This week on Perl 6, week ending 2004-03-28

... and we're back! Another interesting week in Perl 6. Your Summarizer even wrote some [parrot] code and it's been simply ages since he did that. In accordance with ancient custom, we'll start the summary with perl6-internals.

Building with miniparrot

Back in the early days Dan proposed, and it was generally agreed that the Parrot build process wouldn't be Perl dependent, but instead there would be a few OS specific 'bootstrap' scripts, enough to get miniparrot up and running. Miniparrot would have just enough smarts to be able to complete the configuration and build the final full parrot.

After last week's discussion about reinventing metaconfig, I wondered if the miniparrot plan was still in place. It seems I'd missed the discussion of stat that ended up talking about how miniparrot would be able to do its job. I find myself wondering what else is needed to get miniparrot to the point where it can start doing configuration work.

Continuations continued (and fun with stacks)

Warning: The following discussion of the Continuation discussions is irrevocably biased; I find it very hard to be objective about discussions I participate in, and I was rather loud mouthed in this one.

The previous discussions of the uses and semantics of continuations carried over into this week. Piers Cawley argued that the current stack architecture seemed to be optimized for the wrong thing, with the special case RetContinuations being symptoms. He argued that current architecture (where a single stack frame can accommodate multiple pushes, with copy on write semantics being used to handle full continuations) should be replaced with a 'naïve' architecture using linked lists of immutable, simple stack frames, one frame per push. Switching to this approach, he argued, would do away with a great deal of code complexity, and issues of high object creation overhead could be offset by using free lists and preallocation to reuse stack frames. Oh yes, and there'd be no difference between a RetContinuation and a full Continuation with this scheme.

Leo Tötsch wasn't convinced. Dan was though, and made the decision to switch to single item per frame, immutable, non COW stacks. Leo implemented it. His first cut was rather slow; later refinements added freelists and other handy stuff to start pulling the performance back up. I'm sure there's more refinement to come though.

Variadic functions

Ilya Martynov had some questions about how to handle variadic functions. Leo clarified some things and pointed Ilya at the foldup op. Jens Rieks suggested aliasing the registers I[1-4] to argc[ISPN], which Leo liked. I'm not sure he's implemented it yet though.

GCC compiling to Parrot

In previous weeks Gerard Butler had posted wondering about getting GCC to target Parrot. The initial response was rather negative, pointing out that GCC and Parrot saw memory very differently, to the extent that there would probably be a need have special PMCs for GCC managed memory, which would make communication between GCC implemented languages and Parrot implemented ones rather tricky.

Undeterred, Gerald mapped out a way forward and asked for opinions. Dan thought the scheme looked reasonable, but fenced that with the caveat that he knows nothing about GCC's internals.

Safe execution core and ops classification

Leo checked in some patches from Jens Rieks to allow classification of ops. He thought that this meant we were a good way along the road to providing a 'Safe' run-core option, though there was still a lot to do. He outlined a road map and asked for comments (and implementations). Comments were forthcoming, and Dan eventually bundled his comments up into a single post with some design in it. For some reason this induced some industrial hand waving about Proof Carrying Code from Steve Fink (he accused himself of hand waving, not me).

Jarkko Hietaniemi (Emacs' dynamic completion suggested 'Jarkko Himself' for that one. Well, it made me smile) offered the pathological

    eval 'while(push@a,0){}'

as an example of the kind of bad things that can happen if you allow eval EXPR in your 'safe' code, even with strict rules on what it's allowed to compile (Dan pointed out that quotas would help in this particular case...)

If there's one lesson to take from the discussion it's this: Code Safety is Hard. Whether it's AI Hard or not is left as an exercise for the interested reader. -- Dan's big(gish) post

UNO (Universal Network Objects) interface for Parrot?

Tim Bunce pointed everyone at OpenOffice's Universal Network Objects and wondered if anyone had had a look to see what is needed to plug Parrot into them. And was promptly Warnocked. -- More on UNO

Load paths

In Perl, it's possible to write require Some::Module, and Perl will go off and hunt for the appropriate file in the various directories in its @INC. You can do something similar in most languages.

Right now, you can't do it in Parrot though; Parrot's load_bytecode and other such ops take filesystem paths so, if things aren't set up exactly as the programmer expects, Bad Things can happen.

As Dan (and others on IRC and elsewhere I'm sure) points out, this is sub optimal. He posted an overview of the issue and a few possible ways forward and asked for comments. There were several. Mostly along the lines of 'core support for full over the net URIs for bytecode loading would be unutterably Bad'.

Tcl, looking for a few good people

Will Coleda's Tcl implementation has apparently reached the point where he'd appreciate assistance. He said as much on the list. If you're interested in helping getting a full Tcl implementation that targets Parrot up and running then drop him a line.

Ulterior Reference Counting for DoD

Andre Pang pointed the list at a paper on yet another Garbage Collection strategy called 'Ulterior Reference Counting' that looks potentially interesting. However, it turns out that it doesn't quite work that well with Parrot since Parrot guarantees that objects don't move around.

Multi Method Dispatch vtable functions in bytecode

Dan announced that he'd started adding opcode support for multimethod dispatch. Leo had a bunch of questions with no answers so far.

So that's where Jürgen's been

After a long absence, Jürgen Bömmels appeared on the list and explained that he'd got a new job, moved to a new town and had had no connection to the Internet. He's currently working through a huge backlog of mail and trying to get familiar with the current state of Parrot. It sounds like it might be a while before he starts contributing patches to ParrotIO again. Still, welcome back Jürgen.


Piers Cawley posted his initial version of ParrotUnit, a port of the xUnit OO testing framework. Warnock applies.

Behaviour of PMCs on assignment

Dan noted that, right now, binary vtable functions take three arguments, the destination, the left hand side and the right hand side, which allows them to either take the type of the destination into account, or simply to replace it with a new value. The advantage of this approach is that vtable functions have the potential to be more efficient when, say, the left hand side is the same as the destination. The disadvantage is that you have to make a PMC to receive the results of the operation before you can actually do the operation, which can be a pain (and suboptimal). Dan offered 3 different options and asked for opinions.

TOGoS argued that the 3 argument form was actually the Wrong Thing in general and that vtable methods should simply create a new PMC and replace the destination with it. He argued that this behaviour is what most HLLs expect, and having it would make the compiler's life a great deal easier.

Meanwhile, in perl6-language

They talked about Unicode a good deal, but (per my announcement a few weeks back) I won't be covering those bits.

Outer product considered useful

Luke Palmer proposed an outer(*@ranges) function to allow for what he called 'dynamically nested loops'. He even provided an implementation for it which used a coroutine. As Simon Cozens pointed out, the fact that something as powerful as Luke's proposal can be implemented in (initially buggy) pure Perl 6 with no need for any additions to the language itself is very nice, but really more of a side issue for the time being.

Announcements, Acknowledgements, Apologies

No announcements (apart from "Look! ParrotUnit! It's jolly good! You should use it and send me patches!") this week. And if you think I'm apologizing...

If you find these summaries useful or enjoyable, please consider contributing to the Perl Foundation to help support the development of Perl. You might also like to send me feedback at , or drop by my website, maybe I'll really add some content to it this week. -- The Perl Foundation -- Perl 6 Development site -- My website, "Just a Summary"

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