Johnny von Neumann, who was in the room that day.

The experiment so far

I've gotten Marpa to grind through this sentence, using the same framework as the Stanford NLP demo. That demo, in fact, refuses to even attempt any sentence longer than 70 words, so my Ada quote needs to be broken up. Even on the smaller pieces, the Stanford demo becomes quite slow. Marpa, by contrast, grinds through the whole thing quickly. The Stanford demo is based on a CYK parser, and presumably is O(n3) -- cubic. Marpa seems to be exhibiting linear behavior.

Promising as this seems for Marpa, its first results may not hold up as the experiment gets more realistic. So far, I've only given Marpa enough English grammar and vocabulary to parse this one sentence. That is enough to make the grammar very complex and ambiguous, but even so it must be far less complex and ambiguous than the one behind the Stanford demo. Marpa will never have time worse than O(n3), but it's quite possible that if Marpa's grammar were as ambiguous as the Stanford one, Marpa would be no faster. Marpa, in fact, could turn out to be slower by some linear factor.

There may never be a final decision based on speed. Marpa might turn out to represent one approach, good for certain purposes. And, especially when speed is indecisive, other abilities can prove more important.

To learn more

Marpa::R2 is available on CPAN. A list of my Marpa tutorials can be found here. There are new tutorials by Peter Stuifzand and amon. The Ocean of Awareness blog focuses on Marpa, and it has an annotated guide. Marpa also has a web page. For questions, support and discussion, there is the "marpa parser" Google Group. Comments on this post can be made there. : Localization for Exception Messages

Ok, my previous blog post wasn't quite as final as I thought.. My exceptions grant said that the design should make it easy to enable localization and internationalization hooks. I want to discuss some possible approaches and thereby demonstrate that the design is flexible enough as it is.

At this point I'd like to mention that much of the flexibility comes from either Perl 6 itself, or from the separation of stringifying and exception and generating the actual error message.

Mixins: the sledgehammer

One can always override a method in an object by mixing in a role which contains the method on question. When the user requests error messages in a different language, one can replace method Str or method message with one that generates the error message in a different language.

Where should that happen? The code throws exceptions is fairly scattered over the code base, but there is a central piece of code in Rakudo that turns Parrot-level exceptions into Perl 6 level exceptions. That would be an obvious place to muck with exceptions, but it would mean that exceptions that are created but not thrown don't get the localization. I suspect that's a fairly small problem in the real world, but it still carries code smell. As does the whole idea of overriding methods.

Another sledgehammer: alternative setting

Perl 6 provides built-in types and routines in an outer lexical scope known as a "setting". The default setting is called CORE. Due to the lexical nature of almost all lookups in Perl 6, one can "override" almost anything by providing a symbol of the same name in a lexical scope.

One way to use that for localization is to add another setting between the user's code and CORE. For example a file DE.setting:

my class X::Signature::Placeholder does X::Comp {
    method message() {
        'Platzhaltervariablen können keine bestehenden Signaturen überschreiben';

After compiling, we can load the setting:

$ ./perl6 --target=pir --output=DE.setting.pir DE.setting
$ ./install/bin/parrot -o DE.setting.pbc DE.setting.pir
$ ./perl6 --setting=DE -e 'sub f() { $^x }'
Platzhaltervariablen können keine bestehenden Signaturen überschreiben
at -e:1

That works beautifully for exceptions that the compiler throws, because they look up exception types in the scope where the error occurs. Exceptions from within the setting are a different beast, they'd need special lookup rules (though the setting throws far fewer exceptions than the compiler, so that's probably manageable).

But while this looks quite simple, it comes with a problem: if a module is precompiled without the custom setting, and it contains a reference to an exception type, and then the l10n setting redefines it, other programs will contain references to a different class with the same name. Which means that our precompiled module might only catch the English version of X::Signature::Placeholder, and lets our localized exception pass through. Oops.

Tailored solutions

A better approach is probably to simply hack up the string conversion in type Exception to consider a translator routine if present, and pass the invocant to that routine. The translator routine can look up the error message keyed by the type of the exception, and has access to all data carried in the exception. In untested Perl 6 code, this might look like this:

# required change in CORE
my class Exception {
    multi method Str(Exception:D:) {
        return self.message unless defined $*LANG;
        if %*TRANSLATIONS{$*LANG}{self.^name} -> $translator {
            return $translator(self);
        return self.message; # fallback

# that's what a translator could write:

%*TRANSLATIONS<de><X::TypeCheck::Assignment> = {
        "Typenfehler bei Zuweisung zu '$_.symbol()': "
        ~ "'{$_.expected.^name}' erwartet, aber '{$^name} bekommen"

And setting the dynamic language $*LANG to 'de' would give a German error message for type check failures in assignment.

Another approach is to augment existing error classes and add methods that generate the error message in different languages, for example method message-fr for French, and check their existence in Exception.Str if a different language is requested.


In conclusion there are many bad and enough good approaches; we will decide which one to take when the need arises (ie when people actually start to translate error messages).

Dave's Free Press: Journal: Graphing tool

Dave's Free Press: Journal: Travelling in time: the CP2000AN

Dave's Free Press: Journal: XML::Tiny released

Dave's Free Press: Journal: YAPC::Europe 2007 report: day 1

Ocean of Awareness: Significant newlines? Or semicolons?

Should statements have explicit terminators, like the semicolon of Perl and the C language? Or should they avoid the clutter, and separate statements by giving whitespace syntactic significance and a real effect on the semantics, as is done in Python and Javascript?

Actually we don't have to go either way. As an example, let's look at some BNF-ish DSL. It defines a small calculator. At first glance, it looks as if this language has taken the significant-whitespace route -- there certainly are no explicit statement terminators.

:default ::= action => ::first
:start ::= Expression
Expression ::= Term
Term ::=
    | Term '+' Term action => do_add
Factor ::=
    | Factor '*' Factor action => do_multiply
Number ~ digits
digits ~ [\d]+
:discard ~ whitespace
whitespace ~ [\s]+

The rule is that there isn't one

If we don't happen to like the layout of the above DSL, and rearrange it in various ways, we'll find that everything we try works. If we become curious about what exactly what the rules for newlines are, and look at the documentation, we won't find any. That's because there aren't any.

We can see this by thoroughly messing up the line structure:

:default ::= action => ::first :start ::= Expression Expression ::= Term
Term ::= Factor | Term '+' Term action => do_add Factor ::= Number |
Factor '*' Factor action => do_multiply Number ~ digits digits ~
[\d]+ :discard ~ whitespace whitespace ~ [\s]+

The script will continue to run just fine.

How does it work?

How does it work? Actually, pose the question this way: Can a human reader tell where the statements end? If the reader is not used to reading BNF, he might have trouble with this particular example but, for a language that he knows, the answer is simple: Yes, of course he can. So really the question is, why do we expect the parser to be so stupid that it cannot?

The only trick is that this is done without trickery. Marpa's DSL is written in itself, and Marpa's self-grammar describes exactly what a statement is and what it is not. The Marpa parser is powerful enough to simply take this self-describing DSL and act on it, finding where statements begin and end, much as a human reader is able to.

To learn more

This example was produced with the Marpa parser. Marpa::R2 is available on CPAN. The code for this example is based on that in the synopsis for its top-level document, but it is isolated conveniently in a Github gist.

A list of my Marpa tutorials can be found here. There are new tutorials by Peter Stuifzand and amon. The Ocean of Awareness blog focuses on Marpa, and it has an annotated guide. Marpa has a web page that I maintain and Ron Savage maintains another. For questions, support and discussion, there is the "marpa parser" Google Group. Comments on this post can be made there.

Dave's Free Press: Journal: Thanks, Yahoo!

Dave's Free Press: Journal: YAPC::Europe 2007 report: day 2 : Pattern Matching and Unpacking

When talking about pattern matching in the context of Perl 6, people usually think about regex or grammars. Those are indeed very powerful tools for pattern matching, but not the only one.

Another powerful tool for pattern matching and for unpacking data structures uses signatures.

Signatures are "just" argument lists:

sub repeat(Str $s, Int $count) {
    #     ^^^^^^^^^^^^^^^^^^^^  the signature
    # $s and $count are the parameters
    return $s x $count

Nearly all modern programming languages have signatures, so you might say: nothing special, move along. But there are two features that make them more useful than signatures in other languages.

The first is multi dispatch, which allows you to write several routines with the name, but with different signatures. While extremely powerful and helpful, I don't want to dwell on them. Look at Chapter 6 of the "Using Perl 6" book for more details.

The second feature is sub-signatures. It allows you to write a signature for a sigle parameter.

Which sounds pretty boring at first, but for example it allows you to do declarative validation of data structures. Perl 6 has no built-in type for an array where each slot must be of a specific but different type. But you can still check for that in a sub-signature

sub f(@array [Int, Str]) {
    say @array.join: ', ';
f [42, 'str'];      # 42, str
f [42, 23];         # Nominal type check failed for parameter '';
                    # expected Str but got Int instead in sub-signature
                    # of parameter @array

Here we have a parameter called @array, and it is followed by a square brackets, which introduce a sub-signature for an array. When calling the function, the array is checked against the signature (Int, Str), and so if the array doesn't contain of exactly one Int and one Str in this order, a type error is thrown.

The same mechanism can be used not only for validation, but also for unpacking, which means extracting some parts of the data structure. This simply works by using variables in the inner signature:

sub head(*@ [$head, *@]) {
sub tail(*@ [$, *@tail]) {
say head <a b c >;      # a
say tail <a b c >;      # b c

Here the outer parameter is anonymous (the @), though it's entirely possible to use variables for both the inner and the outer parameter.

The anonymous parameter can even be omitted, and you can write sub tail( [$, *@tail] ) directly.

Sub-signatures are not limited to arrays. For working on arbitrary objects, you surround them with parenthesis instead of brackets, and use named parameters inside:

multi key-type ($ (Numeric :$key, *%)) { "Number" }
multi key-type ($ (Str     :$key, *%)) { "String" }
for (42 => 'a', 'b' => 42) -> $pair {
    say key-type $pair;
# Output:
# Number
# String

This works because the => constructs a Pair, which has a key and a value attribute. The named parameter :$key in the sub-signature extracts the attribute key.

You can build quite impressive things with this feature, for example red-black tree balancing based on multi dispatch and signature unpacking. (More verbose explanation of the code.) Most use cases aren't this impressive, but still it is very useful to have occasionally. Like for this small evaluator. : YAPC Europe 2013 Day 3

The second day of YAPC Europe climaxed in the river boat cruise, Kiev's version of the traditional conference dinner. It was a largish boat traveling on the Dnipro river, with food, drinks and lots of Perl folks. Not having fixed tables, and having to get up to fetch food and drinks led to a lot of circulation, and thus meeting many more people than at traditionally dinners. I loved it.

Day 3 started with a video message from next year's YAPC Europe organizers, advertising for the upcoming conference and talking a bit about the oppurtunities that Sofia offers. Tempting :-).

Monitoring with Perl and Unix::Statgrab was more about the metrics that are available for monitoring, and less about doing stuff with Perl. I was a bit disappointed.

The "Future Perl Versioning" Discussion was a very civilized discussion, with solid arguments. Whether anybody changed their minds remain to be seen.

Carl Mäsak gave two great talks: one on reactive programming, and one on regular expressions. I learned quite a bit in the first one, and simply enjoyed the second one.

After the lunch (tasty again), I attended Jonathan Worthington's third talk, MoarVM: a metamodel-focused runtime for NQP and Rakudo. Again this was a great talk, based on great work done by Jonathan and others during the last 12 months or so. MoarVM is a virtual machine designed for Perl 6's needs, as we understand them now (as opposed to parrot, which was designed towards Perl 6 as it was understood around 2003 or so, which is considerably different).

How to speak manager was both amusing and offered a nice perspective on interactions between managers and programmers. Some of this advice assumed a non-tech-savy manager, and thus didn't quite apply to my current work situation, but was still interesting.

I must confess I don't remember too much of the rest of the talks that evening. I blame five days of traveling, hackathon and conference taking their toll on me.

The third session of lightning talks was again an interesting mix, containing interesting technical tidbits, the usual "we are hiring" slogans, some touching and thoughtful moments, and finally a song by Piers Cawley. He had written the lyrics in the previous 18 hours (including sleep), to (afaict) a traditional irish song. Standing up in front of ~300 people and singing a song that you haven't really had time to practise takes a huge amount of courage, and I admire Piers both for his courage and his great performance. I hope it was recorded, and makes it way to the public soon.

Finally the organizers spoke some closing words, and received their well-deserved share of applause.

As you might have guess from this and the previous blog posts, I enjoyed this year's YAPC Europe very much, and found it well worth attending, and well organized. I'd like to give my heart-felt thanks to everybody who helped to make it happen, and to my employer for sending me there.

This being only my second YAPC, I can't make any far-reaching comparisons, but compared to YAPC::EU 2010 in Pisa I had an easier time making acquaintances. I cannot tell what the big difference was, but the buffet-style dinners at the pre-conference meeting and the river boat cruise certainly helped to increase the circulation and thus the number of people I talked to.

Dave's Free Press: Journal: YAPC::Europe 2007 travel plans : A small regex optimization for NQP and Rakudo

Recently I read the course material of the Rakudo and NQP Internals Workshop, and had an idea for a small optimization for the regex engine. Yesterday night I implemented it, and I'd like to walk you through the process.

As a bit of background, the regex engine that Rakudo uses is actually implemented in NQP, and used by NQP too. The code I am about to discuss all lives in the NQP repository, but Rakudo profits from it too.

In addition one should note that the regex engine is mostly used for parsing grammar, a process which involves nearly no scanning. Scanning is the process where the regex engine first tries to match the regex at the start of the string, and if it fails there, moves to the second character in the string, tries again etc. until it succeeds.

But regexes that users write often involve scanning, and so my idea was to speed up regexes that scan, and where the first thing in the regex is a literal. In this case it makes sense to find possible start positions with a fast string search algorithm, for example the Boyer-Moore algorithm. The virtual machine backends for NQP already implement that as the index opcode, which can be invoked as start = index haystack, needle, startpos, where the string haystack is searched for the substring needle, starting from position startpos.

From reading the course material I knew I had to search for a regex type called scan, so that's what I did:

$ git grep --word scan
3rdparty/libtommath/bn_error.c:   /* scan the lookup table for the given message
3rdparty/libtommath/bn_mp_cnt_lsb.c:   /* scan lower digits until non-zero */
3rdparty/libtommath/bn_mp_cnt_lsb.c:   /* now scan this digit until a 1 is found
3rdparty/libtommath/bn_mp_prime_next_prime.c:                   /* scan upwards 
3rdparty/libtommath/changes.txt:       -- Started the Depends framework, wrote d
src/QRegex/P5Regex/Actions.nqp:            :rxtype<sca
src/QRegex/P6Regex/Actions.nqp:            :rxtype<sca
src/vm/jvm/QAST/Compiler.nqp:    method scan($node) {
src/vm/moar/QAST/QASTRegexCompilerMAST.nqp:    method scan($node) {
Binary file src/vm/moar/stage0/NQPP6QRegexMoar.moarvm matches
Binary file src/vm/moar/stage0/QASTMoar.moarvm matches
src/vm/parrot/QAST/Compiler.nqp:    method scan($node) {
src/vm/parrot/stage0/P6QRegex-s0.pir:    $P5025 = $P5024."new"("scan" :named("rx
src/vm/parrot/stage0/QAST-s0.pir:.sub "scan" :subid("cuid_135_1381944260.6802") 
src/vm/parrot/stage0/QAST-s0.pir:    push $P5004, "scan"

The binary files and .pir files are generated code included just for bootstrapping, and not interesting for us. The files in 3rdparty/libtommath are there for bigint handling, thus not interesting for us either. The rest are good matches: src/QRegex/P6Regex/Actions.nqp is responsible for compiling Perl 6 regexes to an abstract syntax tree (AST), and src/vm/parrot/QAST/Compiler.nqp compiles that AST down to PIR, the assembly language that the Parrot Virtual Machine understands.

So, looking at src/QRegex/P6Regex/Actions.nqp the place that mentions scan looked like this:

    $block<orig_qast> := $qast;
    $qast := :rxtype<concat>,
        :rxtype<scan> ),
                      ?? :rxtype<pass> )
                      !! (nqp::substr(%*RX<name>, 0, 12) ne '!!LATENAME!!'
                            ?? :rxtype<pass>, :name(%*RX<name>) )
                            !! :rxtype<pass>,
                                       :name(nqp::substr(%*RX<name>, 12)),

So to make the regex scan, the AST (in $qast) is wrapped in<concat>, :rxtype<scan> ), $qast, ...), plus some stuff I don't care about.

To make the optimization work, the scan node needs to know what to scan for, if the first thing in the regex is indeed a constant string, aka literal. If it is, $qast is either directly of rxtype literal, or a concat node where the first child is a literal. As a patch, it looks like this:

--- a/src/QRegex/P6Regex/Actions.nqp
+++ b/src/QRegex/P6Regex/Actions.nqp
@@ -667,9 +667,21 @@ class QRegex::P6Regex::Actions is HLL::Actions {
     self.store_regex_nfa($code_obj, $block,$qast))
     self.alt_nfas($code_obj, $block, $qast);
+    my $scan := :rxtype<scan> );
+    {
+        my $q := $qast;
+        if $q.rxtype eq 'concat' && $q[0] {
+            $q := $q[0]
+        }
+        if $q.rxtype eq 'literal' {
+            nqp::push($scan, $q[0]);
+            $scan.subtype($q.subtype);
+        }
+    }
     $block<orig_qast> := $qast;
     $qast := :rxtype<concat>,
-        :rxtype<scan> ),
+                 $scan,

Since concat nodes have always been empty so far, the code generators don't look at their child nodes, and adding one with nqp::push($scan, $q[0]); won't break anything on backends that don't support this optimization yet (which after just this patch were all of them). Running make test confirmed that.

My original patch did not contain the line $scan.subtype($q.subtype);, and later on some unit tests started to fail, because regex matches can be case insensitive, but the index op works only case sensitive. For case insensitive matches, the $q.subtype of the literal regex node would be ignorecase, so that information needs to be carried on to the code generation backend.

Once that part was in place, and some debug nqp::say() statements confirmed that it indeed worked, it was time to look at the code generation. For the parrot backend, it looked like this:

    method scan($node) {
        my $ops := self.post_new('Ops', :result(%*REG<cur>));
        my $prefix := self.unique('rxscan');
        my $looplabel := self.post_new('Label', :name($prefix ~ '_loop'));
        my $scanlabel := self.post_new('Label', :name($prefix ~ '_scan'));
        my $donelabel := self.post_new('Label', :name($prefix ~ '_done'));
        $ops.push_pirop('repr_get_attr_int', '$I11', 'self', %*REG<curclass>, '"$!from"');
        $ops.push_pirop('ne', '$I11', -1, $donelabel);
        $ops.push_pirop('goto', $scanlabel);
        $ops.push_pirop('inc', %*REG<pos>);
        $ops.push_pirop('gt', %*REG<pos>, %*REG<eos>, %*REG<fail>);
        $ops.push_pirop('repr_bind_attr_int', %*REG<cur>, %*REG<curclass>, '"$!from"', %*REG<pos>);
        self.regex_mark($ops, $looplabel, %*REG<pos>, 0);

While a bit intimidating at first, staring at it for a while quickly made clear what kind of code it emits. First three labels are generated, to which the code can jump with goto $label: One as a jump target for the loop that increments the cursor position ($looplabel), one for doing the regex match at that position ($scanlabel), and $donelabel for jumping to when the whole thing has finished.

Inside the loop there is an increment (inc) of the register the holds the current position (%*REG<pos>), that position is compared to the end-of-string position (%*REG<eos>), and if is larger, the cursor is marked as failed.

So the idea is to advance the position by one, and then instead of doing the regex match immediately, call the index op to find the next position where the regex might succeed:

--- a/src/vm/parrot/QAST/Compiler.nqp
+++ b/src/vm/parrot/QAST/Compiler.nqp
@@ -1564,7 +1564,13 @@ class QAST::Compiler is HLL::Compiler {
         $ops.push_pirop('goto', $scanlabel);
         $ops.push_pirop('inc', %*REG<pos>);
-        $ops.push_pirop('gt', %*REG<pos>, %*REG<eos>, %*REG<fail>);
+        if nqp::elems($node.list) && $node.subtype ne 'ignorecase' {
+            $ops.push_pirop('index', %*REG<pos>, %*REG<tgt>, self.rxescape($node[0]), %*REG<pos>);
+            $ops.push_pirop('eq', %*REG<pos>, -1, %*REG<fail>);
+        }
+        else {
+            $ops.push_pirop('gt', %*REG<pos>, %*REG<eos>, %*REG<fail>);
+        }
         $ops.push_pirop('repr_bind_attr_int', %*REG<cur>, %*REG<curclass>, '"$!from"', %*REG<pos>);
         self.regex_mark($ops, $looplabel, %*REG<pos>, 0);

The index op returns -1 on failure, so the condition for a cursor fail are slightly different than before.

And as mentioned earlier, the optimization can only be safely done for matches that don't ignore case. Maybe with some additional effort that could be remedied, but it's not as simple as case-folding the target string, because some case folding operations can change the string length (for example ß becomes SS while uppercasing).

After successfully testing the patch, I came up with a small, artifical benchmark designed to show a difference in performance for this particular case. And indeed, it sped it up from 647 ± 28 µs to 161 ± 18 µs, which is roughly a factor of four.

You can see the whole thing as two commits on github.

What remains to do is implementing the same optimization on the JVM and MoarVM backends, and of course other optimizations. For example the Perl 5 regex engine keeps track of minimal and maximal string lengths for each subregex, and can anchor a regex like /a?b?longliteral/ to 0..2 characters before a match of longliteral, and generally use that meta information to fail faster.

But for now I am mostly encouraged that doing a worthwhile optimization was possible in a single evening without any black magic, or too intimate knowledge of the code generation.

Update: the code generation for MoarVM now also uses the index op. The logic is the same as for the parrot backend, the only difference is that the literal needs to be loaded into a register (whose name fresh_s returns) before index_s can use it. : Quo Vadis Perl?

The last two days we had a gathering in town named Perl (yes, a place with that name exists). It's a lovely little town next to the borders to France and Luxembourg, and our meeting was titled "Perl Reunification Summit".

Sadly I only managed to arrive in Perl on Friday late in the night, so I missed the first day. Still it was totally worth it.

We tried to answer the question of how to make the Perl 5 and the Perl 6 community converge on a social level. While we haven't found the one true answer to that, we did find that discussing the future together, both on a technical and on a social level, already brought us closer together.

It was quite a touching moment when Merijn "Tux" Brand explained that he was skeptic of Perl 6 before the summit, and now sees it as the future.

We also concluded that copying API design is a good way to converge on a technical level. For example Perl 6's IO subsystem is in desperate need of a cohesive design. However none of the Perl 6 specification and the Rakudo development team has much experience in that area, and copying from successful Perl 5 modules is a viable approach here. Path::Class and IO::All (excluding the crazy parts) were mentioned as targets worth looking at.

There is now also an IRC channel to continue our discussions -- join #p6p5 on if you are interested.

We also discussed ways to bring parallel programming to both perls. I missed most of the discussion, but did hear that one approach is to make easier to send other processes some serialized objects, and thus distribute work among several cores.

Patrick Michaud gave a short ad-hoc presentation on implicit parallelism in Perl 6. There are several constructs where the language allows parallel execution, for example for Hyper operators, junctions and feeds (think of feeds as UNIX pipes, but ones that allow passing of objects and not just strings). Rakudo doesn't implement any of them in parallel right now, because the Parrot Virtual Machine does not provide the necessary primitives yet.

Besides the "official" program, everybody used the time in meat space to discuss their favorite projects with everybody else. For example I took some time to discuss the future of with Patrick and Gabor Szabgab, and the relation to perl6maven with the latter. The Rakudo team (which was nearly completely present) also discussed several topics, and I was happy to talk about the relation between Rakudo and Parrot with Reini Urban.

Prior to the summit my expectations were quite vague. That's why it's hard for me to tell if we achieved what we and the organizers wanted. Time will tell, and we want to summarize the result in six to nine months. But I am certain that many participants have changed some of their views in positive ways, and left the summit with a warm, fuzzy feeling.

I am very grateful to have been invited to such a meeting, and enjoyed it greatly. Our host and organizers, Liz and Wendy, took care of all of our needs -- travel, food, drinks, space, wifi, accommodation, more food, entertainment, food for thought, you name it. Thank you very much!

Update: Follow the #p6p5 hash tag on twitter if you want to read more, I'm sure other participants will blog too.

Other blogs posts on this topic: PRS2012 – Perl5-Perl6 Reunification Summit by mdk and post-yapc by theorbtwo

Dave's Free Press: Journal: Wikipedia handheld proxy

Dave's Free Press: Journal: Bryar security hole

Dave's Free Press: Journal: POD includes

Dave's Free Press: Journal: cgit syntax highlighting : First day at YAPC::Europe 2013 in Kiev

Today was the first "real" day of YAPC Europe 2013 in Kiev. In the same sense that it was the first real day, we had quite a nice "unreal" conference day yesterday, with a day-long Perl 6 hackathon, and in the evening a pre-conference meeting a Sovjet-style restaurant with tasty food and beverages.

The talks started with a few words of welcome, and then the announcement that the YAPC Europe next year will be in Sofia, Bulgaria, with the small side note that there were actually three cities competing for that honour. Congratulations to Sofia!

Larry's traditional keynote was quite emotional, and he had to fight tears a few times. Having had cancer and related surgeries in the past year, he still does his perceived duty to the Perl community, which I greatly appreciate.

Afterwards Dave Cross talked about 25 years of Perl in 25 minutes, which was a nice walk through some significant developments in the Perl world, though a bit hasty. Maybe picking fewer events and spending a bit more time on the selected few would give a smoother experience.

Another excellent talk that ran out of time was on Redis. Having experimented a wee bit with Redis in the past month, this was a real eye-opener on the wealth of features we might have used for a project at work, but in the end we didn't. Maybe we will eventually revise that decision.

Ribasushi talked about how hard benchmarking really is, and while I was (in principle) aware of that fact that it's hard to get right, there were still several significant factors that I overlooked (like the CPU's tendency to scale frequency in response to thermal and power-management considerations). I also learned that I should use Dumbbench instead of the core module. Sadly it didn't install for me (Capture::Tiny tests failing on Mac OS X).

The Perl 6 is dead, long live Perl 5 talk was much less inflammatory than the title would suggest (maybe due to Larry touching on the subject briefly during the keynote). It was mostly about how Perl 5 is used in the presenter's company, which was mildly interesting.

After tasty free lunch I attended jnthn's talk on Rakudo on the JVM, which was (as is typical for jnthn's talk) both entertaining and taught me something, even though I had followed the project quite a bit.

Thomas Klausner's Bread::Board by example made me want to refactor the OTRS internals very badly, because it is full of the anti-patterns that Bread::Board can solve in a much better way. I think that the OTRS code base is big enough to warrant the usage of Bread::Board.

I enjoyed Denis' talk on Method::Signatures, and was delighted to see that most syntax is directly copied from Perl 6 signature syntax. Talk about Perl 6 sucking creativity out of Perl 5 development.

The conference ended with a session of lighning talks, something which I always enjoy. Many lightning talks had a slightly funny tone or undertone, while still talking about interesting stuff.

Finally there was the "kick-off party", beverages and snacks sponsored by There (and really the whole day, and yesterday too) I not only had conversations with my "old" Perl 6 friends, but also talked with many interesting people I never met before, or only met online before.

So all in all it was a nice experience, both from the social side, and from quality and contents of the talks. Venue and food are good, and the wifi too, except when it stops working for a few minutes.

I'm looking forward to two more days of conference!

(Updated: Fixed Thomas' last name)

Ocean of Awareness: Marpa v. Parse::RecDescent: a rematch

The application

In a recent post, I looked at an unusual language which serializes arrays and strings, using a mixture of counts and parentheses. Here is an example:

A2(A2(S3(Hey)S13(Hello, World!))S5(Ciao!))

The language is of special interest for comparison against recursive descent because, while simple, it requires procedural parsing -- a purely declarative BNF approach will not work. So it's a chance to find out if Marpa can play the game that is recursive descent's specialty.

The previous post focused on how to use Marpa to mix procedural and declarative parsing together smoothly, from a coding point of view. It only hinted at another aspect: speed. Over the last year, Marpa has greatly improved its speed for this kind of application. The latest release of Marpa::R2 now clocks in almost 100 times faster than Parse::RecDescent for long inputs.

The benchmark

Marpa::R2 Marpa::XS Parse::RecDescent
1000 1.569 2.938 13.616
2000 2.746 7.067 62.083
3000 3.935 13.953 132.549
10000 12.270 121.654 1373.171

Parse::RecDescent is pure Perl, while Marpa is based on a parse engine in a library written in hand-optimized C. You'd expect Marpa to win this race and it did.

And it is nice to see that the changes from Marpa::XS to Marpa::R2 have paid off. Included in the table are the Marpa numbers from my 2012 benchmark of Marpa::XS. Marpa::R2 has a new interface and an internal lexer, and now beats Marpa::XS by a factor of up to 10.

While the benchmarked language is ideally suited to show recursive descent to advantage, the input lengths were picked to emphasize Marpa's strengths. Marpa optimizes by doing a lot of precomputation, and is written with long inputs in mind. Though these days, a 500K source, longer than the longest tested, would not exactly set a new industry record.

To learn more

There are fuller descriptions of the language in Flavio's post and code, and my recent post on how to write a parser for this language. I talk more about the benchmark's methodology in my post on the 2012 benchmark.

Marpa::R2 is available on CPAN. A list of my Marpa tutorials can be found here. There is a new tutorial by Peter Stuifzand. The Ocean of Awareness blog focuses on Marpa, and it has an annotated guide. Marpa also has a web page. For questions, support and discussion, there is a Google Group: Comments on this post can be made there.

Dave's Free Press: Journal: CPAN Testers' CPAN author FAQ : Correctness in Computer Programs and Mathematical Proofs

While reading On Proof and Progress in Mathematics by Fields Medal winner Bill Thurston (recently deceased I was sorry to hear), I came across this gem:

The standard of correctness and completeness necessary to get a computer program to work at all is a couple of orders of magnitude higher than the mathematical community’s standard of valid proofs. Nonetheless, large computer programs, even when they have been very carefully written and very carefully tested, always seem to have bugs.

I noticed that mathematicians are often sloppy about the scope of their symbols. Sometimes they use the same symbol for two different meanings, and you have to guess from context which on is meant.

This kind of sloppiness generally doesn't have an impact on the validity of the ideas that are communicated, as long as it's still understandable to the reader.

I guess on reason is that most mathematical publications still stick to one-letter symbol names, and there aren't that many letters in the alphabets that are generally accepted for usage (Latin, Greek, a few letters from Hebrew). And in the programming world we snort derisively at FORTRAN 77 that limited variable names to a length of 6 characters.

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Dave's Free Press: Journal: YAPC::Europe 2006 report: day 3 : iPod nano 5g on linux -- works!

For Christmas I got an iPod nano (5th generation). Since I use only Linux on my home computers, I searched the Internet for how well it is supported by Linux-based tools. The results looked bleak, but they were mostly from 2009.

Now (December 2012) on my Debian/Wheezy system, it just worked.

The iPod nano 5g presents itself as an ordinary USB storage device, which you can mount without problems. However simply copying files on it won't make the iPod show those files in the play lists, because there is some meta data stored on the device that must be updated too.

There are several user-space programs that allow you to import and export music from and to the iPod, and update those meta data files as necessary. The first one I tried, gtkpod 2.1.2, worked fine.

Other user-space programs reputed to work with the iPod are rhythmbox and amarok (which both not only organize but also play music).

Although I don't think anything really depends on some particular versions here (except that you need a new enough version of gtkpod), here is what I used:

  • Architecture: amd64
  • Linux: 3.2.0-4-amd64 #1 SMP Debian 3.2.35-2
  • Userland: Debian GNU/Linux "Wheezy" (currently "testing")
  • gtkpod: 2.1.2-1


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