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Sun, 04 Dec 2016

Perl 6 By Example: Formatting a Sudoku Puzzle

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This blog post is part of my ongoing project to write a book about Perl 6.

If you're interested, please sign up for the mailing list at the bottom of the article, or here. It will be low volume (less than an email per month, on average).

As a gentle introduction to Perl 6, let's consider a small task that I recently encountered while pursuing one of my hobbies.

Sudoku is a number-placement puzzle played on a grid of 9x9 cells, subdivided into blocks of 3x3. Some of the cells are filled out with numbers from 1 to 9, some are empty. The objective of the game is to fill out the empty cells so that in each row, column and 3x3 block, each digit from 1 to 9 occurs exactly once.

An efficient storage format for a Sudoku is simply a string of 81 characters, with 0 for empty cells and the digits 1 to 9 for pre-filled cells. The task I want to solve is to bring this into a friendlier format.

The input could be:


On to our first Perl 6 program:

# file sudoku.p6
use v6;
my $sudoku = '000000075000080094000500600010000200000900057006003040001000023080000006063240000';
for 0..8 -> $line-number {
    say substr $sudoku, $line-number * 9, 9;

You can run it like this:

$ perl6 sudoku.p6

There's not much magic in there, but let's go through the code one line at a time.

The first line, starting with a # is a comment that extends to the end of the line.

use v6;

This line is not strictly necessary, but good practice anyway. It declares the Perl version you are using, here v6, so any version of the Perl 6 language. We could be more specific and say use v6.c; to require exactly the version discussed here. If you ever accidentally run a Perl 6 program through Perl 5, you'll be glad you included this line, because it'll tell you:

$ perl sudoku.p6
Perl v6.0.0 required--this is only v5.22.1, stopped at sudoku.p6 line 1.
BEGIN failed--compilation aborted at sudoku.p6 line 1.

instead of the much more cryptic

syntax error at sudoku.p6 line 4, near "for 0"
Execution of sudoku.p6 aborted due to compilation errors.

The first interesting line is

my $sudoku = '00000007500...';

my declares a lexical variable. It is visible from the point of the declaration to the end of the current scope, which means either to the end of the current block delimited by curly braces, or to the end of the file if it's outside any block. As it is in this example.

Variables start with a sigil, here a '$'. Sigils are what gave Perl the reputation of being line noise, but there is signal in the noise. The $ looks like an S, which stands for scalar. If you know some math, you know that a scalar is just a single value, as opposed to a vector or even a matrix.

The variable doesn't start its life empty, because there's an initialization right next to it. The value it starts with is a string literal, as indicated by the quotes.

Note that there is no need to declare the type of the variable beyond the very vague "it's a scalar" implied by the sigil. If we wanted, we could add a type constraint:

my Str $sudoku = '00000007500...';

But when quickly prototyping, I tend to forego type constraints, because I often don't know yet how exactly the code will work out.

The actual logic happens in the next lines, by iterating over the line numbers 0 to 8:

for 0..8 -> $line-number {

The for loop has the general structure for ITERABLE BLOCK. Here the iterable is a range, and the block is a pointy block. The block starts with ->, which introduces a signature. The signature tells the compiler what arguments the blocks expects, here a single scalar called $line-number.

Perl 6 allows to use a dash - or a single quote ' inside identifier, as long as there is a letter on both sides (to disambiguate it from subtraction).

Again, type constraints are optional. If you chose to include them, it would be for 0..9 -> Int $line-number { ... }.

$line-number is again a lexical variable, and visible inside the block that comes after the signature. Blocks are delimited by curly braces.

say substr $sudoku, $line-number * 9, 9;

Both say and substr are functions provided by the Perl 6 standard library. substr($string, $start, $chars) extracts a substring of (up to) $chars characters length from $string, starting from index $start. Oh, and indexes are zero-based in Perl 6.

say then prints this substring, followed by a line break.

As you can see from the example, function invocations don't need parenthesis, though you can add them if you want:

say substr($sudoku, $line-number * 9, 9);

or even

say(substr($sudoku, $line-number * 9, 9));

Making the Sudoku playable

As the output of our script stands now, you can't play the resulting Sudoku even if you printed it, because all those pesky zeros get in your way of actually entering the numbers you carefully deduce while solving the puzzle.

So, let's substitute each 0 with a blank:

# file sudoku.p6
use v6;

my $sudoku = '000000075000080094000500600010000200000900057006003040001000023080000006063240000';
$sudoku = $sudoku.trans('0' => ' ');

for 0..8 -> $line-number {
    say substr $sudoku, $line-number * 9, 9;

trans is a method of the Str class. Its argument is a Pair. The boring way to create a Pair would be'0', ' '), but since it's so commonly used, there is a shortcut in the form of the fat arrow, =>. The method trans replaces each occurrence of they pair's key with the pair's value, and returns the resulting string.

Speaking of shortcuts, you can also shorten $sudoku = $sudoku.trans(...) to $sudoku.=trans(...). This is a general pattern that turns methods that return a result into mutators.

With the new string substitution, the result is playable, but ugly:

$ perl6 sudoku.p6
    8  94
   5  6  
 1    2  
   9   57
  6  3 4 
  1    23
 8      6

A bit ASCII art makes it bearable:

|   | 1 |   |
|   |   |79 |
| 9 |   | 4 |
|   |  4|  5|
|   |   | 2 |
|3  | 29|18 |
|  4| 87|2  |
|  7|  2|95 |
| 5 |  3|  8|

To get the vertical dividing lines, we need to sub-divide the lines into smaller chunks. And since we already have one occurrence of dividing a string into smaller strings of a fixed size, it's time to encapsulate it into a function:

sub chunks(Str $s, Int $chars) {
    gather for 0 .. $s.chars / $chars - 1 -> $idx {
        take substr($s, $idx * $chars, $chars);

for chunks($sudoku, 9) -> $line {
    say chunks($line, 3).join('|');

The output is:

$ perl6 sudoku.p6
   |   | 75
   | 8 | 94
   |5  |6  
 1 |   |2  
   |9  | 57
  6|  3| 4 
  1|   | 23
 8 |   |  6
 63|24 |   

But how did it work? Well, sub (SIGNATURE) BLOCK declares a subroutine, short sub. Here I declare it to take two arguments, and since I tend to confuse the order of arguments to functions I call, I've added type constraints that make it very likely that Perl 6 catches the error for me.

gather and take work together to create a list. gather is the entry point, and each execution of take adds one element to the list. So

gather {
    take 1;
    take 2;

would return the list 1, 2. Here gather acts as a statement prefix, which means it collects all takes from within the for loop.

A subroutine returns the value from the last expression, which here is the gather for ... thing discussed above.

Coming back to the program, the for-loop now looks like this:

for chunks($sudoku, 9) -> $line {
    say chunks($line, 3).join('|');

So first the program chops up the full Sudoku string into lines of nine characters, and then for each line, again into a list of three strings of three characters length. The join method turns it back into a string, but with pipe symbols inserted between the chunks.

There are still vertical bars missing at the start and end of the line, which can easily be hard-coded by changing the last line:

    say '|', chunks($line, 3).join('|'), '|';

Now the output is

|   |   | 75|
|   | 8 | 94|
|   |5  |6  |
| 1 |   |2  |
|   |9  | 57|
|  6|  3| 4 |
|  1|   | 23|
| 8 |   |  6|
| 63|24 |   |

Only the horizontal lines are missing, which aren't too hard to add:

my $separator = '+---+---+---+';
my $index = 0;
for chunks($sudoku, 9) -> $line {
    if $index++ %% 3 {
        say $separator;
    say '|', chunks($line, 3).join('|'), '|';
say $separator;

Et voila:

|   |   | 75|
|   | 8 | 94|
|   |5  |6  |
| 1 |   |2  |
|   |9  | 57|
|  6|  3| 4 |
|  1|   | 23|
| 8 |   |  6|
| 63|24 |   |

There are two new aspects here: the if conditional, which structurally very much resembles the for loop. The second new aspect is the divisibility operator, %%. From other programming languages you probably know % for modulo, but since $number % $divisor == 0 is such a common pattern, $number %% $divisor is Perl 6's shortcut for it.

Shortcuts, Constants, and more Shortcuts

Perl 6 is modeled after human languages, which have some kind of compression scheme built in, where commonly used words tend to be short, and common constructs have shortcuts.

As such, there are lots of ways to write the code more succinctly. The first is basically cheating, because the sub chunks can be replaced by a built-in method in the Str class, comb:

# file sudoku.p6
use v6;

my $sudoku = '000000075000080094000500600010000200000900057006003040001000023080000006063240000';
$sudoku = $sudoku.trans: '0' => ' ';

my $separator = '+---+---+---+';
my $index = 0;
for $sudoku.comb(9) -> $line {
    if $index++ %% 3 {
        say $separator;
    say '|', $line.comb(3).join('|'), '|';
say $separator;

The if conditional can be applied as a statement postfix:

say $separator if $index++ %% 3;

Except for the initialization, the variable $index is used only once, so there's no need to give it name. Yes, Perl 6 has anonymous variables:

my $separator = '+---+---+---+';
for $sudoku.comb(9) -> $line {
    say $separator if $++ %% 3;
    say '|', $line.comb(3).join('|'), '|';
say $separator;

Since $separator is a constant, we can declare it as one:

`constant $separator = '+---+---+---+';

If you want to reduce the line noise factor, you can also forego the sigil, so constant separator = '...'.

Finally there is a another syntax for method calls with arguments: instead of $obj.method(args) you can say $obj.method: args, which brings us to the idiomatic form of the small Sudoku formatter:

# file sudoku.p6
use v6;

my $sudoku = '000000075000080094000500600010000200000900057006003040001000023080000006063240000';
$sudoku = $sudoku.trans: '0' => ' ';

constant separator = '+---+---+---+';
for $sudoku.comb(9) -> $line {
    say separator if $++ %% 3;
    say '|', $line.comb(3).join('|'), '|';
say separator;

IO and other Tragedies

A practical script doesn't contain its input as a hard-coded string literal, but reads it from the command line, standard input or a file.

If you want to read the Sudoku from the command line, you can declare a subroutine called MAIN, which gets all command line arguments passed in:

# file sudoku.p6
use v6;

constant separator = '+---+---+---+';

sub MAIN($sudoku) {
    my $substituted = $sudoku.trans: '0' => ' ';

    for $substituted.comb(9) -> $line {
        say separator if $++ %% 3;
        say '|', $line.comb(3).join('|'), '|';
    say separator;

This is how it's called:

$ perl6-m sudoku-format-08.p6 000000075000080094000500600010000200000900057006003040001000023080000006063240000
|   |   | 75|
|   | 8 | 94|
|   |5  |6  |
| 1 |   |2  |
|   |9  | 57|
|  6|  3| 4 |
|  1|   | 23|
| 8 |   |  6|
| 63|24 |   |

And you even get a usage message for free if you use it wrongly, for example by omitting the argument:

$ perl6-m sudoku.p6 
  sudoku.p6 <sudoku> 

You might have noticed that the last example uses a separate variable for the substituted Sudoku string.This is because function parameters (aka variables declared in a signature) are read-only by default. Instead of creating a new variable, I could have also written sub MAIN($sudoku is copy) { ... }.

Classic UNIX programs such as cat and wc, follow the convention of reading their input from file names given on the command line, or from the standard input if no file names are given on the command line.

If you want your program to follow this convention, lines() provides a stream of lines from either of these source:

# file sudoku.p6
use v6;

constant separator = '+---+---+---+';

for lines() -> $sudoku {
    my $substituted = $sudoku.trans: '0' => ' ';

    for $substituted.comb(9) -> $line {
        say separator if $++ %% 3;
        say '|', $line.comb(3).join('|'), '|';
    say separator;

Get Creative!

You won't learn a programming language from reading a blog, you have to actually use it, tinker with it. If you want to expand on the examples discussed earlier, I'd encourage you to try to produce Sudokus in different output formats.

SVG offers a good ratio of result to effort. This is the rough skeleton of an SVG file for a Sudoku:

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "">
<svg width="304" height="304" version="1.1"
    <line x1="0" x2="300" y1="33.3333" y2="33.3333" style="stroke:grey" />
    <line x1="0" x2="300" y1="66.6667" y2="66.6667" style="stroke:grey" />
    <line x1="0" x2="303" y1="100" y2="100" style="stroke:black;stroke-width:2" />
    <line x1="0" x2="300" y1="133.333" y2="133.333" style="stroke:grey" />
    <!-- more horizontal lines here -->

    <line y1="0" y2="300" x1="33.3333" x2="33.3333" style="stroke:grey" />
    <!-- more vertical lines here -->

    <text x="43.7333" y="124.5"> 1 </text>
    <text x="43.7333" y="257.833"> 8 </text>
    <!-- more cells go here -->
    <rect width="304" height="304" style="fill:none;stroke-width:1;stroke:black;stroke-width:6"/>

If you have a Firefox or Chrome browser, you can use it to open the SVG file.

If you are adventurous, you could also write a Perl 6 program that renders the Sudoku as a Postscript (PS) or Embedded Postscript document. It's also a text-based format.

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Sun, 27 Nov 2016

Perl 6 By Example: Running Rakudo

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This blog post is part of my ongoing project to write a book about Perl 6.

If you're interested, please sign up for the mailing list at the bottom of the article, or here. It will be low volume (less than an email per month, on average).

Before we start exploring Perl 6, you should have an environment where you can run Perl 6 code. So you need to install Rakudo Perl 6, currently the only actively developed Perl 6 compiler. Or even better, install Rakudo Star, which is a distribution that includes Rakudo itself, a few useful modules, and an installer that can help you install more modules.

Below a few options for getting Rakudo Star installed are discussed. Chose whatever works for you.

The examples here use Rakudo Star 2016.10.


You can download installers from for Mac OS (.dmg) and Windows (.msi). After download, you can launch them, and they walk you through the installation process.

Note that Rakudo is not relocatable, which means you have to install to a fix location that was decided by the creator of the installer. Moving the installation to a different directory.

On Windows, the installer offers you need to add C:\rakudo\bin and C:\rakudo\share\perl6\site\bin to your PATH environment. You should chose that option, as it allows you to call rakudo (and programs that the module installer installs on your behalf) without specifying full paths.


On platforms that support Docker, you can pull an existing Docker container from the docker hub:

$ docker pull mj41/perl6-star

Then you can get an interactive Rakudo shell with this command:

$ docker run -it mj41/perl6-star perl6

But that won't work for executing scripts, because the container has its own, separate file system. To make scripts available inside the container, you need to tell Docker to make the current directory available to the container:

$ docker run -v $PWD:/perl6 -w /perl6 -it mj41/perl6-star perl6

The option -v $PWD:/perl6 instructs Docker to mount the current working directory ($PWD) into the container, where it'll be available as /perl6. To make relative paths work, -w /perl6 instructs Docker to set the working directory of the rakudo process to /perl6.

Since this command line starts to get unwieldy, I created an alias (this is Bash syntax; other shells might have slightly different alias mechanisms):

alias p6d='docker run -v $PWD:/perl6 -w /perl6 -it mj41/perl6-star perl6'

I put this line into my ~/.bashrc files, so new bash instances have a p6d command, short for "Perl 6 docker".

As a short test to see if it works, you can run

$ p6d -e 'say "hi"'

If you go the Docker route, just the p6d alias instead of perl6 to run scripts.

Building from Source

To build Rakudo Star from source, you need make, gcc or clang and perl 5 installed. This example installs into $HOME/opt/rakudo-star:

$ wget
$ tar xzf rakudo-star-2016.10.tar.gz
$ cd rakudo-star-2016.10/
$ perl --prefix=$HOME/opt/rakudo-star --gen-moar
$ make install

You should have about 2GB of RAM available for the last step; building a compiler is a resource intensive task.

You need to add paths to two directories to your PATH environment variable, one for Rakudo itself, one for programs installed by the module installer:


If you are a Bash user, you can put that line into your ~/.bashrc file to make it available in new Bash processes.

Testing your Rakudo Star Installation

You should now be able to run Perl 6 programs from the command line, and ask Rakudo for its version:

$ perl6 --version
This is Rakudo version 2016.10-2-gb744de3 built on MoarVM version 2016.09-39-g688796b
implementing Perl 6.c.

$ perl6 -e "say <hi>"

If, against all odds, all of these approaches have failed you to produce a usable Rakudo installation, you should describe your problem to the friendly Perl 6 community, which can usually provide some help. describes ways to interact with the community.

Next week we'll take a look at the first proper Perl 6 example, so stay tuned for updates!

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Sun, 20 Nov 2016

What is Perl 6?

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This blog post is part of my ongoing project to write a book about Perl 6.

If you're interested, please sign up for the mailing list at the bottom of the article, or here. It will be low volume (less than an email per month, on average).

Perl 6 is a programming language. It is designed to be easily learned, read and written by humans, and it is inspired by natural language. It allows the beginner to write in "baby Perl", while giving the experienced programmer freedom of expression, from concise to poetic.

Perl 6 is gradually typed. It mostly follows the paradigm of dynamically typed languages in that it accepts programs whose type safety it can't guarantee during compilation. Unlike many dynamic languages, it accepts and enforces type constraints. Where possible, the compiler uses type annotations to make decisions at compile time that would otherwise only be possible at run time.

Many programming paradigms have influenced Perl 6. You can write imperative, object-oriented and functional programs in Perl 6. Declarative programming is supported through the regex and grammar engine.

Most lookups in Perl 6 are lexical, and the language avoids global state. This makes parallel and concurrent execution of programs easier, as does Perl 6's focus on high-level concurrency primitives. Instead of threads and locks, you tend to think about promises and message queues when you don't want to be limited to one CPU core.

Perl 6 as a language is not opinionated about whether Perl 6 programs should be compiled or interpreted. Rakudo Perl 6, the main implementation, precompiles modules on the fly, and interprets scripts.

Perl 5, the Older Sister

Around the year 2000, Perl 5 development faced major strain from the conflicting desires to evolve and to keep backwards compatibility.

Perl 6 was the valve to release this tension. All the extension proposals that required a break in backwards compatibility were channeled into Perl 6, leaving it in a dreamlike state where everything was possible and nothing was fixed. It took several years of hard work to get into a more solid state.

During this time, Perl 5 also evolved, and the two languages are different enough that most Perl 5 developers don't consider Perl 6 a natural upgrade path anymore, to the point that Perl 6 does not try to obsolete Perl 5 (at least not more than it tries to obsolete any other programming language :-), and the first stable release of Perl 6 in 2015 does not indicate any lapse in support for Perl 5.

Library Availability

Being a relatively young language, Perl 6 lacks the mature module ecosystem that languages such as Perl 5 and Python provide.

To bridge this gap, interfaces exist that allow you to call into libraries written in C, Python, Perl 5 and Ruby. The Perl 5 and Python interfaces are sophisticated enough that you can write a Perl 6 class that subclasses one written in either language, and the other way around.

So if you like a particular Python library, for example, you can simply load it into your Perl 6 program through the Inline::Python module.

Why Should I Use Perl 6?

If you like the quick prototyping experience from dynamically typed programming languages, but you also want enough safety features to build big, reliable applications, Perl 6 is a good fit for you. Its gradual typing allows you to write code without having a full picture of the types involved, and later introduce type constraints to guard against future misuse of your internal and external APIs.

Perl has a long history of making text processing via regular expressions (regexes) very easy, but more complicated regexes have acquired a reputation of being hard to read and maintain. Perl 6 solves this by putting regexes on the same level as code, allowing you to name it like subroutines, and even to use object oriented features such as class inheritance and role composition to manage code and regex reuse. The resulting grammars are very powerful, and easy to read. In fact, the Rakudo Perl 6 compiler parses Perl 6 source code with a Perl 6 grammar!

Speaking of text, Perl 6 has amazing Unicode support. If you ask your user for a number, and they enter it with digits that don't happen to be the Arabic digits from the ASCII range, Perl 6 still has you covered. And if you deal with graphemes that cannot be expressed as a single Unicode code point, Perl 6 still presents it as a single character.

There are more technical benefits that I could list, but more importantly, the language is designed to be fun to use. An important aspect of that is good error messages. Have you ever been annoyed at Python for typically giving just SyntaxError: invalid syntax when something's wrong? This error could come from forgetting a closing parenthesis, for example. In this case, a Perl 6 compiler says

Unable to parse expression in argument list; couldn't find final ')'

which actually tells you what's wrong. But this is just the tip of the iceberg. The compiler catches common mistakes and points out possible solutions, and even suggests fixes for spelling mistakes.

Finally, Perl 6 gives you the freedom to express your problem domain and solution in different ways and with different programming paradigms. And if the options provided by the core language are not enough, Perl 6 is designed with extensibility in mind, allowing you to introduce both new semantics for object oriented code and new syntax.

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Sat, 19 Nov 2016

Perl 6 By Example, Another Perl 6 Book

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proposed book cover, starring a Butterfly, by Sebastian Riedl

I'm taking a shot a writing a Perl 6 book. Let's see how this goes.

My working title is Perl 6 by Example, and I want to recycle the approach taken in Using Perl 6 to introduce topics by example. Contrary to the now abandoned previous effort, I want to introduce the examples in small chunks, developing and explaining them bit by bit.

I expect the result to be rather limited in size (maybe 70 to 100 pages), and not a comprehensive guide to the Perl 6 language, but enough to get you started.

Target Audience

The reader should have previous programming experience. I assume familiarity with conditionals, variables, loops and basic data types such as various numbers, strings and lists. Some OO concepts such as classes, instances and methods will also be assumed, but I will drop some clarifying words about how I use the terminology.


I will follow roughly the same process as in my previous book: First I write blog posts about the topics I intend to cover, and later distill them into chapters.

When I have some significant portion of the manuscript available, I will pre-publish it on Leanpub, and start to sell it.

If there is a lot of interest, I might investigate options to publish it in print form.

The Team

Luckily, several people have offered to help in some form or another, typically by writing or proof-reading (in no particular order):

  • [ptc]
  • DrForr
  • Coke
  • tbowder
  • AlexDaniel
  • seatek

Knowing the warm and helpful nature of the Perl 6 community, it is likely that more folks will step up to help. Any help, as well co-authors, will be highly appreciated.

Other Perl 6 Books

Information on other Perl 6 book projects is rather sparse and vague. I try my best to keep informed about these projects.

  • Laurent Rosenfeld is working on a book which is basically a Perl 6 version of Think Python. It is targeted at programming beginners, and has been accepted by a well-respected publisher.
  • There are rumors that Larry Wall is writing a Programming Perl 6 book, and I wish that to be true. I know nothing about the progress or time horizon of that project.
  • brian d foy has started a kickstarter to fund his project to write "Learning Perl 6". Since "Learning Perl" is a book focused on beginners with no prior programming experience, I assume the same is true for the Perl 6 version of this book, so I don't see much competition between his and mine project, and wish him luck and success.
  • Ken Youens-Clark has written about metagenomics, a subset of bioinformatics, using Perl 6 example code.


If you are interested in progress updates or milestones about my book project, please sign up for the mailing list below. It will be low volume (probably less than one email per month). I will also try my best to inform you about news of the other Perl 6 book projects.

Numerous signups will also boost my motivation to work on the book.

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Tue, 01 Nov 2016

Perl 6 Advent Calendar 2016 -- Call for Authors

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Every year since 2009, the Perl 6 community publishes a Perl 6 advent calendar, in the form of blog posts on

To keep up this great tradition, we need 24 blog posts, and volunteers who write them. If you want to contribute a blog post about anything related to Perl 6, please add your name (and potentially also a topic already) to the schedule, and if you don't yet have a login on the advent blog, please tell me your email address so that I can send you an invitation.

Perl 6 advent blog posts should be finished the day before they are due, and published with midnight (UTC) of the due date as publishing date.

If you have any questions, or want to discuss blog post ideas, please join on the #perl6 IRC channel on, or drop me an email at

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Sun, 26 Apr 2015

Writing docs helps you take the user's perspective

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This year, most of my contributions to Perl 6 have been to the documentation, or were directly inspired by writing the documentation.

Quite often when I write documentation, I start thinking things like this is a bit awkward to explain, wouldn't it be more consistent if ... or what happens when I use a negative number here? The implementation disallows it, but does it actually need to? or if I tell people to just pass this particular value most of the time, why not make it the default?.

Like most people who aspires to be a good programmer, I'm lazy. In particular, I hate doing pointless work. And documenting inconsistencies or missing default values or arbitrary restrictions definitively feels like doing work that shouldn't be necessary. So with a sigh I overcome my laziness, and try to fix stuff in the code, the tests, and sometimes the design docs so I can be more lazy in documenting the features. And of course, to make the overall experience more pleasant for the end user.

I've been skeptical of README-driven development in the past, dismissing it as part of the outdated (or at least for software not suitable) waterfall model, or as "no plan survives contact with the enemy". But now that I'm writing more docs, I see the value of writing docs early (of course with the provision that if things turn out to be impractical a documented, the docs may still be revised). Because it's very easy as a developer to lose the user's perspective, and writing docs makes it easier (at least for me) to look at the project from that perspective again.


With the philosophy part done, I'd like to bring some examples.

The missing default value

In Perl 6 land, we distinguish meta classes, which control behavior of a type, and representations, which control memory layout of objects.

Most Perl 6 objects have the representation P6opaque, which provides opaque, efficient storage for objects with attributes, properties, slots, or however you call per-object storage in your favorite language. Special representations exists for interfacing with C libraries, concurrency control and so on.

The class Metamodel::Primitives provides primitives for writing meta classes, with this method:

method create_type(Mu $how, $repr) { ... }

$how is our standard name for Meta stuff (from "Higher Order Workings", or simply from controlling how stuff works), and $repr is the name of the representation.

Somebody new to meta object stuff doesn't need to know much about representations (except when they want to very low-level stuff), so the docs for create_type could have said if you don't know what representation to use, use P6opaque. Or I could just establish P6opaque as a default:

method create_type(Mu $how, $repr = 'P6opaque') { ... }

There, less to document, and somebody new to this stuff can ignore the whole representations business for a while longer.

Arbitrary restrictions

The method rotor on the List was intended to create a list of sublists with fixed number of elements from the original list, potentially with overlap. So the old API was:

method rotor($elems = 2, $overlap = 1) { ... }

And one would use it a

.say for (1..7).rotor(3, 1);
# 1 2 3
# 3 4 5
# 5 6 7

Again I had an issue with default values: It wasn't clear to me why $elems defaulted to 2 (so I removed that default), or whe $overlap defaulted to 1. Wouldn't 0 be a more intuitive default?

But my main issue was that the implementation disallowed negative overlaps, and the design docs were silent on the issue. If you visualize how rotor works (take $elems elements from the list, then step back $overlap elements, then rinse and repeat), it's clear what negative overlaps mean: they are steps forward instead of backwards, and create gaps (that is, some list elements aren't included in the sublists).

And once you allow negative steps backwards, why not go work with steps forward in the first place, which are more intuitive to the user, and explicitly allow negative steps to create overlaps?

So that's what we did, though the end result is even more general.

The crucial question here was why disallow negative overlaps?, or recognizing that a restriction was arbitrary. And then lifting it.

Wording of error messages

Error messages are important to communicate why something went wrong.

We used to have the error message Could not find an appropriate parametric role variant for $role. A test for a good error message is: ask why?, and if the piece of code that threw the error can know the answer, the error message needs improving.

In this case: why can't the runtime environment find an appropriate variant? Because it didn't try hard enough? No. Because it's buggy? I hope not. It can't find the candidate because it's not there. So, include that answer in the error message: No appropriate parametric role variant available for $role.

(Uninformative/lazy error messages are one of my favorite topics for rants; consider the good old SIOCADDRT: No such process that route(8) sometimes emits, or python's Cannot import name X -- why not? ...)

So, write those docs. Write them at a time where you can still change semantics. Keep asking yourself what you could change so the documentation becomes shorter, sweeter, easier understandable.

[/perl-6] Permanent link

Sat, 14 Mar 2015

Why is it hard to write a compiler for Perl 6?

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Russian translation available; Пост доступен на сайте Почему так трудно написать компилятор для Perl 6?.

Today's deceptively simple question on #perl6: is it harder to write a compiler for Perl 6 than for any other programming language?

The answer is simple: yes, it's harder (and more work) than for many other languages. The more involved question is: why?

So, let's take a look. The first point is organizational: Perl 6 isn't yet fully explored and formally specified; it's much more stable than it used to be, but less stable than, say, targeting C89.

But even if you disregard this point, and target the subset that for example the Rakudo Perl 6 compiler implements right now, or the wait a year and target the first Perl 6 language release, the point remains valid.

So let's look at some technical aspects.

Static vs. Dynamic

Perl 6 has both static and dynamic corners. For example, lexical lookups are statical, in the sense that they can be resolved at compile time. But that's not optional. For a compiler to properly support native types, it must resolve them at compile time. We also expect the compiler to notify us of certain errors at compile time, so there must be a fair amount of static analysis.

On the other hand, type annotations are optional pretty much anywhere, and methods are late bound. So the compiler must also support features typically found in dynamic languages.

And even though method calls are late bound, composing roles into classes is a compile time operation, with mandatory compile time analysis.

Mutable grammar

The Perl 6 grammar can change during a parse, for example by newly defined operators, but also through more invasive operations such as defining slangs or macros. Speaking of slangs: Perl 6 doesn't have a single grammar, it switches back and forth between the "main" language, regexes, character classes inside regexes, quotes, and all the other dialects you might think of.

Since the grammar extensions are done with, well, Perl 6 grammars, it forces the parser to be interoperable with Perl 6 regexes and grammars. At which point you might just as well use them for parsing the whole thing, and you get some level of minimally required self-hosting.

Meta-Object Programming

In a language like C++, the behavior of the object system is hard-coded into the language, and so the compiler can work under this assumption, and optimize the heck out of it.

In Perl 6, the object system is defined by other objects and classes, the meta objects. So there is another layer of indirection that must be handled.

Mixing of compilation and run time

Declarations like classes, but also BEGIN blocks and the right-hand side of constant declarations are run as soon as they are parsed. Which means the compiler must be able to run Perl 6 code while compiling Perl 6 code. And also the other way round, through EVAL.

More importantly, it must be able to run Perl 6 code before it has finished compiling the whole compilation unit. That means it hasn't even fully constructed the lexical pads, and hasn't initialized all the variables. So it needs special "static lexpads" to which compile-time usages of variables can fall back to. Also the object system has to be able to work with types that haven't been fully declared yet.

So, lots of trickiness involved.

Serialization, Repossession

Types are objects defined through their meta objects. That means that when you precompile a module (or even just the setting, that is, the mass of built-ins), the compiler has to serialize the types and their meta objects. Including closures. Do you have any idea how hard it is to correctly serialize closures?

But, classes are mutable. So another module might load a precompiled module, and add another method to it, or otherwise mess with it. Now the compiler has to serialize the fact that, if the second module is loaded, the object from the first module is modified. We say that the serialization context from the second module repossesses the type.

And there are so many ways in which this can go wrong.

General Featuritis

One of the many Perl 6 mottos is "torture the implementor on behalf of the user". So it demands not only both static and dynamic typing, but also functional features, continuations, exceptions, lazy lists, a powerful grammar engine, named arguments, variadic arguments, introspection of call frames, closures, lexical and dynamic variables, packed types (for direct interfacing with C libraries, for example), and phasers (code that is automatically run at different phases of the program).

All of these features aren't too hard to implement in isolation, but in combination they are a real killer. And you want it to be fast, right?

[/perl-6] Permanent link

Sun, 22 Feb 2015

Profiling Perl 6 code on IRC

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On the #perl6 IRC channel, we have a bot called camelia that executes small snippets of Perl 6 code, and prints the output that it produces. This is a pretty central part of our culture, and we use it to explain or demonstrate features or even bugs in the compiler.

Here is an example:

10:35 < Kristien> Can a class contain classes?
10:35 < Kristien> m: class A { class B { } }; say
10:35 <+camelia> rakudo-moar 114659: OUTPUT«No such method 'B' for invocant of 
                 type 'A'␤  in block <unit> at /tmp/g81K8fr9eY:1␤␤»
10:35 < Kristien> :(
10:36 < raydiak> m: class A { class B { } }; say
10:36 <+camelia> rakudo-moar 114659: OUTPUT«␤»

Yesterday and today I spent some time teaching this IRC bot to not only run the code, but optionally also run it through a profiler, to make it possible to determine where the virtual machine spends its time running the code. an example:

12:21 < moritz> prof-m: for ^100; say "done"
12:21 <+camelia> prof-m 9fc66c: OUTPUT«done␤»
12:21 <+camelia> .. Prof:

The Rakudo Perl 6 Compiler on the MoarVM backend has a profile, which produces a fancy HTML + Javascript page, and this is what is done. It is automatically uploaded to a webserver, producing this profile.

Under the hood, it started with a patch that makes it possible to specify the output filename for a profile run, and another one to clear up the fallout from the previous patch.

Then came the bigger part: setting up the Apache virtual host that serves the web files, including a restricted user that only allows up- and downloads via scp. Since the IRC bot can execute arbitrary code, it is very likely that an attacker can steal the private SSH keys used for authentication against the webserver. So it is essential that if those keys are stolen, the attacker can't do much more than uploading more files.

I used rssh for this. It is the login shell for the upload user, and configured to only allow scp. Since I didn't want the attacker to be able to modify the authorized_keys file, I configured rssh to use a chroot below the home directory (which sadly in turn requires a setuid-root wrapper around chroot, because ordinary users can't execute it. Well, nothing is perfect).

Some more patching and debugging later, the bot was ready.

The whole thing feels a bit bolted on; if usage warrants it, I'll see if I can make the code a bit prettier.

[/perl-6] Permanent link

Fri, 06 Feb 2015 some stats, future directions

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In June 2012 I started the perl6/doc repository with the intent to collect/write API documentation for Perl 6 built-in types and routines. Not long afterwards, the website was born, generated from the aforementioned repository.

About 2.5 years later, the repository has seen more than one thousand commits from more than 40 contributors, 14 of which contributed ten patches or more. The documentation encompasses about 550 routines in 195 types, with 15 documents for other things than built-in types (for example an introduction to regexes, descriptions of how variables work).

In terms of subjective experience, I observed an increase in the number of questions on our IRC channel and otherwise that could be answered by pointing to the appropriate pages of, or augmenting the answer with a statement like "for more info, see ..."

While it's far from perfect, I think both the numbers and the experience is very encouraging, and I'd like to thank everybody who helped make that happen, often by contributing skills I'm not good at: front-end design, good English and gentle encouragement.

Plans for the Future

Being a community-driven project, I can't plan anybody else's time on it, so these are my own plans for the future of

Infrastructural improvements

There are several unsolved problems with the web interface, with how we store our documents, and how information can be found. I plan to address them slowly but steadily.

  • The search is too much centered around types and routines, searching for variables, syntactic constructs and keywords isn't easily possible. I want it to find many more things than right now.
  • Currently we store the docs for each type in a separate file called Type.pod. Which will break when we start to document native types, which being with lower case letters. Having int.pod and Int.pod is completely unworkable on case-insensitive or case-preserving file system. I want to come up with a solution for that, though I don't yet know what it will look like.
  • is served from static pages, which leads to some problems with file names conflicting with UNIX conventions. You can't name a file infix:</>.html, and files with two consecutive dots in their names are also weird. So in the long run, we'll have to switch to some kind of dynamic URL dispatching, or a name escaping scheme that is capable of handling all of Perl 6's syntax.
  • Things like the list of methods and what they coerce to in class Cool don't show up in derived types; either the tooling needs to be improved for that, or they need to be rewritten to use the usual one-heading-per-method approach.


Of course my plan is to improve coverage of the built-in types and routines, and add more examples. In addition, I want to improve and expand on the language documentation (for example syntax, OO, regexes, MOP), ideally documenting every Perl 6 feature.

Once the language features are covered in sufficient breadth and depth (though I won't wait for 100% coverage), I want to add three tutorial tracks:

  • A track for beginners
  • A quick-start for programmers from other languages
  • A series of intermediate to advanced guides covering topics such as parsing, how to structure a bigger application, the responsible use of meta programming, or reactive programming.

Of course I won't be able to do that all on my own, so I hope to convince my fellow and future contributors that those are good ideas.

Time to stop rambling about the future, and off to writing some docs, this is yours truly signing off.

[/perl-6] Permanent link

Thu, 05 Feb 2015

All Perl 6 modules in a box

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Sometimes when we change things in the Perl 6 language or the Rakudo Perl 6 compiler that implements it, we want to know if the planned changes will cause fallout in the library modules out there, and how much.

To get a quick estimate, we can now do a git grep in the experimental perl6-all-modules repository.

This is an attempt to get all the published module into a single git repository. It is built using git subrepo, an unofficial git extension module that I've been wanting to try for some time, and that seems to have some advantages over submodules in some cases. The notable one in this case being that git grep ignores submodules, but descends into subrepos just fine.

Here is the use case that made me create this repository: Rakudo accesses low-level operations through the nqp:: pseudo namespace. For example nqp::concat_s('a', 'b') is a low-level way to concatenate two strings. User-level programs can also use nqp:: ops, though it is generally a bad idea, because it ties the program to the particular compiler used, and what's more, the nqp:: ops are not part of the public API, and thus neither documented in the same place as the rest of Perl 6, nor are there any promises for stability attached.

So we want to require module authors to use a pragma, use nqp; in order to make their use of compiler internal explicit and deliberate. And of course, where possible, we want them to not use them at all :-)

To find out how many files in the ecosystem use nqp:: ops, a simple command, combined with the power of the standard UNIX tools, will help:

$ git grep -l 'nqp::'|wc -l

That's not too bad, considering we have... how many modules/distributions again?

Since they are added in author/repo structure, counting them with ls and wc isn't hard:

ls -1d */*/|wc -l

Ok, but number of files in relation to distributions isn't really useful. So let's ask: how many distributions directly use nqp:: ops?

$ git grep -l nqp:: | cut -d/ -f1,2 |sort -u|wc -l

23 out of 282 (or about 8%) distributions use the nqp:: syntax.

By the way, there is a tool (written in Perl 6, of course) to generate and update the repository. Not perfect yet, very much a work in progress. It's in the _tools folder, so you should probably filter out that directory in your queries (though in the examples above, it doesn't make a difference).

So, have fun with this new toy!

[/perl-6] Permanent link

Wed, 10 Dec 2014

New Perl 6 community server now live, accepting signups

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The new Perl 6 community server is now alive and kicking.

As planned, I've set up KVM virtualization, and so far there are two guest systems. is meant for general Perl 6 development activity (which also includes irssi/weechat sessions), and is equipped with 20GB RAM to handle multiple concurrent rakudo-jvm compilations :-). It runs a pretty bare-bones Debian Jessie.

Update: there is now a website for the new server. is the web server where I plan to host and related (sub-)domains. It's not as beefy as hack, but sufficiently large to compile and run Rakudo, in preparation for future Perl 6-based web hosting. Currently I'm running a copy of several subdomains on it (with the domain name p6c instead of perl6 for test purposes); the plan is to switch the DNS over once all of the websites have been copied/migrated.

If you have a Perl 6 related use for a shell account or for serving websites, please request an account by email ( or IRC (moritz on freenode and magnet), including:

  1. Your desired username
  2. What you want to do on the machine(s) (not necessary for #perl6 regulars)
  3. Which of the machine(s) you need access to
  4. Optionally an openssh public key
  5. Whether you'd be willing to help a bit with sysadmin tasks (mostly apt-get update && apt-get dist-upgrade, restarting hung services, killing huge processes)
  6. Software you need installed (it's OK to not know this up-front)

Note that will shut down soon (no fixed date yet, but "end of 2014" is expected), so if you rely on feather now, you should consider migrating to the new server.

The code of conduct is pretty simple:

  1. Be reasonable in your resource usage.
  2. Use technical means to limit your resource usage so that it doesn't accidentally explode (ulimit comes to mind).
  3. Limit yourself to legal and Perl 6-related use cases (no warez).
  4. Help your fellow hackers.

The standard disclaimer applies:

  • Expect no privacy. There will potentially be many root users, who could all read your files and memory.
  • There are no promises of continued service or even support. Your account can be terminated without notice.
  • Place of jurisdiction in Nürnberg, Germany. You have to comply with German law while using the server. (Note that this puts pretty high standards on privacy for any user data you collect, including from web applications). It's your duty to inform yourself about the applicable laws. Illegal activities will be reported to the authorities.

With all that said, happy hacking!.

[/perl-6] Permanent link

Tue, 02 Dec 2014

A new Perl 6 community server - update

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In my previous post I announced my plans for a new Perl 6 community server (successor to, and now I'd like to share some updates.

Thanks to the generosity of the Perl 6 community, the server has been ordered and paid. I am now in the process of contacting those donors who haven't paid yet, leaving them the choice to re-purpose their pledge to ongoing costs (traffic, public IPv4 addresses, domain(s), SSL certs if necessary) and maintenance, or withdraw their pledges.

Some details of the hardware we'll get:

  • CPU: Intel® Xeon® Haswell-EP Series Processor E5-2620 v3, 2.40 GHz, 6-Core Socket 2011-3, 15MB Cache
  • RAM: 4x8GB DDR4 DDR4 PC2133 Reg. ECC 2R
  • HD: 2x 2TB SATA3-HD

The vendor has told me that all parts have arrived, and will be assembled today or tomorrow.

Currently I lean towards using KVM to create three virtual hosts: one for websites (*, perlcabal.syn), one for general hacking and IRC activity, and one for high-risk stuff (evalbots,, ...).

I've secured the domain (for "perl 6 community"), and the IPv4 range - and the IPv6 net 2001:780:101:ff00::/64.

So the infrastructure is in place, now I'm waiting for the delivery of the hardware.

[/perl-6] Permanent link

Wed, 05 Nov 2014

A new Perl 6 community server - call for funding

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So far, many Perl 6 developers have used feather as a generic development server. Juerd, who has genereously provided this server for us for free for many years, has announced that it will be shut down at the end of the year.

My daytime job is at a b2b IT outsourcing and hosting company called noris network, and they have agreed to sponsor the hosting/housing of a 1U 19" server in one of their state-of-the-art data centers in Nürnberg, Germany.

What's missing is the actual hardware. Some folks in the community have already agreed to participate in funding the hardware, though I have few concrete pledges.

So here is the call to action: If you want to help the Perl 6 community with a one-time donation towards a new community server, please send me an e-mail to moritz at faui2k3 dot org, specifying the amount you're willing do pledge, and whether you want to stay private as a donor. I accept money transfer by paypal and wire transfer (SWIFT). Direct hardware donations are also welcome. (Though actual money will be deferred until the final decision what hardware to buy, and thus the total amount required).

How much do we need?

Decent, used 1U servers seem to start at about 250€, though 350€ would get us a lot more bang (mostly RAM and hard disk space). And in general, the more the merrier. (Cheaper offers exist, for example on ebay, but usually they are without hard disks, so the need for extra drives makes them more expensive in total).

With more money, even beefier hardware and/or spare parts and/or a maintainance contract and/new hardware would be an option.

What do we need it for?

The main tasks for the server are:

  • Hosting websites like and the synopses
  • Hosting infrastructure like the panda metadata server
  • Be available for smoke runs of the compilers, star distributions and module ecosystem.
  • Be available as a general development machine for people who don't have linux available and/or not enough resources to build some Perl 6 compilers on their own machines comfortably.
  • A place for IRC sessions for community memebers
  • A backup location for community services like the IRC logs, the camelia IRC eval bot etc. Those resources are currently hosted elswewhere, though having another option for hosting would be very valuable.
  • A webspace for people who want to host Perl 6-related material.
  • It is explicitly not meant as a general hosting platform, nor as a mail server.


    If the hardware we get is beefy enough, I'd like to virtualize the server into two to three components. One for hosting the and related websites that should be rather stable, and one for the rest of the system. If resources allow it, and depending on feedback I get, maybe a third virtual system for high-risk stuff like evalbot.

    As operating system I'll install Debian Jessie (the current testing), simply because I'll end up maintaing the system, and it's the system I'm most familiar with.

[/perl-6] Permanent link

Sat, 19 Oct 2013

A small regex optimization for NQP and Rakudo

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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.

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Wed, 14 Aug 2013

YAPC Europe 2013 Day 3

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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.

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