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Perl 6 in 2011 - A Retrospection

Moritz Lenz — Sat, Dec 31 10:00:00 2011

The change of year is a good occasion to look back. Here I want to reflect on the development of Perl 6, its compilers and ecosystem.

At the start of the year, masak's Perl 6 Coding Contest continued from 2010, concluding in the announcement of the winner. I must admit that I still haven't read all the books I won :-)

Specification

2011 was a rather quiet year in terms of spec changes; they were a mixture of responses to compiler writer and user feedback, and some simplifications and cleanups.

Positional parameters used to be allowed to be called by name; this feature is now gone. That both makes the signature binder simpler, and removes accidental dependencies on names that weren't meant to be public. Read the full justification for more background.

A small change that illustrates the cleanup of old, p5-inherited features was the change that made &eval stop catching exceptions. There is really no good reason for it to catch them, except Perl 5 legacy.

say now uses a different stringification than print. The reasoning is that print is aimed at computer-readable output, whereas say is often used for debugging. As an example, undefined values stringify to the empty string (and produce a warning), whereas say calls the .gist method on the object to be said, which produces the type name on undefined values.

An area that has been greatly solidified due to implementation progress is Plain Old Documentation or Pod. Tadeusz Sośnierz' Google Summer of Code project ironed out many wrinkles and inconsistencies, and changed my perception of this part of the spec from "speculative" to "under development".

Rakudo

Rakudo underwent a huge refactoring this year; it is now bootstrapped by a new compiler called "nqp", and uses a new object model (nom).

It allows us to gain speed and memory advantages from gradual typing; for example the mandelbrot fractral generator used to take 18 minutes to run on a machine of mine, and now takes less than 40 seconds. Speedups in other areas are not as big, but there is still much room for improvement in the optimizer.

With the nom branch came support for different object representations. It makes it possible to store object attributes in simple C-like structs, which in turn makes it much easier and more convenient to interoperate with C libraries.

Tadeusz' work on Pod gave Rakudo support for converting Pod to plain text and HTML, and attach documentation objects to routines and other objects.

Rakudo now also has lazy lists, much better role handling, typed exceptions for a few errors, the -n and -p command line options, support for big integers, NFA-based support for proto regexes and improvements to many built-in functions, methods and operators.

Niecza

It is hard to accurately summarize the development of Niecza in a few sentences; instead of listing the many, many new features I should give an impression on how it feels and felt for the user.

At the start of 2011, programming in niecza was a real adventure. Running some random piece of Perl 6 code that worked with Rakudo rarely worked, most of the time it hit a missing built-in, feature or bug.

Now it often just works, and usually much faster than in Rakudo. There are still some missing features, but Stefan O'Rear and his fellow contributors work tirelessly on catching up to Rakudo, and it some areas Niecza is clearly ahead (for example Unicode support in regexes, and longest-token matching).

Since Niecza is implemented on top of the Common Language Runtime (CLR) (which means .NET or mono), it makes it easy to use existing CLR-based libraries. Examples include an interactive fractal generator and a small Tetris game in Perl 6.

Perlito

Perlito aims to be a minimal compiler with multiple backends, which can be used for embedding and experimenting with Perl 6. It had several releases in 2011, and has interesting features like a Javascript backend.

Ecosystem

The presence of two usable compilers (and in the case of Rakudo, two viable but very different branches) has led to many questions about the different compilers. The new Perl 6 Compiler Feature matrix tries to answer the questions about the state of the implemented features in the compilers.

With Panda we now have a module installer that actually works with Rakudo. It still has some lengths to go in terms of stability and feature completeness, but it is fun to work with.

The new Perl 6 Modules page gives an overview of existing Perl 6 modules; we hope to evolve it into a real CPAN equivalent.

Community

This year we had another Perl 6 Advent Calendar, with much positive feedback both from the Perl 6 community and the wider programming community.

We were also happy to welcome several new prolific contributors to the Perl 6 compilers and modules. The atmosphere in the community still feels relaxed, friendly and productive -- I quite enjoy it.

The year ends like it started: with a Perl 6 Coding Contest. This is a good opportunity to dive into Perl 6, provide feedback to compiler writers, and most of all have fun.

Fourth Grant Report: Structured Error Messages

Moritz Lenz — Mon, Dec 19 10:28:48 2011

Progress on my grant for error message is slow but steady. Since my last report, I've done the following things:

Merged the nom-exceptions Rakudo branch, so now you can reliably throw Perl 6 objects as exceptions.
Implemented several error classes and roles in Rakudo
Started to throw typed errors both from runtime libraries and from inside the compiler
Hacked the default exception printer Rakudo to be much more flexible, for example you can now write exception classes that supress backtraces from the standard handler.
Wrote tests for typed run time and compile time errors, and at the same time developed a test function that makes it easy to write such tests.

It's time for a quick review of how far I am along the various deliverables in the original grant proposal.

D1: Specification. I think the hard work here is done already, what remains to do is finding good default and how to manipulate them (for example, how to generally switch on/off printing of backtraces?).
D2: Error catalogue, tests: I've not worked on this one too much. The error classes and roles so far mostly served to exercise the implementation; going through the existing errors from the various compilers and formalizing them will be quite a bit of work, but only moderately complicated.
D3: Implementation, documentation. Like D1, the hard part is mostly done. We can now throw errors from within the compiler actions and from the setting, next up will be the grammar. Then all places where errors are thrown need to be changed to use the new error classes. Again that'll be much work, but easy to do. Documentation is still missing.

All in all I feel I'm well on the way, and most complex decisions have been made.

For a more user oriented view of the new exception system I'd like to point you to my Perl 6 advent calendar post on exceptions.

The Three-Fold Function of the Smart Match Operator

Moritz Lenz — Wed, Oct 12 11:37:08 2011

In Perl 5, if you want to match a regex against a particular string, you write $string =~ $regex.

In the design process of Perl 6, people have realized that you cannot only match against regexes, but lots of other things can act as patterns too: types (checking type conformance), numbers, strings, junctions (composites of values), subroutine signatures and so on. So smart matching was born, and it's now written as $topic ~~ $pattern. Being a general comparison mechanism is the first function of the smart match operator.

But behold, there were problems. One of them was the perceived need for special syntactic forms on the right hand side of the smart match operator to cover some cases. Those were limited and hard to implement. There was also the fact that now we had two different ways to invoke regexes: smart matching, and direct invocation as m/.../, which matches against the topic variable $_. That wasn't really a problem as such, but it was an indicator of design smell.

And that's where the second function of the smart match operator originated: topicalization. Previously, $a ~~ $b mostly turned into a method call, $b.ACCEPTS($a). The new idea was to set the topic variable to $a in a small scope, which allowed many special cases to go away. It also nicely unified with given $topic { when $matcher { ... } }, which was already specified as being a topicalizer.

In the new model, MATCH ~~ PAT becomes something like do { $_ = MATCH; PAT.ACCEPTS($_) } -- which means that if MATCH accesses $_, it automatically does what the user wants.

Awesomeness reigned, and it worked out great.

Until the compiler writers actually started to implement a few more cases of regex matching. The first thing we noticed was that if $str ~~ $regex { ... } behaved quite unexpectedly. What happend was that $_ got set to $str, the match was conducted and returned a Match object. And then called $match.ACCEPTS($str), which failed. A quick hack around that was to modify Match.ACCEPTS to always return the invocant (ie the Match on which it was called), but of course that was only a stop gap solution.

The reason it doesn't work for other, more involved cases of regex invocations is that they don't fit into the "does $a match $b?" schema. Two examples:

# :g for "global", all matches
my @matches = $str ~~ m:g/pattern/; 

if $str ~~ s/pattern/substitution/ { ... }

People expect those to work. But global matching of a regex isn't a simple conformance check, and that is reflected in the return value: a list. So should we special-cases smart-matching against a list, just because we can't get global matching to work in smart-matching otherwise? (People have also proposed to return a kind of aggregate Match object instead of a list; that comes with the problem that Match objects aren't lazy, but lists are. You could "solve" that with a LazyMatch type; watch the pattern of workarounds unfold...)

A substitution is also not a simple matching operation. In Perl 5, a s/// returns the number of successful substitutions. In Perl 6, that wouldn't work with the current setup of the smart match operator, where it would then smart-match the string against the returned number of matches.

So to summarize, the smart match operator has three functions: comparing values to patterns, topicalization, and conducting regex matches.

These three functions are distinct enough to start to interact in weird ways, which limits the flexibility in choice of return values from regex matches and substitutions.

I don't know what the best way forward is. Maybe it is to reintroduce a dedicated operator for regex matching, which seems to be the main feature with which topicalization interacts badly. Maybe there are other good ideas out there. If so, I'd love to hear about them.

Third Grant Report: Structured Error Messages

Moritz Lenz — Mon, Aug 29 10:56:41 2011

Progress on my grant for error message is slower than expected, as expected :-). Yes, you've read that sentence before.

In the past months, general hacking on the nom branch of Rakudo was just too much fun -- and partially a prerequisite for the exceptions work.

I did manage to redo the backtraces that are generated from error messages.

Backtraces are now generated mostly in Perl 6 code, making them much more hackable. There's a Backtrace class, which is a list of Backtrace::Frame objects, each knowing the code object associated with it, as well as line number and file. (This is both specced and works in Rakudo)

Routines can have the is hidden_from_backtrace trait, which makes them not show up in the default backtrace stringification (one can still request a .full string representation). This is useful for routines which are internally used to generate exceptions, like die().

Rakudo also has a --ll-exceptions command line option which provides PIR-level backtraces, in the rare case the Perl 6 level backtraces hide too much information.

I've also started the nom-exceptions branch in Rakudo, which aims at lifting current limitations in Rakudo's exception handling. Currently die() and friends generate a parrot exception, and then there's a routine that fills the error variable $!. This routine generates a new Exception object, and sticks the parrot exception into it.

This practice means that if you create a subclass of Exception, instantiate it and throw it, you still only get an Exception in the error handler, not an object of the subclass. Since the actual exception type is very important for the ongoing work, that has to change. The branch mentioned earlier allows one to generate a Perl 6 exception, and pass that on as the payload of the parrot exception, which is then unwrapped when filling $!.

As a proof of concept this works, but it suffers from not being robust enough -- as it is, we could accidentally unwrap the payload of a CONTROL exception, placing meaningless junk into $!. So this needs a bit more work, which I plan to do this week (or next, if it proves to be more difficult than anticipated).

As always, your feedback is very welcome.

Why Rakudo needs NQP

Moritz Lenz — Wed, Aug 24 13:43:55 2011

Rakudo, a popular Perl 6 compiler, is built on top of a smaller compiler called "NQP", short for Not Quite Perl.

Reading through a recent ramble by chromatic, I felt like he said "Rakudo needs NQP to be able to ditch Parrot, once NQP runs on a different platform" (NQP is the "another layer", which sits between Rakudo and Parrot, mentioned in the next-to-final paragraph).

I'm sure chromatic knows that VM independence is the least important reason for having NQP at all, but the casual reader might not, so let me explain the real importance of NQP for Rakudo here.

The short version is just a single word: bootstrapping.

The longer version is that large parts of Rakudo are written in Perl 6 itself (or a subset thereof), and something is needed to break the circularity.

In particular the base of the compiler is written in a subset of Perl 6, and NQP compiles those parts to bytecode, which can then compile the rest of the compiler.

This is not just because we have a fancy for Perl 6, and thus want to write as much of the code in Perl 6, but there are solid technical reasons for writing the compiler in Perl 6.

In Perl 6, the boundary between run time and compile time is blurred, as well as the boundary between the compiler, the run time library and user-space code. For example you alter the grammar with which your source code is parsed, by injecting your own grammar rules.

"Your own grammar rules" above refers to user-space code, while the grammar that is being altered is part of the compiler. If we had written the compiler in something else than Perl 6 (for example Java), it would be horribly difficult to inject user-space Perl 6 code into compiled code from a different language.

And the code not only needs to be injected, but the data passed back and forth between the compiler and the user space need to be Perl 6 objects, so all important data structures in the compiler need to be Perl 6 based anyway.

And it's not just for grammar modifications: At its heart, Perl 6 is an object oriented language. When the compiler sees a class definition, it translates them to a series of method calls on the metaobject, which again needs to be a Perl 6 object, otherwise it wouldn't be easily usable and extensible from the user space.

Now you might think that grammar modifications and changes to the Metaobject are pretty obscure features, and you could get along just fine with an incomplete Perl 6 compiler that neglected those two areas. But even then you'd have lots of interactions between run time and compile time. For example consider a numeric literal like 42. Obviously that needs to be constructed of type Int. What's less obvious is that it needs to be constructed to be of type Int at compile time already, because Perl 6 code can run interleaved with the compilation. So the compiler needs to be able to handle Perl 6 objects in all their generality, which is a huge pain if the compiler is not written in Perl 6.

Rakudo has cheated on that front in the past, and consequently has had lots of bugs and limitations due to non-Perl 6 objects leaking out at unexpected ends. If you ever got a "Null PMC Access" from Rakudo, you know what I mean.

The lesson we learned was that you need a Perl 6 compiler to implement a Perl 6 compiler, even if that first Perl 6 compiler can handle only a rather limited subset of Perl 6.

And there are also quite some benefits to this approach. For example NQP's new regex engine is implemented as a role in NQP. It is mixed into an NQP class which allows us to build Rakudo, but it is also mixed in a Perl 6 class, which allows the generation of Perl 6-level Match objects without any need to create NQP-level match objects first, and then wrap them in Perl 6 Match objects.

That's what NQP does for us. It allows us to actually write a Perl 6 compiler.

Perl 6 Compiler Feature Matrix

Moritz Lenz — Tue, Jul 26 12:07:37 2011

We now have a nice table that tells you which Perl 6 compiler implements what..

Such a thing was long overdue. When the topic came up in the past, people have suggested mostly automated solutions that compared test coverage of compiles to generate such a table. Nothing came out of it, it would have been a rather large endeavor. Now Eevee blogged about the lack of some easy overview that tells you what is implemented in Rakudo., and I thought it was time to tackle the problem.

Instead of some advanced automated system, we now have a simple text file, and a short perl script that converts it to a HTML page.

I'd like to thanks Will Coleda, Patrick Michaud and Stefan O'Rear for their contributions, and encourage everybody to keep the data up to date.

How fast is Rakudo's "nom" branch?

Moritz Lenz — Sat, Jul 2 08:43:01 2011

Nearly one year ago, the Rakudo Perl 6 developers proudly released the first Rakudo Star, a distribution aimed at showing the world what Perl 6 can look like, and in turn get feedback from more early adaptors.

And feedback we got. While the overall response was very positive, people had one main concern: it was too slow. That didn't come as a surprise, considering that we had focused on features first. Now it was time to change that, and work on massive performance improvements.

That is easier said than done. One of the reasons is that Rakudo is tightly coupled to the parrot virtual machine, but there is a lot of mismatch between the two. For example parrot provides multi dispatch built-in, but not quite with the semantics that Perl 6 needs. Same for parameter binding, objects and a number of other areas.

In the following year, parrot got a new, faster garbage collector, and Jonathan Worthington came up with a cache for type checks at routine call time.

This sped up this simple mandelbrot fractal generator at size 201x201 from 18 minutes to 16 minutes 14 seconds. Actually the speedup was better than that, but we paid a performance penalty for new features, bug fixes and parrot performance regressions.

But it was clear that more substantial improvements where needed. One of the most promising candidates for speedups is a complete redesign of the object model, resulting in the "nom" (new object model) branch of Rakudo. Additionally to providing much more well suited OO primitives than parrot can offer right now, it also allows to share more information between compile time and run time, making a lot of optimizations possible.

Yesterday I sped up some operations on Complex numbers, and implemented a built-in that was missing to run the mandelbrot script. And today I timed it: 3 Minutes. From originally 18 Minutes.

Now that's a speedup by more than a factor of 5. I'm not sure if it will extend to other operators, but it sure is encouraging.

And that's without the optimizations that will now be possible, for example inlining operators. So after a literally slow start, Rakudo Perl 6 has a bright and fast future ahead. And it's already here, just not evenly distributed.

Introducing my new project: Quelology organizes books

Moritz Lenz — Sun, Jun 26 12:39:43 2011

For about half a year I've been working on a website called quelology, which collects book series and translations.

It is intended to answer questions of the form: I've now read "Harry Potter and the Order of the Phoenix", which is the next book in that series? or What's the name of the French translation of that book?

The website and data mining behind it are written in Perl, and it is based on book meta data by isfdb, amazon and worldcat.

I'm working on importing data from more sources, next up will be the Swedish National Library.

After completing the data mining stage, I'll add an interfaces that allows the visitor to edit the book, series and translations data, so that users can extend the data body.

Why is my /tmp/ directory suddenly only 1MB big?

Moritz Lenz — Tue, Jun 14 18:48:00 2011

Today I got a really weird error on my Debian "Squeeze" Linux box -- a processes tried to write a temp file, and it complained that there was No space left on device.

The weird thing is, just yesterday my root parition was full, and I had made about 7GB free space in it.

I checked, there was still plenty of room today. But behold:

$ df -h /tmp/
Filesystem            Size  Used Avail Use% Mounted on
overflow              1.0M  632K  392K  62% /tmp

So, suddenly my /tmp/ directory was a ram disc with just 1MB of space. And it didn't show up in /etc/fstab, so I had no idea what cause it.

After googling a bit around, I found the likely reason: as a protection against low disc space, some daemon automatically "shadows" the current /tmp/ dir with a ram disc if the the root partition runs out of disc space. Sadly there's no automatic reversion of that process once enough disc space is free again.

To remove the mount, you can say (as root)

umount -l /tmp/

And to permanently disable this feature, use

echo 'MINTMPKB=0' > /etc/default/mountoverflowtmp

Second Grant Report: Structured Error Messages

Moritz Lenz — Sun, May 22 08:28:21 2011

Progress on my grant for error message is slower than expected, as expected :-). Life and work, though rewarding, take their toll. Also many of my original ideas turned out to be not very good, and got shot down by @Larry (with good reason).

However I did get around to lay out some specification for exceptions in S32::Exception (rendered) - some basic roles, and the relationships between exception, Failure (lazy/unthrown exceptions) and backtraces.

More importantly I think I'm at a point where I could start to actually implement some of that stuff, and use that experience to update the specification.

First Grant Report: Structured Error Messages

Moritz Lenz — Sun, Apr 10 13:31:31 2011

My Hague Grant proposal for designing, implementing and testing structured error messages for Perl 6 has been acceepted, and I've started my work on it in my copious free time.

Before the grant started I've unified the error messages of several compilers to use "Cannot" instead of a wild mixture of "Cannot", "Can not" and "Can't".

In the past week I created a repository for the initial work on the error message spec, and added a list of existing error messages across different compilers, and some notes regarding the upcoming spec.

So far I've outlined some thoughts about separation of concerns, classification of the error messages, testing error messages for certain properties, and calling syntax for die() and fail().

Any constructive feedback on it is very welcome.

Thanks go to Ian Hague and The Perl Foundation for supporting my work financially.

Perl 6 notes from February 2011

Moritz Lenz — Mon, Feb 14 17:59:15 2011

Lately real life has prevented me from blogging, so here are just a few random notes from the Perl 6 developers:

The Perl bug tracker now has tags testneeded and testcommitted, which can mark tests that need or have tests in the spectest suite. Since the URLs for querying these tags are unwieldy and non-obvious, I've created some aliases: http://rakudo.de/testneeded and http://rakudo.de/testcommitted.

Development of the new nqp and rakudo-on-the-new-object-model is progressing nicely. I had some fun porting some PIR code to NQP, and writing some new code. Most interesting to read is the source of the new meta model, much of which is written in a subset of Perl 6 (so quite readable, if you happen to know Perl 6. For example you can see how the method resolution order for multiple inheritance is calculated.

There is a parrot branch that adds a generation garbage collector to parrot. Its release is planned for shortly after the 3.1.0 release due tomorrow. Initial benchmarks show that Rakudo is between 25% and 30% faster on that parrot, as measured by a spectest run. I very much look forward to having that in the parrot main line.

Writing code for niecza is quite a nice experience. It still has a big startup cost, but then runs much faster than rakudo (at least it feels that way). There are still lots of features missing (for example non-integer number literals), but feature requests are usually implement quite quickly.

An offer for software developers: free IRC logging

Moritz Lenz — Mon, Feb 7 18:20:43 2011

The Perl 6 developers communicate a lot through IRC. Some of the conversation is still valuable later on, so we have public IRC logs.

The software powering these logs was written especially for #perl6, but works fine for other channels too. Among the other users are TestML, CDK (Chemistry Development Kit), darcs, mojo, Padre, the Perl IDE, Parrot and Rosetta Code.

If you are also developing software, and would like public logs for your channel (either on freenode or irc.perl.org; other servers might be added on demand), feel free to contact me (moritz on freenode, or per email: moritz at faui2k3.org)

Features include: linking to individual lines, permanent URLs and volatile URLs for the current day, automatic linking of URLs and readable color schemes.

A current limitation is that you can't have two channels with the same name from different networks, in case of conflict "first come, first served" holds.

Thoughts on masak's Perl 6 Coding Contest

Moritz Lenz — Sun, Jan 23 19:38:48 2011

Masak's Perl 6 Coding Contest (short p6cc) is now in its final stage - the public commentary of solutions. So far masak has commented on p1, p2, and p3. I enjoyed the reviews and explanations so far, and look forward to more (except to the p4 review, because I botched the solution to this one).

The reviews made a good read, and here are only a few very minor points that I find worth mentioning. No criticism intended (neither to the author nor the reviewer).

Matthias' p1 solution uses side effects in subroutine named to-string. I would avoid that in "production" code (for whatever I might mean with that word...), since it's not what I would expect from the name. Instead of a counter, the array indexes could be used to identify which matrix to stringify.
Masak's review of fox' p1 solution contains the question I wonder why the @items array deserved a plural but the @matrix only a singular.... I'd say that's because @matrix holds one matrix, but @items holds many items. Speaking of which, I don't like the name @matrix - it describe a structure rather than the contents. The structure becomes pretty obvious through the access, but what is in the matrix?
After viewing my own p1 submission without syntax hilighting, I wonder why I chose so many double blank lines. My vim color scheme uses a dark blue for comments, which means that comment blocks need more visual distance from code, in my personal opinion. But when sharing code, I shouldn't make layout decisions based on non-shared syntax hilighting.
colomon's p2 submission uses complex numbers for coordinates. I considered that myself, and there's nothing wrong with it. Just strange that he then reinvented subtraction of two complex numbers in sub lines-intersect. Maybe he didn't want to come up with names for the intermediate results, $slope and $axes-intercept might have been viable ones.
Matthias could have simplified comb: /<&number>/ to just comb: &number -- a regex is just a callable, and when it's explicit in the current scope, you can just use a hard reference to it.
colomon's p3 solution could, as far as I can tell, replace all regexes with tokens - less backtracking, fewer surprises.
Reviewing my own p3 solution, masak asked I wonder what stopped moritz from, rather than doing $_ && .including on line 39, doing .?including instead. Either he considered that too cute, or he didn't consider it. I did consider it, but I decided against it, because the two things are subtly different. Mine only calls the method if the topic is true, whereas .?including always tries the call - even on a type object, where accessing an attribute leads to a fatal error. I think in my code that case doesn't show up, but it did appear during debugging in earlier versions.

Does Your Bug Tracker Do This For You?

Moritz Lenz — Sun, Jan 9 11:22:17 2011

While working on a fast moving Open Source project, we sometimes get some platform specific bug reports. And even if nobody explicitly works on the problem, sometimes they go away due to general improvements of robustness.

So sometimes an intrepid volunteer goes through the list of old bugs, and asks the reporter if these platform specific bugs still persist. Quite often, we don't get a reply. Then the reports aren't very useful to us, and should be closed. Which requires a second pass through the bug queue. With more than 600 open bugs this quite a bit of work.

So I'd like to have an option to automatically close a bug within a given time span (say, four weeks or two months) if no feedback occurs. Our bug tracker (or at least our installation) doesn't have such a button.

Does your favorite bug tracker have it? Would you also sometimes like such a button?