Perl 6 Archives

WANTED: Perl 6 Historical Items

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The Perl 6 programming language had a turbulent birth. It was announced in the summer of 2000 and the first stable language release shipped out only 2 years ago, on Christmas, 2015. A lot has happened during that decade and a half, yet the details are hard to piece together.

After my recent facelift to rakudo.org, I'm working on a (second) facelift to perl6.org website.

Part of the work involves bringing all the Perl 6 deliverables under one umbrella, so the user isn't thrown around multiple websites, trying to find what to install. At the same time, we want to strengthen the distinction between Perl 6 the language and the compilers that implement it, as well as encourage more implementors to give it a go at implementing a Perl 6 programming language compiler.

The Perl 6 Programming Language Museum will be part of that effort and along with interesting tidbits of Perl 6 history, it'll showcase past implementation attempts that may no longer be in active development today. Since I don't know much about what happened before I came to the language sometime in 2015, I need your help in collecting those tidbits.

Larry Wall at FOSDEM 2015, photo by Klapi

In my mind's eye, I'm imagining a few pages on perl6.org; something in the same vein as Computer History Museum's pages—pictures, years, and info, and potentially links to code repositories. Depending on the content we collect, it's possible there will be a digital PDF version of the Museum that can also be printed and handed out at events, if desired.

I'm looking for:

  • Descriptions of interesting/significant events (like the mug throwing incident).
  • Descriptions of interesting/significant implementations of Perl 6 or influential Perl 6 projects. Having links to repos/tarballs of their code is a plus.
  • Samples of interesting/significant email threads or chat logs.
  • Pictures of interesting/significant objects (first sight at plush Camelias?).
  • Pictures of interesting/significant humans (a filled out model release form is required).
  • Anything else that's Museum worthy.

If you have any of these items, please submit them to the appropriate year directory in the Perl 6 Museum Items repository. If you're a member of Perl 6 GitHub org, you should already have a commit bit to that repo. Otherwise, submit your items via a pull request.

Let's build something cool and interesting for the people using Perl 6 a hundred years from now to look at and remember!

If you have any questions or need help, talk to a human on our IRC chat.

-OFun

Perl 6: On Specs, Versioning, Changes, and... Breakage

Read this article on Rakudo.Party

Recently, I came across a somewhat-frantic comment on StackOverflow that describes a 2017.01 change to the type of return value of .sort:

"you just can't be sure what ~~ returns" Ouch. […] .list the result of a sort is presumably an appropriate work around. But, still, ouch. I don't know of a blog post or whatever that explains how P6 approaches changes to the language; and to roast; and to Rakudo. Perhaps someone will write one that also explains how this aspect of 2017.01 was conceived, considered and applied; what was right about the change; what was wrong; etc.

Today, I decided to answer that call to write a blog post and reply to all of the questions posed in the comment, as well as explain how it's possible that such an "ouch" change made it in.

On Versioning

The '6' in Perl 6 is just part of the name. The language version itself is encoded by a sequential letter, which is also the starting letter of a codename for that release. For example, the current stable language version is 6.c "Christmas". The next language release will be 6.d with one of the proposed codenames being "Diwali". The version after that will be 6.e, then 6.f, and so on.

If you've used Perl 6 sometime between 2015 and 2018, you likely used the "Rakudo" compiler, which is often packaged as "Rakudo Star" distribution and is versioned with the year and the month of the release, e.g. release 2017.01.

In some languages, like Perl 6's sister language Perl 5, what the compiler does is what the language itself is. Bugs aside, if the latest (2017.09) Perl 5 compiler gives 4 for 2+2, then that's the definition of what 2+2 is in the Perl 5 language.

In Perl 6, however, how a compiler (e.g. "Rakudo") behaves or what it implements does not define the Perl 6 language. The Perl 6 language specification does. The specification consists of a test suite of about 155,000 tests and anything that passes that test suite can call itself a "Perl 6 compiler".

It's to this specification version 6.c "Christmas" refers. It was released on December 25, 2015 and at the time of this writing, it's the first and only release of a stable language spec. Aside from a few error corrections, there were no changes to that specification… The latest version of Rakudo still passes every single test—it's a release requirement.

On Changes

Ardent Perl 6 users would likely recall that there have been many changes in the Rakudo compiler since Christmas 2015. Including the "ouch" change referenced by that StackOverflow comment. If the specification did not change and core devs are not allowed to make changes that break 6.c specification, how is it possible that the return type of .sort could have changed?

The reason is—and I hope the other core devs will forgive me for my choice of imagery—the specification is full of holes!

It doesn't (yet) cover every imaginable use and combination of features. What happens when you try to print a Junction of strings? As far as 6.c version of Perl 6 language is concerned, that's undefined behaviour. What object do you get if you call .Numeric on an Rat type object rather than an instance? Undefined behaviour. What about the return value of .sort? You'll get sorted values in an Iterable type, but whether that type is a Seq or a List is not specified by the 6.c specification.

This is how 2017.01 version of Rakudo managed to change the return type of .sort, despite being a compliant implementation of the 6.c language—the spec was not precise about what Iterable type .sort must return; both Seq and List are Iterable, thus both conform to the spec. (It's worth noting that since 2017.01 we implemented an extended testing framework that also guides our decisions on whether we actually allow changes that don't violate the spec).

In my personal opinion, the 6.c spec is overly sparse in places, which is why we saw a number of large changes in 2016 and early 2017, including the "ouch" change the commenter on StackOverlow referred to. But… it won't stay that way forever.

The Future of the Spec

At the time of this writing, there have been 3,129 commits to the spec, since 6.c language release. These are the proposals for the 6.d language specification. While some of these commits address new features, a lot of them close those holes the 6.c spec contains. The main goal is not to write a "whole new spec" but to refine and clarify the previous version.

Thus, when 6.d is released, it'll look something like this:

A few more slices of new features, but largely the same thing. Still some holes (undefined behaviour) in it, but a lot less than in 6.c language. It now defines that printing a Junction will thread it; that calling .Numeric on a Numeric type object gives a numeric equivalent of zero of that type and a warning; and that the .sort's Iterable return type is a Seq, not a List.

As more uses of combinations original designers haven't thought of come around, even more holes will be covered in future language versions.

Breaking Things

The cheese metaphor covers refinements to the specification, but there's another set of changes the core developers sometimes have to make: changes that violate previous versions of the specification. For 6.d language, the list of such changes is available in our 6.d-prep repository (some of the listed changes don't violate 6.c spec, but still have significant impact so we pushed them to the next language version).

This may seem to be a contradiction: didn't I say earlier that passing 6.c specification is part of the compiler's release requirements? The key to resolving that contradiction lies in ability to request different language versions in different comp units (e.g. in different modules) that are used by the same program.

A single compiler can support multiple language versions. Specifying use v6.c pragma loads 6.c language. Specifying use v6.d (currently available as use v6.d.PREVIEW) loads 6.d language. Not specifying anything loads the newest version the compiler supports.

One of the changes between 6.c and 6.d languages is that await no longer blocks the thread in 6.d. We can observe this change using a single small script that loads two modules. The code between the two modules is the same, except they request different language versions:

# file ./C.pm6
use v6.c;
sub await-c is export {
    await ^10 .map: {
        start await ^5 .map: { start await Promise.in: 1 }
    }
    say "6.c version took $(now - ENTER now) secs";
}

# file ./D.pm6
use v6.d.PREVIEW;
sub await-d is export {
    await ^10 .map: {
        start await ^5 .map: { start await Promise.in: 1 }
    }
    say "6.d version took $(now - ENTER now) secs";
}

# $ perl6 -I. -MC -MD -e 'await-c; await-d'
# 6.c version took 2.05268528 secs
# 6.d version took 1.038609 secs

When we run the program, we see that no-longer blocked threads let 6.d version complete a lot faster (in fact, if you bump the loop numbers by a factor, 6.d would still complete, while 6.c would deadlock).

So this is the Perl 6 mechanism that lets the core developers make breaking changes without breaking user's programs. There are some limitations to it (e.g. methods on classes)—so for some things there still will be standard deprecation procedures. We also try to limit the number of such spec-breaking changes, to reduce the maintenance burden and impact on users who don't want to lock their code down to some older version. Thus, don't worry about getting some weird new language on the next language release—the differences will be minimal.

Who Decides?

This all brings us to one of the questions posed by that StackOverflow user: how do language changes get conceived, considered, and applied—in short: who decides what the behaviour is to be like? What is the process?

As far as conception goes, many of the current ideas are based on seeing what our users need. Some proposals come directly from users; others get inspired as more elegant solutions to problems users showed they were trying to solve. Some of the changes proposed for 6.d language were informed by problematic areas of currently-implemented features that weren't foreseen during original implementation.

When it comes to implementation, the scope of the feature and core developer's expertise with the given area of the codebase generally drive the process. With the "ouch" change, the expert in the area of Iterables deemed Seq to be a superior type for .sort to return, due to its non-caching behaviour as well as its ease of degenerating into a caching List.

Some changes get opened as an Issue on the bugtracker first, to notify other devs of the impending change. Large changes usually get a proposed design written down first. The proposal is shared with the core devs and feedback is gathered before the proposal is actually implemented. The implementation of significant things is also merged far away from the date of the next release, to let the bleeding-edge users find any potential problems in the work.

Geth, our IRC bot, announces all commits in our development IRC channel. Most of the core devs backlog that channel, so any of the potentially problematic commits—even if one of the devs goes ahead and commits the change—get discussed and at times reverted.

The Perl 6 pumpking (Jonathan Worthington) and the BDFL (Larry Wall) are available to provide feedback on controversial, questionable, or large changes being proposed. They also have the veto power on any changes. Our messaging bot helps us request feedback from them, even if they're currently not in the chat.

When it comes to errata to previous specifications, unless the test to be changed is "obviously wrong", the decision on whether the errata can be applied is delegated to the Release Manager (AlexDaniel), and informed by the pumpking/BDFL, if required.

The Future

The current process is a bit loose in places. A test that's "obviously wrong" to one person might have some valid reasons behind it to someone else. This is why the TODO for 6.d release lists several documents to be written that will refine the procedures for various types of changes.

It won't be on the scale of PEP, but simply something more concrete for the core devs to refer to, when performing changes that have some impact on the users. It's a balancing act between organization and procedure and letting through a consistent flow of contributions.

And if breaking changes have to be made, an alert will be pushed to the the P6lert service for users of Perl 6 to get informed of them in advance.

Conclusion

Today, we gleaned an insight into how Perl 6 core devs introduce changes to the compiler and the language.

The language specification and the compiler's behaviour are separate entities. The 6.c language specification has places of unspecified behaviour, which is how changes that have large impact on the users slipped through in the past.

The extended testing framework as well as specification clarifications offered by 6.d language proposal tests that refine the specification and close the holes with undefined behaviour reduce unforeseen impact on the users.

The core dev team informs their decisions based on user's feedback and the way the language is used by the community. Large changes get written up as proposals and the pumking/BDFL offer advise on anything controversial.

In the future, more refined practices for how changes are made will be defined, as we work on making upgrade experience more predictable and non-breaking for our users. The P6lert service helps that goal and is already available today.

Hope this answers all the questions :)

Perl 6 Core Hacking: QASTalicious

Read this article on Rakudo.Party

Over the past month, I spent some time in Rakudo's QAST land writing a few optimizations, fixing bugs involving warnings, as well as squashing a monster hive of 10 thunk scoping bugs with a single commit. In today's article, we'll go over that last feat in detail, as well as learn what QAST is and how to work with it.

PART I: The QAST

"QAST" stands for "Q" Abstract Syntax Tree. The "Q" is there because it's comes after letter "P", and "P" used to be in "PAST" to stand for "Parrot", the name of an earlier, experimental Perl 6 implementation (or rather, its virtual machine). Let's see what QAST is all about!

Dumping QAST

Every Rakudo Perl 6 program compiles down to a tree of QAST nodes and you can dump that tree if you specify --target=ast or --target=optimize command line option to perl6 when compiling a program or a module:

$ perl6 --target=ast -e 'say "Hello, World!"'
[...]
- QAST::Op(call &say) <sunk> :statement_id<?> say \"Hello, World!\"
  - QAST::Want <wanted> Hello, World!
    - QAST::WVal(Str)
    - Ss
    - QAST::SVal(Hello, World!)
[...]

The difference between the --target=ast and --target=optimize is that the former shows the QAST tree as soon as it has been generated, while the later shows the QAST tree after the static optimizer has had a go at it.

While the command line option gives you the QAST for the entire program (excluding modules pre-compiled separately), each QAST::Node object has a .dump method you can use to dump specific QAST pieces of interest from within Rakudo's source code.

For example, to examine the QAST generated by the statement token, I'd find method statement in src/Perl6/Actions.nqp and stick nqp::say('statement QAST: ' ~ $past.dump) close to the end of the method.

Since Rakudo's compilation takes a couple of minutes for each go, I like to key my debug dumps on env variables, like this:

nqp::atkey(nqp::getenvhash(),'ZZ1') && nqp::say('ZZ1: something or other');
...
nqp::atkey(nqp::getenvhash(),'ZZ2') && nqp::say('ZZ2: something else');

Then, I can execute the compiled ./perl6 as if I didn't add anything, and enable my dumps by running ZZ1=1 ./perl6, ZZ2=1 ./perl6, or both dumps at the same time with ZZ1=1 ZZ2=1 ./perl6.

Viewing QAST

Looking at the output of --target dumps in the terminal is sufficient for a quickie glance at the trees, but for extra assistance you can install CoreHackers::Q module that brings in q command line utility.

Simply prefix your regular perl6 invocation with q a or q o to produce --target=ast and --target=optimize QAST dumps respectively. The program will generate out.html file in the current directory:

$ q a perl6 -e 'say "Hello, World!"'
$ firefox out.html

Pop open the generated HTML file and reap these benefits:

  • Color-coded QAST nodes
  • Color hints for sunk nodes
  • Ctrl+Click on any node to collapse it
  • Muted view of QAST::Want alternatives, makes it easier to ignore them

Eventually, I hope to extend this tool and make it more helpful, but at the time of this writing, that's all it does.

The QAST Forest

There are four main files in rakudo's source where you'd expect to be working with QAST nodes: src/Perl6/Grammar.nqp, src/Perl6/Actions.nqp, src/Perl6/World.nqp, and src/Perl6/Optimizer.nqp. If you're using Z-Script utility, you can even run z q command to open these four files in Atom editor.

Grammar.nqp is the Perl 6 grammar. Actions.nqp are the actions for it. World.nqp contains all sorts of helpful routines used by both Grammar.nqp and Actions.nqp that access them via the $*W dynamic variable containing a Perl6::World object. Lastly, Optimizer.nqp contains Rakudo's static optimizer.

The root (of all evil) is the QAST::Node object, with all the other QAST nodes being its subclasses. Let's review some of the popular ones:

QAST::Op

QAST::Op nodes are the workhorse of the QAST world. The :op named argument specifies the name of an NQP op or the name of a Rakudo's NQP extension op and its children are the arguments:

Here's a say op printing a string value:

QAST::Op.new: :op<say>,
  QAST::SVal.new: :value('Hello, World!');

And here's a QAST node for a call op that calls Perl 6's infix:<+> operator; notice how the name of the routine we call is given via :name named argument:

QAST::Op.new: :op<call>, :name('&infix:<+>'),
  QAST::IVal.new( :value(2)),
  QAST::IVal.new: :value(2)

QAST::*Val

The QAST::SVal, QAST::IVal, QAST::NVal, and QAST::WVal nodes, specify string, integer, float, and "World" object values respectively. The first three are the "unboxed" raw values, while World objects are everything else, such as DateTime, Block, or Str objects.

QAST::Want

Some of the objects can be represented by multiple QAST::*Val nodes, where the most appropriate value is used depending on what is wanted in the current context. QAST::Want node contains these alternatives, interleaved with string markers indicating what those alternatives are.

For example, numeric value 42 in Perl 6 could be wanted as an object to call some method on, or as a raw value to be assigned to a native int variable. The QAST::Want node for it would look like this:

QAST::Want.new:
  QAST::WVal.new(:value($Int-obj))),
  'Ii',
  QAST::IVal.new: :value(42)

The $Int-obj above would contain an instance of Int type with value set to 42. The Ii marker indicates the following alternative is an integer value and we provide a QAST::IVal object containing it. The other possible markers are Nn (float), Ss (string), and v (void context) alternatives.

When these nodes are later converted to bytecode, the most appropriate value will be selected, with the first child being the "default" value, to be used when none of the available alternatives make the cut.

QAST::Var

These nodes are used for variables and parameters. The :name named argument specifies the name of the variable and :scope its scope:

QAST::Op.new: :op('bind'),
  QAST::Var.new(:name<$x>, :scope<lexical>, :decl<var>, :returns(int)),
  QAST::IVal.new: :value(0)

The :decl named arg is present when the node is used for the variable's declaration (when it's absent, we simply reference the variable) and its value dictates what sort of variable it is: var for variables and param for routine parameters. Several other :decl types, as well as optional arguments specifying additional configuration of the variable exist. You can find them discussed in the QAST documentation

QAST::Stmt / QAST::Stmts

These are statement grouping constructs. For example, here, the truthy branch of an nqp::if contains three nqp::say statements, all grouped inside QAST::Stmts:

QAST::Op.new: :op<if>,
  QAST::IVal.new(:value(42)),
  QAST::Stmts.new(
    QAST::Op.new( :op<say>, QAST::SVal.new: :value<foo>),
    QAST::Op.new( :op<say>, QAST::SVal.new: :value<bar>),
    QAST::Op.new: :op<say>, QAST::SVal.new: :value<ber>),
  QAST::Op.new: :op<say>, QAST::SVal.new: :value<meow>,

The singular QAST::Stmt is similar. The difference is it marks a register allocation boundary, beyond which, any temporaries are free to be reused. When used correctly, this alternative can result in better code generation.

QAST::Block

This node is both a unit of invocation and a unit of lexical scoping. For example, code sub foo { say "hello" } might compile to a QAST::Block like this:

Block (:cuid(1)) <wanted> :IN_DECL<sub> { say \"hello\" }
[...]
  Stmts <wanted> say \"hello\"
    Stmt <wanted final> say \"hello\"
      Want <wanted>
        Op (call &say) <wanted> :statement_id<?> say \"hello\"
          Want <wanted> hello
            WVal (Str)
            - Ss
            SVal (hello)
        - v
        Op (p6sink)
          Op (call &say) <wanted> :statement_id<?> say \"hello\"
            Want <wanted> hello
              WVal (Str)
              - Ss
              SVal (hello)
[...]

Each block demarcates a lexical scope boundary—this detail comes into play in Part II of this article, when we'll be going over a fix for a bug.

Others

A few more QAST nodes exist. They're out of scope of this article, but you may wish to read the documentation or, since some of them are not appear in those docs, go straight to the source.

Executing QAST Trees

Having a decent familarity with nqp ops (as well as Rakudo's nqp extensions) is helpful when working with QAST. A sharp eye would notice in QAST dumps that many QAST::Op nodes correspond to nqp::* op calls, where :op named argument specifies the name of the op.

When writing large QAST trees, it's handy to write them down using pure NQP ops first, and then translate the result into a tree of QAST node objects. Let's look at a simplified example:

nqp::if(
  nqp::isgt_n(nqp::rand_n(1e0), .5e0),
  nqp::say('Glass half full'),
  nqp::say('Glass half empty'));

We have NQP op, so we'll start with QAST::Op node, using 'if' as the value for :op. The op takes three positional arguments—the three ops used for the conditional, the truthy branch, and the falsy branch. Some of the ops also take float and string values, so we'll use QAST::NVal and QAST::SVal nodes for those. The result is:

QAST::Op.new(:op('if'),
  QAST::Op.new(:op('isgt_n'),
    QAST::Op.new(:op('rand_n'),
      QAST::NVal.new(:value(1e0))
    ),
    QAST::NVal.new(:value(.5e0))
  ),
  QAST::Op.new(:op('say'),
    QAST::SVal.new(:value('Glass half full'))
  ),
  QAST::Op.new(:op('say'),
    QAST::SVal.new(:value('Glass half empty'))
  )
)

I find it easier to track the tree's nesting by using parentheses only when necessary, preferring colon method call syntax whenever possible:

QAST::Op.new: :op<if>,
  QAST::Op.new(:op<isgt_n>,
    QAST::Op.new(:op<rand_n>,
      QAST::NVal.new: :value(1e0)),
    QAST::NVal.new: :value(.5e0)),
  QAST::Op.new(:op<say>,
    QAST::SVal.new: :value('Glass half full')),
  QAST::Op.new: :op<say>,
    QAST::SVal.new: :value('Glass half empty')

If a .new is followed by a colon, there aren't any more nodes on the same level. If .new is followed by an opening parentheses, there are more sister nodes yet to come.

Due to Rakudo's lengthy compilation, it can be handy to execute your QAST tree without having to stick it into src/Perl6/Actions.nqp or similar file first. To some extent, it's possible to do that with a regular Perl 6 program. We'll simply access Perl6::World object in $*W variable inside a BEGIN block, where it still exists, and call .compile_time_evaluate method, giving it an empty variable as the first positional (it expects a Match object for the tree) and our QAST tree as the second positional:

use QAST:from<NQP>;
BEGIN $*W.compile_time_evaluate: $,
    QAST::Op.new: :op<if>,
      QAST::Op.new(:op<isgt_n>,
        QAST::Op.new(:op<rand_n>,
          QAST::NVal.new: :value(1e0)),
        QAST::NVal.new: :value(.5e0)),
      QAST::Op.new(:op<say>,
        QAST::SVal.new: :value('Glass half full')),
      QAST::Op.new: :op<say>,
        QAST::SVal.new: :value('Glass half empty')

The one caveat with this method is we're using full-blown Perl 6 language, whereas in src/Perl6/Actions.nqp and related files, as .nqp extension suggests, we're using NQP language only. Keep an eye out for strange explosions; it's possible your QAST tree that explodes in Perl 6 will compile just fine in the land of pure NQP.

Annotating QAST Nodes

All QAST nodes support annotations that allow you to attach an arbitrary value to a node and then read that value elsewhere. To add an annotation, use .annotate method, which takes two positional arguments—a string containing name of the annotation and the value to attach to it—and returns that value. Recent versions of NQP also have .annotate_self method that works the same, except it returns the QAST node itself:

$qast.annotate_self('foo', 42).annotate: 'bar', 'meow';

Later, you can read that value using .ann method that takes the name of the annotation as the argument. If the annotation doesn't exist, NQPMu is returned instead:

note($qast.ann: 'foo'); # OUTPUT: «42␤»

You can also check for whether an annotation merely exists using .has_ann method that returns 1 (true) or 0 (false):

note($qast.has_ann: 'bar'); # OUTPUT: «1␤»

Or dump all of the annotations on the node (to prevent potential flood of output, most values will be dumped as simply a question mark):

note($qast.dump_annotations); # OUTPUT: « :bar<?> :foo<?>␤»);

Lasty, to clear all annotations on the node, simply call .clear_annotations method.

Mutating QAST Nodes

A handy thing to do with QAST node objects is to mutate them into something better. That's essentially all the static optimizer in src/Perl6/Optimizer.nqp does. Named arguments can be mutated by calling them as methods and providing a value. For example, $qast.op('callstatic') will change the value of :op from whatever it is to callstatic. Positional arguments can be altered by re-assignment to a positional index, as well as shift, push, unshift, pop operations performed either via method calls with those names or nqp:: ops. Some nodes also support nqp::elems calls on them, which is slightly faster than the generic pattern of +@($qast) that can be used on all nodes to find out the number of children a node contains.

As an exercise, let's write a small optimization: some operations, like $foo < $bar < $ber compile to nqp::chain ops. That is so even if we have only two children, e.g. $foo < $bar. In such cases, rewriting the op to be nqp::call has performance advantages: not only nqp::call on its own is a little bit faster than nqp::chain, the static optimizer knows how to do further optimizations on nqp::call ops.

Let's take a look at what both 2-child and 2+-child nqp::chain chains look like:

$ perl6 --target=ast -e '2 < 3 < 4; 2 < 3'

The first statement compiled to this (I removed QAST::Wants for clarity):

- QAST::Op(chain &infix:«<»)  :statement_id<?> <
  - QAST::Op(chain &infix:«<») <wanted> <
    - QAST::IVal(2)
    - QAST::IVal(3)
  - QAST::IVal(4)

And the second one to:

- QAST::Op(chain &infix:«<»)  :statement_id<?> <
  - QAST::IVal(2)
  - QAST::IVal(3)

Thus, to target our optimization correctly, we need to ensure neither child of our chain op is a chain op. In addition, we need to ensure that the op we're optimizing is not itself a child of another chain op.

Raking the code of the optimizer, we can spot that chain depth is already tracked via $!chain_depth attribute, so we merely need to ensure we're at the first link of the chain. The code then becomes:

$qast.op: 'call'
  if nqp::istype($qast, QAST::Op)
  && $qast.op eq 'chain'
  && $!chain_depth == 1
  && ! (nqp::istype($qast[0], QAST::Op) && $qast[0].op eq 'chain')
  && ! (nqp::istype($qast[1], QAST::Op) && $qast[1].op eq 'chain');

Once we find a chain QAST::Op, we index into it and use nqp::istype to check the type of kid nodes, and if those happen to be QAST::Op nodes, we ensure the :op parameter is not a chain op. If all of the conditions are met, we simply call .op method on our node with value 'call' to convert it into a call op.

We then stick our optimization early enough into .visit_op method of the optimizer and its later portions will further optimize our call.

A fairly easy and straightforward optimization that can bring a lot of benefit.

PART II: A Thunk in The Trunk


Note: it took me three evenings to debug and fix the following tickets. To learn the solution I tried many dead ends that I won't be covering, to keep you from getting bored, and instead will instantly jump to conclusions. The point I'm making is that fixing core bugs is a lot easier than may seem from reading this article—you just need to be willing to spend some time on them.


Now that we have some familiarity with QAST, let's try to fix a bug that existed in Rakudo v2018.01.30.ga.5.c.2398.cc and earlier. The ticket in question is R#1212, that shows the following problem:

$ perl6 -e 'say <a b c>[$_ xx 2] with 1'

Use of Nil in string context
  in block  at -e line 1
Unable to call postcircumfix [ (Any) ] with a type object
Indexing requires a defined object
  in block <unit> at -e line 1

It looks like the $_ topical variable inside the indexing brackets fails to get the value from with statement modifier and ends up being undefined. Sounds like a challenge!

It's A Hive!

Both with and xx operator create thunks (thunks are like blocks of code, without having explicit blocks in the code; this, for example, lets rand xx 10 to produce 10 different random values; rand is thunked and the thunk is called for each iteration). This reminded me of some other tickets I've seen, so I went to fail.rakudo.party and looked through open tickets for anything that mentioned thunking or wrong scoping.

I ended up with a list of 7 tickets, and with the help of dogbert++ later increased the number to 9, which with the original Issue gives us a total of 10 different manifestations of a bug. The other tickets are RT#130575, RT#132337, RT#131548, RT#132211, RT#126569, RT#128054, RT#126413, RT#126984, and RT#132172. Quite a bug hive!

Test It Out

Our starting point is to cover each manifestation of the bug with a test. Make all the test pass and you know you've fixed the bug, plus you already have something to place into roast, to cover the tickets. My tests ended up looking like this, where I've used gather/take duo to capture what the tickets' code printed to the screen:

use Test;
plan 1;
subtest 'thunking closure scoping' => {
    plan 10;

    # https://github.com/rakudo/rakudo/issues/1212
    is-deeply <a b c>[$_ xx 2], <b b>.Seq, 'xx inside `with`' with 1;

    # RT #130575
    is-deeply gather {
        sub itcavuc ($c) { try {take $c} andthen 42 };
        itcavuc $_ for 2, 4, 6;
    }, (2, 4, 6).Seq, 'try with block and andthen';

    # RT #132337
    is-deeply gather {
        sub foo ($str) { { take $str }() orelse Nil }
        foo "cc"; foo "dd";
    }, <cc dd>.Seq, 'block in a sub with orelse';

    # RT #131548
    is-deeply gather for ^7 {
        my $x = 1;
        1 andthen $x.take andthen $x = 2 andthen $x = 3 andthen $x = 4;
    }, 1 xx 7, 'loop + lexical variable plus chain of andthens';

    # RT #132211
    is-deeply gather for <a b c> { $^v.uc andthen $v.take orelse .say },
        <a b c>.Seq, 'loop + andthen + orelse';

    # RT #126569
    is-deeply gather { (.take xx 10) given 42 }, 42 xx 10,
        'parentheses + xx + given';

    # RT #128054
    is-deeply gather { take ("{$_}") for <aa bb> }, <aa bb>.Seq,
        'postfix for + take + block in a string';

    # RT #126413
    is-deeply gather { take (* + $_)(32) given 10 }, 42.Seq,
        'given + whatever code closure execution';

    # RT #126984
    is-deeply gather {
        sub foo($x) { (* ~ $x)($_).take given $x }; foo(1); foo(2)
    }, ("11", "22").Seq, 'sub + given + whatevercode closure execution';

    # RT #132172
    is-deeply gather { sub {
        my $ver =.lines.uc with "totally-not-there".IO.open
            orelse "meow {$_ ~~ Failure}".take and return 42;
    }() }, 'meow True'.Seq, 'sub with `with` + orelse + block interpolation';
}

When I brought up the first bug in our dev chatroom, jnthn++ pointed out that such bugs are often due to mis-scoped blocks, as p6capturelex op that's involved needs to be called in the immediate outer of the block it references.

Looking through the tickets, I also spotted skids++'s note that changing a conditional for statement_id in block migrator predicate fixed one of the tickets. This wasn't the full story of the fix, as the many still-failing tests showed, but it was a good start.

What's Your Problem?

In order to find the best solution for a bug, it's important to understand what exactly is the problem. We know mis-scoped blocks are the cause of the bug, so lets grab each of our tests, dump their QAST (--target=ast), and write out how mis-scoped the blocks are.

To make it easier to match the QAST::Blocks with the QAST::WVals referencing them, I made a modification to QAST::Node.dump to include CUID numbers and statement_id annotations in the dumps.

Going through mosts of the buggy code chunks, we have these results:

is-deeply <a b c>[$_ xx 2], <b b>.Seq, 'xx inside `with`' with 1;
# QAST for `xx` is ALONGSIDE RHS `andthen` thunk, but needs to be INSIDE

is-deeply gather {
    sub itcavuc ($c) { try {take $c} andthen 42 };
    itcavuc $_ for 2, 4, 6;
}, (2, 4, 6).Seq, 'try with block and andthen';
# QAST for try block is INSIDE RHS `andthen` thunk, but needs to be ALONGSIDE

is-deeply gather {
    sub foo ($str) { { take $str }() orelse Nil }
    foo "cc"; foo "dd";
}, <cc dd>.Seq, 'block in a sub with orelse';
# QAST for block is INSIDE RHS `andthen` thunk, but needs to be ALONGSIDE

is-deeply gather for ^7 {
    my $x = 1;
    1 andthen $x.take andthen $x = 2 andthen $x = 3 andthen $x = 4;
}, 1 xx 7, 'loop + lexical variable plus chain of andthens';
# each andthen thunk is nested inside the previous one, but all need to be
# ALONGSIDE each other

is-deeply gather for <a b c> { $^v.uc andthen $v.take orelse .say },
    <a b c>.Seq, 'loop + andthen + orelse';
# andthen's block is INSIDE orelse's but needs to be ALONGSIDE each other

is-deeply gather { (.take xx 10) given 42 }, 42 xx 10,
    'parentheses + xx + given';
# .take thunk is ALONGSIDE given's thunk, but needs to be INSIDE of it

is-deeply gather { take ("{$_}") for <aa bb> }, <aa bb>.Seq,
    'postfix for + take + block in a string';
# the $_ is ALONGSIDE `for`'s thunk, but needs to be INSIDE

is-deeply gather { take (* + $_)(32) given 10 }, 42.Seq,
    'given + whatever code closure execution';
# the WhateverCode ain't got no statement_id and is ALONGSIDE given
# block but needs to be INSIDE of it

So far, we can see a couple of patterns:

  • xx and WhateverCode thunks don't get migrated, even though they should
  • andthen thunks get migrated, even though they shouldn't

The first one is fairly straightforward. Looking at the QAST dump, we see xx thunk has a higher statement_id than the block it was meant to be in. This is what skids++'s hint addresses, so we'll change the statement_id conditional from == to >= to look for statement IDs higher than our current one as well, since those would be from any substatements, such as our xx inside the positional indexing operator:

($b.ann('statement_id') // -1) >= $migrate_stmt_id

The cause is very similar for the WhateverCode case, as it's missing statement_id annotation altogether, so we'll just annotate the generated QAST::Block with the statement ID. Some basic detective work gives us the location where that node is created: we search src/Perl6/Actions.nqp for word "whatever" until we spot whatever_curry method and in its guts we find the QAST::Block we want. For the statement ID, we'll grep the source for statement_id:

$ grep -FIRn 'statement_id' src/Perl6/
src/Perl6/Actions.nqp:1497:            $past.annotate('statement_id', $id);
src/Perl6/Actions.nqp:2326:                $_.annotate('statement_id', $*STATEMENT_ID);
src/Perl6/Actions.nqp:2488:                -> $b { ($b.ann('statement_id') // -1) == $stmt.ann('statement_id') });
src/Perl6/Actions.nqp:9235:                && ($b.ann('statement_id') // -1) >= $migrate_stmt_id
src/Perl6/Actions.nqp:9616:            ).annotate_self: 'statement_id', $*STATEMENT_ID;
src/Perl6/World.nqp:256:            $pad.annotate('statement_id', $*STATEMENT_ID);

From the output, we can see the ID is stored in $*STATEMENT_ID dynamic variable, so we'll use that for our annotation on the WhateverCode's QAST::Block:

my $block := QAST::Block.new(
    QAST::Stmts.new(), $past
).annotate_self: 'statement_id', $*STATEMENT_ID;

Let's compile and run our bug tests. If you're using Z-Script, you can re-compile Rakudo by running z command with no arguments:

$ z
[...]
$ ./perl6 bug-tests.t
1..1
    1..10
    ok 1 - xx inside `with`
    not ok 2 - try with block and andthen
    # Failed test 'try with block and andthen'
    # at bug-tests.t line 10
    # expected: $(2, 4, 6)
    #      got: $(2, 2, 4)
    not ok 3 - block in a sub with orelse
    # Failed test 'block in a sub with orelse'
    # at bug-tests.t line 16
    # expected: $("cc", "dd")
    #      got: $("cc", "cc")
    not ok 4 - loop + lexical variable plus chain of andthens
    # Failed test 'loop + lexical variable plus chain of andthens'
    # at bug-tests.t line 22
    # expected: $(1, 1, 1, 1, 1, 1, 1)
    #      got: $(1, 4, 3, 3, 3, 3, 3)
    not ok 5 - loop + andthen + orelse
    # Failed test 'loop + andthen + orelse'
    # at bug-tests.t line 28
    # expected: $("a", "b", "c")
    #      got: $("a", "a", "a")
    ok 6 - parentheses + xx + given
    ok 7 - postfix for + take + block in a string
    ok 8 - given + whatever code closure execution
    ok 9 - sub + given + whatevercode closure execution
    not ok 10 - sub with `with` + orelse + block interpolation
    # Failed test 'sub with `with` + orelse + block interpolation'
    # at bug-tests.t line 49
    # expected: $("meow True",)
    #      got: $("meow False",)
    # Looks like you failed 5 tests of 10
not ok 1 - thunking closure scoping
# Failed test 'thunking closure scoping'
# at bug-tests.t line 3
# Looks like you failed 1 test of 1

Looks like that fixed half of the issues already. That's pretty good!

Extra Debugging

Let's now look at the remaining failures and figure out why block migration isn't how we want it in those cases. To assists with our sleuthing efforts, let's make a couple of changes to produce more debugging info.

First, let's modify QAST::Node.dump method in NQP's repo to dump the value of in_stmt_mod annotation, by telling it to dump out the value verbatim if the key is in_stmt_mod:

if $k eq 'IN_DECL' || $k eq 'BY' || $k eq 'statement_id'
|| $k eq 'in_stmt_mod' {
    ...

Next, let's go to sub migrate_blocks in Actions.nqp and add a bunch of debug dumps inside most of the conditionals. This will let us track when a block is compared and to see whether migration occurs. As mentioned earlier, I like to key my dumps on env vars using nqp::getenvhash op, so after modifications my migrate_blocks routine looks like this; note the use of .dump method to dump QAST node guts (tip: .dump method also exists on Perl6::Grammar's match objects!):

sub migrate_blocks($from, $to, $predicate?) {
    my @decls := @($from[0]);
    my int $n := nqp::elems(@decls);
    my int $i := 0;
    while $i < $n {
        my $decl := @decls[$i];
        if nqp::istype($decl, QAST::Block) {
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: -----------------');
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: trying to grab ' ~ $decl.dump);
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: to move to ' ~ $to.dump);
            if !$predicate || $predicate($decl) {
                nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: grabbed');
                $to[0].push($decl);
                @decls[$i] := QAST::Op.new( :op('null') );
            }
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: -----------------');
        }
        elsif (nqp::istype($decl, QAST::Stmt) || nqp::istype($decl, QAST::Stmts)) &&
              nqp::istype($decl[0], QAST::Block) {
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: -----------------');
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: trying to grab ' ~ $decl[0].dump);
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: to move to ' ~ $to.dump);
            if !$predicate || $predicate($decl[0]) {
                nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: grabbed');
                $to[0].push($decl[0]);
                $decl[0] := QAST::Op.new( :op('null') );
            }
            nqp::atkey(nqp::getenvhash(),'ZZ') && nqp::say('ZZ1: -----------------');
        }
        elsif nqp::istype($decl, QAST::Var) && $predicate && $predicate($decl) {
            $to[0].push($decl);
            @decls[$i] := QAST::Op.new( :op('null') );
        }
        $i++;
    }
}

After making the changes, we need to recompile both NQP and Rakudo. With Z-Script, we can just run z n to do that:

$ z n
[...]

Now, we'll grab the first failing code and take a look at its QAST. I'm going to use the CoreHackers::Q tool:

$ q a ./perl6 -e '
    sub itcavuc ($c) { try {say $c} andthen 42 };
    itcavuc $_ for 2, 4, 6;'
$ firefox out.html

We can see that our buggy say call lives in QAST::Block with cuid 1, which gets called from within QAST::Block with cuid 3, but is actually located within QAST::Block with cuid 2:

- QAST::Block(:cuid(3)) <wanted> :statement_id<1>
        :count<?> :signatured<?> :IN_DECL<sub>
        :in_stmt_mod<0> :code_object<?>
        :outer<?> { try {say $c} andthen 42 }
    [...]
        - QAST::Block(:cuid(2)) <wanted> :statement_id<2>
                :count<?> :in_stmt_mod<0> :code_object<?> :outer<?>
            [...]
            - QAST::Block(:cuid(1)) <wanted> :statement_id<2>
                    :IN_DECL<> :in_stmt_mod<0> :code_object<?>
                    :also_uses<?> :outer<?> {say $c}
                [...]
                - QAST::Op(call &say)  say $c
    [...]
    - QAST::Op(p6typecheckrv)
        [...]
        - QAST::WVal(Block :cuid(1))

Looks like cuid 2 block steals our cuid 1 block. Let's enable the debug env var and look at the dumps to see why exactly:

$ ZZ=1 ./perl6 -e '
    sub itcavuc ($c) { try {say $c} andthen 42 };
    itcavuc $_ for 2, 4, 6;'

ZZ1: -----------------
ZZ1: trying to grab - QAST::Block(:cuid(1)) <wanted>
    :statement_id<2> :IN_DECL<> :in_stmt_mod<0> :code_object<?>
    :also_uses<?> :outer<?> {say $c}
[...]

ZZ1: to move to - QAST::Block  :statement_id<2>
    :in_stmt_mod<0> :outer<?>

ZZ1: grabbed
ZZ1: -----------------

We can see the theft in progress. Let's take a look at our migration predicate again:

! $b.ann('in_stmt_mod')
&& ($b.ann('statement_id') // -1) >= $migrate_stmt_id

In the dump we can see in_stmt_mod is false. Were it set to a true value, the block would not be migrated—exactly what we're trying to accomplish. Let's investigate the in_stmt_mod annotation, to see when it gets set:

$ G 'in_stmt_mod' src/Perl6/Actions.nqp
2327:                $_.annotate('in_stmt_mod', $*IN_STMT_MOD);
9206:                !$b.ann('in_stmt_mod') && ($b.ann('statement_id') // -1) >= $migrate_stmt_id

$ G '$*IN_STMT_MOD' src/Perl6/Grammar.nqp
1200:        :my $*IN_STMT_MOD := 0;                    # are we inside a statement modifier?
1328:        :my $*IN_STMT_MOD := 0;
1338:        | <EXPR> :dba('statement end') { $*IN_STMT_MOD := 1 }

Looks like it's a marker for statement modifier conditions. Statement modifiers have a lot of relevance to our andthen thunks, because $foo with $bar gets turned into $bar andthen $foo during parsing. Since, as we can see in src/Perl6/Grammar.nqp, in_stmt_mod annotation gets set for with statement modifiers, we can hypothesize that if we turn our buggy andthen into a with, the bug will disappear:

$ ./perl6 -e 'sub itcavuc ($c) { 42 with try {say $c} };
    itcavuc $_ for 2, 4, 6;'
2
4
6

And indeed it does! Then, we have a way forward: we need to set in_stmt_mod annotation to a truthy value for just the first argument of andthen (and its relatives notandthen and orelse).

Glancing at the Grammar it doesn't look like it immediatelly offers a similar opportunity for how in_stmt_mod is set for the with statement modifier. Let's approach it differently. Since we care about this when thunks are created, let's watch for andthen QAST inside sub thunkity_thunk in Actions, then descend into its first kid and add the in_stmt_mod annotation by cheating and using the past_block annotation on QAST::WVal with the thunk that contains the reference to QAST::Block we wish to annotate. The code will look something like this:

sub mark_blocks_as_andnotelse_first_arg($ast) {
    if $ast && nqp::can($ast, 'ann') && $ast.ann('past_block') {
        $ast.ann('past_block').annotate: 'in_stmt_mod', 1;
    }
    elsif nqp::istype($ast, QAST::Op)
    || nqp::istype($ast, QAST::Stmt)
    || nqp::istype($ast, QAST::Stmts) {
        mark_blocks_as_andnotelse_first_arg($_) for @($ast)
    }
}

sub thunkity_thunk($/,$thunky,$past,@clause) {
    [...]

    my $andnotelse_thunk := nqp::istype($past, QAST::Op)
      && $past.op eq 'call'
      && ( $past.name eq '&infix:<andthen>'
        || $past.name eq '&infix:<notandthen>'
        || $past.name eq '&infix:<orelse>');

    while $i < $e {
        my $ast := @clause[$i];
        $ast := $ast.ast if nqp::can($ast,'ast');
        mark_blocks_as_andnotelse_first_arg($ast)
            if $andnotelse_thunk && $i == 0;
        [...]

First, we rake $past argument given to thunkity_thunk for a QAST::Op for nqp::call that calls one of our ops—when we found one, we set a variable to a truthy value. Then, in the loop, when we're iterating over the first child node ($i == 0) of these ops, we'll pass its QAST to our newly minted mark_blocks_as_andnotelse_first_arg routine, inside of which we recurse over any ops that can have kids and mark anything that has past_block annotation with truthy in_stmt_mod annotation.

Let's compile our concoction and give the tests another run. Once again, I'm using Z-Script to recompile Rakudo:

$ z
[...]
$ ./perl6 bug-tests.t
1..1
    1..10
    ok 1 - xx inside `with`
    ok 2 - try with block and andthen
    ok 3 - block in a sub with orelse
    not ok 4 - loop + lexical variable plus chain of andthens
    # Failed test 'loop + lexical variable plus chain of andthens'
    # at bug-tests.t line 23
    # expected: $(1, 1, 1, 1, 1, 1, 1)
    #      got: $(1, 4, 3, 3, 3, 3, 3)
    ok 5 - loop + andthen + orelse
    ok 6 - parentheses + xx + given
    ok 7 - postfix for + take + block in a string
    ok 8 - given + whatever code closure execution
    ok 9 - sub + given + whatevercode closure execution
    not ok 10 - sub with `with` + orelse + block interpolation
    # Failed test 'sub with `with` + orelse + block interpolation'
    # at bug-tests.t line 50
    # expected: $("meow True",)
    #      got: $("meow False",)
    # Looks like you failed 2 tests of 10
not ok 1 - thunking closure scoping
# Failed test 'thunking closure scoping'
# at bug-tests.t line 4
# Looks like you failed 1 test of 1

We got closer to the goal, with 80% of the tests now passing! In the first remaining failure, we already know from our original examination that chained andthen thunks get nested when they should not—we haven't done anything to fix that yet. Let's take care of that first.

Playing Chinese Food Mind Games

Looking back out at the fixes we applied already, we have a marker for when we're working with andthen or its sister ops: the $andnotelse_thunk variable. It seems fairly straight-forward that if we don't want the thunks of these ops to migrate, we just need to annotate them appropriately and stick the check for that annotation into the migration predicate.

In Grammar.nqp, we can see our ops are configured with the .b thunky, so we'll locate that branch in sub thunkity_thunk and pass $andnotelse_thunk variable as a new named param to the make_topic_block_ref block maker:

...
elsif $type eq 'b' {  # thunk and topicalize to a block
    unless $ast.ann('bare_block') || $ast.ann('past_block') {
        $ast := block_closure(make_topic_block_ref(@clause[$i],
          $ast, :$andnotelse_thunk,
          migrate_stmt_id => $*STATEMENT_ID));
    }
    $past.push($ast);
}
...

The block maker) will shove it into the migration predicate, so our block maker code becomes this:

 sub make_topic_block_ref(
    $/, $past, :$copy, :$andnotelse_thunk, :$migrate_stmt_id,
 ) {
    my $block := $*W.push_lexpad($/);

    # Add annotation to thunks of our ops:
    $block.annotate: 'andnotelse_thunk', 1 if $andnotelse_thunk;

    $block[0].push
        QAST::Var.new( :name('$_'), :scope('lexical'), :decl('var') );
    $block.push($past);
    $*W.pop_lexpad();
    if nqp::defined($migrate_stmt_id) {
        migrate_blocks($*W.cur_lexpad(), $block, -> $b {
               ! $b.ann('in_stmt_mod')

            # Don't migrate thunks of our ops:
            && ! $b.ann('andnotelse_thunk')

            && ($b.ann('statement_id') // -1) >= $migrate_stmt_id
        });
    }
    ...

One more compilation cycle and test run:

$ z
[...]
$ ./perl6 bug-tests.t
1..1
    1..10
    ok 1 - xx inside `with`
    ok 2 - try with block and andthen
    ok 3 - block in a sub with orelse
    ok 4 - loop + lexical variable plus chain of andthens
    ok 5 - loop + andthen + orelse
    ok 6 - parentheses + xx + given
    ok 7 - postfix for + take + block in a string
    ok 8 - given + whatever code closure execution
    ok 9 - sub + given + whatevercode closure execution
    not ok 10 - sub with `with` + orelse + block interpolation
    # Failed test 'sub with `with` + orelse + block interpolation'
    # at bug-tests.t line 50
    # expected: $("meow True",)
    #      got: $("meow False",)
    # Looks like you failed 1 test of 10
not ok 1 - thunking closure scoping
# Failed test 'thunking closure scoping'
# at bug-tests.t line 4
# Looks like you failed 1 test of 1

So close! Just a single test failure remains. Let's give it a close look.

Within and Without

Let's repeat our procedure of dumping QASTs as well as enabing the ZZ env var and looking at what's causing the thunk mis-migration. I'm going to run a slightly simplified version of the failing test, to keep the cruft out of QAST dumps. If you're following along, when looking at full QAST dump keep in mind what I mentioned earlier: with gets rewritten into andthen op call during parsing.

$ q a ./perl6 -e '.uc with +"a" orelse "meow {$_ ~~ Failure}".say and 42'
$ firefox out.html

- QAST::Block(:cuid(4)) :in_stmt_mod<0>
    [...]
    - QAST::Block(:cuid(1))  :statement_id<1> :in_stmt_mod<1>
      [...]
      - QAST::Op(chain &infix:<~~>) <wanted> :statement_id<2> ~~
        - QAST::Var(lexical $_) <wanted> $_
        - QAST::WVal(Failure) <wanted> Failure
    - QAST::Block(:cuid(2)) :statement_id<1>
        :in_stmt_mod<1> :andnotelse_thunk<1>
      [...]
      - QAST::Op(callmethod Stringy) <wanted>
        - QAST::Op(call) <wanted> {$_ ~~ Failure}
          - QAST::Op(p6capturelex) <wanted> :code_object<?>
            - QAST::Op(callmethod clone)
              - QAST::WVal(Block)

$ ZZ=1 ./perl6 -e '.uc with +"a" orelse "meow {$_ ~~ Failure}".say and 42'
[...]
ZZ1: -----------------
ZZ1: trying to grab - QAST::Block(:cuid(1))
  :statement_id<1> :in_stmt_mod<1>
  [...]
ZZ1: to move to - QAST::Block
  :statement_id<1> :andnotelse_thunk<1> :in_stmt_mod<1>
  [...]
ZZ1: -----------------

Although QAST::WVal lacks .past_block annotation and so doesn't show the block's CUID in the dump, just by reading the code dumped around that QAST, we can see that the CUID-less block is our QAST::Block :cuid(1), whose immediate outer is QAST::Block :cuid(4), yet it's called from within QAST::Block :cuid(2). It's supposed to get migrated, but that migration never happens, as we can see when we use the ZZ env var to enable our debug dumps in the sub migrate_blocks.

We can see why. Here's our current migration predicate (where $b is the examined block, which in our case is QAST::Block :cuid(1)):

   ! $b.ann('in_stmt_mod')
&& ! $b.ann('andnotelse_thunk')
&& ($b.ann('statement_id') // -1) >= $migrate_stmt_id

The very first condition prevents our migration, as our block has truthy in_stmt_mod annotation, because it's part of the with's condition. At the same time, it does need to be migrated because it's part of the andthen thunk that's inside the statement modifier!

Since we already have $andnotelse_thunk variable in the vicinity of the migration predicate we can use it to tell us whether we're migrating for the benefit of our andthen thunk and not the statement modifier. However, recall that we've used the very same in_stmt_mod annotation to mark the first argument of andthen and its brother ops. We need to alter that first.

And so, the sub mark_blocks_as_andnotelse_first_arg we added earlier becomes:

sub mark_blocks_as_andnotelse_first_arg($ast) {
    if $ast && nqp::can($ast, 'ann') && $ast.ann('past_block') {
        $ast.ann('past_block').annotate: 'in_stmt_mod_andnotelse', 1;
    }
    ...

And then we tweak the migration predicate to watch for this altered annotation and to consider the value of $andnotelse_thunk variable:

migrate_blocks($*W.cur_lexpad(), $block, -> $b {
    (    (! $b.ann('in_stmt_mod_andnotelse') &&   $andnotelse_thunk)
      || (! $b.ann('in_stmt_mod')            && ! $andnotelse_thunk)
    )
    && ($b.ann('statement_id') // -1) >= $migrate_stmt_id
    && ! $b.has_ann('andnotelse_thunk')
});

Thus, we migrate all the blocks with statement_id equal to or higher than ours and are all of the following:

  • Not thunks of actual andthen, notandthen, or orelse
  • Not thunks inside a statement modifier, unless they're inside thunks of andthen or related ops
  • If we're considering migrating them inside one of the andthen's thunks, then also not part of the first argument to andthen (or related ops), .

That's a fancy-pants predicate. Let's compile and see if it gets the job done:

$ z
[...]
$ ./perl6 bug-tests.t
  1..1
    1..10
    ok 1 - xx inside `with`
    ok 2 - try with block and andthen
    ok 3 - block in a sub with orelse
    ok 4 - loop + lexical variable plus chain of andthens
    ok 5 - loop + andthen + orelse
    ok 6 - parentheses + xx + given
    ok 7 - postfix for + take + block in a string
    ok 8 - given + whatever code closure execution
    ok 9 - sub + given + whatevercode closure execution
    ok 10 - sub with `with` + orelse + block interpolation
ok 1 - thunking closure scoping

Success! Now, let's remove all of the debug statements we added. Then, recompile and run make stresstest, to ensure we did not break anything else. With Z-Script, we can do all that by just running z ss:

$ z ss
[...]
All tests successful.
Files=1287, Tests=153127, 159 wallclock secs (21.40 usr  3.27 sys + 3418.56 cusr 179.32 csys = 3622.55 CPU)
Result: PASS

All is green. We can now commit our fix to Rakudo's repo, then commit our tests to the roast repo, and all that remains is closing those 10 tickets we fixed!

Job well done.

Conclusion

Today, we learned quite a bit about QAST: the Abstract Syntax Trees Perl 6 code compiles to in the Rakudo compiler. We examined the common types of QAST and how to create, annotate, mutate, execute, and dump them for examination.

In the second part of the article, we applied our new knowledge to fix a hive of mis-scoped thunking bugs that plagued various Perl 6 constructs. We introspected the generated QAST nodes to specially annotate them, and then used those annotations to reconfigure migration predicate, so that it migrates the blocks correctly.

Hopefully, this knowledge inspires you to fix the many other bugs we have on the RT tracker as well as our GitHub Issue tracker

-Ofun

Announcing P6lert: Perl 6 Alerts Directly From Core Developers

Read this article on Rakudo.Party

Development of Rakudo Perl 6 is quite fast-paced, with hundreds of commits made each month to its five core repositories. Users undoubtedly feel some impact from those commits: bug fixes may break code that relied on them, backend changes may have unforeseen impact on the user code, new useful features may be implemented that users would want to know about.

In the past, for things with very large impact, we made blog posts, but there are lots of small things that fly under the radar, unless you actively pay a lot of attention to Rakudo Perl 6's core development.

To help all of our users to be aware of important issues, we're announcing introduction of P6lert service: tweet-sized alerts from Perl 6 Core Developers.

The Goods

The P6lert service primarily consists of alerts.perl6.org website, but with it come a variety of ways to receive alerts posted on it:

The Content

While we'll make adjustments as we move forward, I foresee most of the non-critical alerts will largely include things that are: (a) more important than simply hoping users-who-care will read about it in the ChangeLog; (b) not as important to warrant a notification blog post.

As a rule-of-thumb, if you picture a user who added p6lert script to their compiler upgrade procedure, the alerts the script will show will inform that user on everything they need to know to perform that upgrade safely.

The alerts are also deliberately length-limited to be easy to process and fast to digest. As they're posted via an IRC bot, the poster has at most about 400 characters to work with.

Each alert has an affects field for it, giving additional info what the alert applies to. I think it'll often be empty, as alerts affecting latest compiler versions imply they affect whatever latest release is at the time alert was posted.

The alerts have a severity rating: low, normal, and high indicating how important an alert is. Along with those, come two out-of-band ratings: info and critical. Info alerts will usually be something the users don't need to act upon, while critical alerts will often simply contain a link to a blog post that details a critical issue.

Of the top of my head, here are some real-life examples from the past and how I'd rate their severity on the P6lert service:

  • info - Telemetry implementation. This was a fairly large implementation of a feature in the core and some users may be interested in it. At the same time, they don't have to act on this alert. Rakudo and Rakudo Star release may also be info alert material.
  • low - implementation of output buffering on IO handles. Once that was implemented, we noticed some minor fallout in code that assumed lack of buffering. A low-severity alert could notify users about this.
  • normal - A real-life normal-severity alert is already posted on the site. During 6.d spec pre-release review, the Str.parse-names method was placed under deprecation and its functionality was moved to live under Str.uniparse. While this method always existed as a 6.d-proposal, it's known to us that some users already use it and an alert will help them ensure their code keeps working past 6.e language release.
  • high - a while ago, .subst was briefly made to die if it couldn't write to $/; same as some methods already do. Upon examination of ecosystem fallout, this change was reverted, pending further review. However, were it to stay, a high-severity alert would be in order.
  • critical - when we finished work on lexical require, conditional loading of modules many users used was silently failing and the users needed to change their code to correct reliance on the old, buggy behavour. We issued a blog post with upgrade instructions. A critical alert would be a link to this blog post.

That's my vision for host the system will be used, but it'll evolve to suit our needs as more core devs and more of our users start using it. The core dev docs for the system along with code for all the pieces is available in perl6/alerts repo.

Conclusion

As part of improving using user experience, Perl 6 core devs now offer alerts.perl6.org service that will list important information about latest developments in the land of Perl 6. There are numerous ways to consume those alerts, such as an RSS or Tweeter feeds, a command line utility, and an easy-to-use API.

The alerts will come in 5 different severity ratings, indicating their importance. We'll continue to improve the system to best suit our users' needs.

If you have any questions or feedback, you can always talk to the core devs on #perl6-dev IRC chat.

-Ofun

Rakudo Perl 6 Advent Calendar 2017 Call for Authors

Every year since 2009, the Rakudo Perl 6 community publishes a Rakudo Perl 6 advent calendar, in the form of blog posts on perl6advent.wordpress.com.

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 Rakudo 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 Zoffix or someone on #perl6 IRC chat your email address so that they can send you an invitation.

Rakudo 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 irc.freenode.org.

CPAN6 Is Here

Read this article on Rakudo.Party

If you've been following Rakudo's development since first language release on Christmas, 2015, you might've heard of numerous people working to bring CPAN support to Rakudo Perl 6.

Good news! It's finally here in usable form and you should start using it!

Let's talk about all the moving parts and how to upload your dists to CPAN.

The Moving Parts and Status Report

All of the heavy lifting has been done awhile back, during Perl Toolchain Summit and other times. I wasn't present for it to know the details, but to catch up you could join #perl6-toolchain chat and talk to humans or read the channel log. PAUSE/CPAN support for Perl 6 dists was implemented and zef module installer was trained to check for CPAN dists as well as our GitHub/GitLab-based ecosystem (called "p6c").

The only bit that was left missing is a front-end to browse available CPAN dists. There is a team who wished to take metacpan.org's codebase and modify it for Rakudo dists. I'm told that project is currently "stalled but not dead".

That's unfortunate, however, earlier this week, modules.perl6.org was taught to handle CPAN dists, so—hooray!—we finally have some sort of a front-end for CPAN dists. If you only want to see CPAN dists in search results, you can use from:cpan search qualifier (just like you can use from:github and from:gitlab ones).

GitHub/GitLab dists URLs still direct to repos, but CPAN dists have a file browser that lets you see what files are up in the dist. The file browser also renders README.md/README.markdown markdown readme files.

The viewer doesn't have all the bells and whistles of metacpan.org and doesn't (yet) render POD6, but it's certainly useable. The person who implemented this viewer will be busy preparing 6.d language release in the near future and won't have the time to make additional improvements to the CPAN dist viewer. So… you're invited to contribute and make it better!

Why Upload to CPAN

CPAN has many mirrors ensuring module installation is not affected whenever GitHub (a single website) has issues. The uploaded dists are also immutable and stay there forever (barring special deletion requests, even deleted dists remain available on BackPAN). This means people are more likely to trust these dists for use in their larger projects that need dependable dependencies. Lastly… it's what the cool kids use!

How to Upload to CPAN

Here's the process for how you can get your dists to CPAN. If these dists are currently listed in our p6c ecosystem, both p6c and CPAN versions will appear on modules.perl6.org, and you're encouraged to remove the p6c version. Some of the described tools are brand-new and others are brand-old, created before Rakudo existed, so treat this guide as part information and part invitation to improve the tools.

Step 1: Get a PAUSE account

PAUSE stands for "The [Perl programming] Authors Upload Server", it's located at pause.perl.org, and it's a site you use to upload dists to CPAN.

Go to request PAUSE account page and subscribe for an account. The "desired ID" field is for your PAUSE ID, and it's currently used as "author" field on modules.perl6.org. For example, mine is ZOFFIX.

I had my account for over a decade, so my memory is a bit fuzzy, but I think you'll need to wait for a human to approve and create your account—it's not instantaneous.

Step 2: Make a Dist Archive

You can manually create a tarball or a zip archive. I don't have all the details on which files you're supposed to have in them; you can take a look at other CPAN dists to see what they're doing or…

Use App::Mi6 module! It's possible you were already using it to create dists, in which case you're in luck, as you can just run mi6 dist to make a dist archive.

I rolled my dists by hand and wrote all the docs in README.md, so when I gave mi6 dist a whirl, it replaced my README.md with emptiness because I wasn't using any POD6—something (currently) to watch out for.

Step 3: Upload Your Dist

The first option is to upload manually: log into pause.perl.org, then go to Upload a file to CPAN, be sure to select Perl6 in the select input and then upload either via an uploaded file or a URL.

The second option is to use App::Mi6's mi6 upload command.

Shortly after the upload, you'll get an email about whether your upload succeeded (you can also see emails on nntp.perl.org). Make sure you have a META6.json file in your dist and that the dist version you're uploading is higher than the currently uploaded version. Those are the most common upload errors.

Step 4: Relax and Wait

If you're on IRC, in about 10 minutes after your upload, our buggable robot will announce it:

<buggable> New CPAN upload: Number-Denominate-1.001001.tar.gz by ZOFFIX
    https://www.cpan.org/authors/id/Z/ZO/ZOFFIX/Perl6/Number-Denominate-1.001001.tar.gz

In about 2 hours, the dist will also appear on modules.perl6.org. Its updater is started in a cron job on 20th and 40th minute of the hour (unless a job is already running) and it takes about 2 hours to finish each run.

Step 5: Celebrate with the Appropriate Amount of Fun

That's about it to the process. I foresee more tools will be created in the future to make the process even easier than it is today. If you have any questions or issues, just talk to a human or a robot on our #perl6 IRC channel.

Conclusion

CPAN support for Rakudo Perl 6 dists is now usably here. You're encouraged to upload your dists to CPAN, to grow a more dependable ecosystem. You're also invited to improve and create tooling that manages and displays CPAN uploads.

-Ofun

6lang: The Naming Discussion Update

Read this article on 6lang.Party

When a couple months ago I rekindled the naming debate—the discussion on whether "Perl 6" should be renamed—I didn't expect anything more than a collective groan. That wasn't the case and today, I figured, I'd post a progress report and list the salient happenings, all the way to my currently being the proud owner of 6lang.party domain name.

The "Rakudo" Language

The "new" name I mentioned in my original post was Rakudo. As many quickly pointed out, it wasn't the greatest of names because it was the name of an implementation. Yes, I agree, but originally I thought few, if any, would be on board with a new name, or extended name, and Rakudo was basically the only name people already were using, so it stood out as something that could be "hijacked."

The Blog Post Fallout

There was quite a bit of discussion on r/perl, r/perl6, and blogs.perl.org. The general mood among the Perl community members who aren't avid 6lang users was that the entirely new name was a good idea. However, the 6lang users, and especially core devs, overall, argued "Perl 6" still had some recognition benefits and should not be removed entirely.

The middle ground was aimed at then: extend the language name. The "official" name would be among the lines of "Blah Perl 6" and users opposed to the 4-letter swear word would just use the name extension on its own, while those who feel the original name has benefits can still reap them.

The decision on the naming extension was placed on the 6.d language release agenda, with the final call on whether and with what the name should to be extended to be done by Larry, when we cut the 6.d language release.

The 6lang

Fast-forward two months. A kind soul (thank you, by the way!) asked Larry what he thought about the naming debate during the last Perl Conference:

Larry opined that we could have other terms by which Perl versions or Perl distributions are marketed as. So that gives us an option to pick an alternative name to be the second name with any "official" standing. Personally, I really like this idea; even more than name extension, because should there indeed be more benefit to the name without "Perl" in it, the alternative name will naturally become the most-used one.

Another core dev, AlexDaniel++, coined an alternative name: spelt 6lang; can be pronounced as slang, if you want to be fancy. I really liked the name, so I jumped in and registered 6lang.party

<AlexDaniel> Zoffix++ for making me recognize the need for
     alternative name. For a long time I was against
<AlexDaniel> and honestly, I can start using something like 6lang
     right away. “Rakudo Perl 6” is infringing on
     language/compiler distinction so I'm feeling reluctant
<Zoffix> OK, I'll too start using 6lang
* Zoffix is now a proud owner of 6lang.party :D
<timotimo> wow
<AlexDaniel> that was quick

And a couple of hours later, our Marketing Department churned out a new poster:

The drawback is that the name can't be used as an identifier… and Larry doesn't think it's a terribly sexy name.

* TimToady notes that 6lang isn't gonna work anywhere an identifier
     needs a leading alpha
<TimToady> it's also not a terribly sexy name
<TimToady> I could go for something more like psix, "where the p is silent
     if you want it to be" :)

Although, on the plus side, the name has the benefit that alphabetically it sorts earlier than pretty much any other language.

<AlexDaniel> If we see “6lang” as a more marketable alternative, then
     the fact that some things may not parse it as an identifier
     practically does not matter. However, this little bit is quite useful:
<AlexDaniel> m: <perl5 golang c# 6lang ruby>.sort.say
<camelia> rakudo-moar 39a4b7: OUTPUT: «(6lang c# golang perl5 ruby)␤»
<AlexDaniel> :)
<AlexDaniel> .oO( AAAlang – batteries included )

To 6.d Release And Beyond

So that's where things progressed to so far. No official decisions have been made yet, but we're thinking about it and playing with the idea. The decision on the naming debate is to be made during 6.d release.

Having learned a painful lesson from The Christmas release, we're reluctant to put down any dates for 6.d release, but I suspect it'll be somewhere between the upcoming New Year's and It's-Ready-When-It's-Ready.

See you then \o

The Rakudo Book Project

Read this article on Rakudo.Party

When I first joined the Rakudo project, we used to say "there are none right now; check back in a year" whenever someone asked for a book about the language. Today, there's a whole website for picking out a book, and the number of available books seems to multiply every time I look at it.

Still, I feel something is amiss, when I talk to folks on our support chat, when I read blog posts about the language, or when I look at our official language documentation. And it's due to that feeling that I wish to join the Rakudo book-writing club and write a few of my own. I dub it: The Rakudo Book Project.


The Books

The Rakudo Book Project involves 3 main books—The White Book, The Gray Book, and The Black Book—as well as 2 half-books—The Green Book and The Cracked Book.

The White Book will aim to provide introductory material to the Rakudo language. The target audience will benefit from prior programming experience, but it won't be strictly necessary for computer-savy people. The target audience is "adept beginners", as some might call it.

The book will cover most of Rakudo's features a typical Rakudo programmer might use in their projects, but it won't cover every little thing about each of them. By the end of the book, the readers will have written several programming projects and will be comfortable making useful, real-world Rakudo programs. More in-depth coverage of the language will be provided by The Gray Book, which is what The White Book's readers would read next. The Black Book will reach even deeper, exploring all of the arcane constructs. The progression through the books can be thought of as a plant growing in a flower pot. Initially, the roots extend through a large area of the pot, but they don't go all the way to all the walls and are rather sparse. As the plant grows, more and more roots shoot out, covering more and more volume of the pot. Same is with the books; while reading The White Book alone will let the plant survive, the root coverage will be sparse. However, by the end of The Black Book, the reader will be an expert Rakudo programmer.

Those three books are the core of my planned project. They're supplemented by two half-books on each end of the knowledge spectrum. The Green Book will target absolute programming beginners and get them up to speed just enough so they would be able to comfortably continue their learning using The White Book. On the other end of the spectrum is The Cracked Book. It's a half-book that follows The Black Book and won't provide more advanced techniques per say, but rather arcane "hacks" or even "bad ideas" that one might not wish to use in real-life code but which nevertheless provide some insight into the language.

The Cracked Book is yet a faint glimmer of an idea. Whether it will actually be made will depend on how much more I will want to say after The Black Book is complete. The Green Book is currently a bit amorphous as well. I have a 12-year old sibling interested in computers, so The Green Book might end up being a Rakudo For Kids.

The likely order in which the books will be produced is White, Gray, Green, Black, and Cracked. It's an ambitious plan, and so I won't be making any promises for producing more than one book at a time. Thus, the current aim is to produce just The White Book.

The Price

The digital versions of the books will be available for free.

Since Rakudo development can always use more funding, I plan to run crowd-funding campaigns during each of the book's development. 100% of all the collected funds will be used to sponsor Rakudo work (sponsoring someone other than me, of course). The campaigns will start once half of the target book has been created and the backers will get early preview digital copies as the book is developed further, as well as honourable mentions as Rakudo sponsors in the book itself.

Thus, the first Rakudo Core Fundraiser will launch once I have the first half of The White Book finished. I'm hoping that will happen soon.

The Why

Other than the obvious reason why people write the books—giving an alternate take on the material—I'd like to do this to cross off an item off my bucket list. Having written a terrible non-fiction book, lackluster fiction book, and a decent illustrated children's book, I hope to add a great technical book to the list, to complete it. I figure, with 5 books to attempt it, I'll be successful.

As for my alternate take, I hope to squash the myth that Rakudo is too big to learn as well as carve out a well-defined path for learners to follow. Just as I could make a living 10 years ago, when I barely spoke English, so a beginner Rakudo programmer can make useful programs with rudimentary knowledge of the language. The key is to not try to learn everything at once as well as have a definite path to walk through. Hence the 5 separate books.

I'm hoping at the end of this journey I will have accomplished all of these goals.

See you at the first Rakudo Core Fundraiser.

Perl 6: Seqs, Drugs, And Rock'n'Roll (Part 2)

Read this article on Perl6.Party

This is the second part in the series! Be sure you read Part I first where we discuss what Seqs are and how to .cache them.

Today, we'll take the Seq apart and see what's up in it; what drives it; and how to make it do exactly what we want.

PART II: That Iterated Quickly

The main piece that makes a Seq do its thing is an object that does the Iterator role. It's this object that knows how to generate the next value, whenever we try to pull a value from a Seq, or push all of its values somewhere, or simply discard all of the remaining values.

Keep in mind that you never need to use Iterator's methods directly, when making use of a Seq as a source of values. They are called indirectly under the hood in various Perl 6 constructs. The use case for calling those methods yourself is often the time when we're making an Iterator that's fed by another Iterator, as we'll see.

Pull my finger...

In its most basic form, an Iterator object needs to provide only one method: .pull-one

my $seq := Seq.new: class :: does Iterator {
    method pull-one {
        return $++ if $++ < 4;
        IterationEnd
    }
}.new;

.say for $seq;

# OUTPUT:
# 0
# 1
# 2
# 3

Above, we create a Seq using its .new method that expects an instantiated Iterator, for which we use an anonymous class that does the Iterator role and provides a single .pull-one method that uses a pair of anonymous state variables to generate 4 numbers, one per call, and then returns IterationEnd constant to signal the Iterator does not have any more values to produce.

The Iterator protocol forbids attempting to fetch more values from an Iterator once it generated the IterationEnd value, so your Iterator's methods may assume they'll never get called again past that point.

Meet the rest of the gang

The Iterator role defines several more methods, all of which are optional to implement, and most of which have some sort of default implementation. The extra methods are there for optimization purposes that let you take shortcuts depending on how the sequence is iterated over.

Let's build a Seq that hashes a bunch of data using Crypt::Bcryptmodule (run zef install Crypt::Bcrypt to install it). We'll start with the most basic Iterator that provides .pull-one method and then we'll optimize it to perform better in different circumstances.

use Crypt::Bcrypt;

sub hash-it (*@stuff) {
    Seq.new: class :: does Iterator {
        has @.stuff;
        method pull-one {
            @!stuff ?? bcrypt-hash @!stuff.shift, :15rounds
                    !! IterationEnd
        }
    }.new: :@stuff
}

my $hashes := hash-it <foo bar ber>;
for $hashes {
    say "Fetched value #{++$} {now - INIT now}";
    say "\t$_";
}

# OUTPUT:
# Fetched value #1 2.26035863
#     $2b$15$ZspycxXAHoiDpK99YuMWqeXUJX4XZ3cNNzTMwhfF8kEudqli.lSIa
# Fetched value #2 4.49311657
#     $2b$15$GiqWNgaaVbHABT6yBh7aAec0r5Vwl4AUPYmDqPlac.pK4RPOUNv1K
# Fetched value #3 6.71103435
#     $2b$15$zq0mf6Qv3Xv8oIDp686eYeTixCw1aF9/EqpV/bH2SohbbImXRSati

In the above program, we wrapped all the Seq making stuff inside a sub called hash-it. We slurp all the positional arguments given to that sub and instantiate a new Seq with an anonymous class as the Iterator. We use attribute @!stuff to store the stuff we need to hash. In the .pull-one method we check if we still have @!stuff to hash; if we do, we shift a value off @!stuff and hash it, using 15 rounds to make the hashing algo take some time. Lastly, we added a say statement to measure how long the program has been running for each iteration, using two now calls, one of which is run with the INIT phaser. From the output, we see it takes about 2.2 seconds to hash a single string.

Skipping breakfast

Using a for loop, is not the only way to use the Seq returned by our hashing routine. What if some user doesn't care about the first few hashes? For example, they could write a piece of code like this:

my $hash = hash-it(<foo bar ber>).skip(2).head;
say "Made hash {now - INIT now}";
say bcrypt-match 'ber', $hash;

# OUTPUT:
# Made hash 6.6813790
# True

We've used Crypt::Bcryptmodule's bcrypt-match routine to ensure the hash we got matches our third input string and it does, but look at the timing in the output. It took 6.7s to produce that single hash!

In fact, things will look the worse the more items the user tries to skip. If the user calls our hash-it with a ton of items and then tries to .skip the first 1,000,000 elements to get at the 1,000,001st hash, they'll be waiting for about 25 days for that single hash to be produced!!

The reason is our basic Iterator only knows how to .pull-one, so the skip operation still generates the hashes, just to discard them. Since the values our Iterator generates do not depend on previous values, we can implement one of the optimizing methods to skip iterations cheaply:

use Crypt::Bcrypt;

sub hash-it (*@stuff) {
    Seq.new: class :: does Iterator {
        has @.stuff;
        method pull-one {
            @!stuff ?? bcrypt-hash @!stuff.shift, :15rounds
                    !! IterationEnd
        }
        method skip-one {
            return False unless @!stuff;
            @!stuff.shift;
            True
        }
    }.new: :@stuff
}

my $hash = hash-it(<foo bar ber>).skip(2).head;
say "Made hash {now - INIT now}";
say bcrypt-match 'ber', $hash;

# OUTPUT:
# Made hash 2.2548012
# True

We added a .skip-one method to our Iterator that instead of hashing a value, simply discards it. It needs to return a truthy value, if it was able to skip a value (i.e. we had a value we'd otherwise generate in .pull-one, but we skipped it), or falsy value if there weren't any values to skip.

Now, the .skip method called on our Seq uses our new .skip-one method to cheaply skip through 2 items and then uses .pull-one to generate the third hash. Look at the timing now: 2.2s; the time it takes to generate a single hash.

However, we can kick it up a notch. While we won't notice a difference with our 3-item Seq, that user who was attempting to skip 1,000,000 items won't get the 2.2s time to generate the 1,000,000th hash. They would also have to wait for 1,000,000 calls to .skip-one and @!stuff.shift. To optimize skipping over a bunch of items, we can implement the .skip-at-least method (for brevity, just our Iterator class is shown):

class :: does Iterator {
    has @.stuff;
    method pull-one {
        @!stuff
            ?? bcrypt-hash( @!stuff.shift, :15rounds )
            !! IterationEnd
    }
    method skip-one {
        return False unless @!stuff;
        @!stuff.shift;
        True
    }
    method skip-at-least (Int \n) {
        n == @!stuff.splice: 0, n
    }
}

The .skip-at-least method takes an Int of items to skip. It should skip as many as it can, and return a truthy value if it was able to skip that many items, and falsy value if the number of skipped items was fewer. Now, the user who skips 1,000,000 items will only have to suffer through a single .splice call.

For the sake of completeness, there's another skipping method defined by Iterator: .skip-at-least-pull-one. It follows the same semantics as .skip-at-least, except with .pull-one semantics for return values. Its default implemention involves just calling those two methods, short-circuiting and returning IterationEnd if the .skip-at-least returned a falsy value, and that default implementation is very likely good enough for all Iterators. The method exists as a convenience for Iterator users who call methods on Iterators and (at the moment) it's not used in core Rakudo Perl 6 by any methods that can be called on users' Seqs.

A so, so count...

There are two more optimization methods—.bool-only and .count-only—that do not have a default implementation. The first one returns True or False, depending on whether there are still items that can be generated by the Iterator (True if yes). The second one returns the number of items the Iterator can still produce. Importantly these methods must be able to do that without exhausting the Iterator. In other words, after finding these methods implemented, the user of our Iterator can call them and afterwards should still be able to .pull-one all of the items, as if the methods were never called.

Let's make an Iterator that will take an Iterable and .rotate it once per iteration of our Iterator until its tail becomes its head. Basically, we want this:

.say for rotator 1, 2, 3, 4;

# OUTPUT:
# [2 3 4 1]
# [3 4 1 2]
# [4 1 2 3]

This iterator will serve our purpose to study the two Iterator methods. For a less "made-up" example, try to find implementations of iterators for combinations and permutations routines in Perl 6 compiler's source code.

Here's a sub that creates our Seq with our shiny Iterator along with some code that operates on it and some timings for different stages of the program:

sub rotator (*@stuff) {
    Seq.new: class :: does Iterator {
        has int $!n;
        has int $!steps = 1;
        has     @.stuff is required;

        submethod TWEAK { $!n = @!stuff − 1 }

        method pull-one {
            if $!n-- > 0 {
                LEAVE $!steps = 1;
                [@!stuff .= rotate: $!steps]
            }
            else {
                IterationEnd
            }
        }
        method skip-one {
            $!n > 0 or return False;
            $!n--; $!steps++;
            True
        }
        method skip-at-least (Int \n) {
            if $!n > all 0, n {
                $!steps += n;
                $!n     −= n;
                True
            }
            else {
                $!n = 0;
                False
            }
        }
    }.new: stuff => [@stuff]
}

my $rotations := rotator ^5000;

if $rotations {
    say "Time after getting Bool: {now - INIT now}";

    say "We got $rotations.elems() rotations!";
    say "Time after getting count: {now - INIT now}";

    say "Fetching last one...";
    say "Last one's first 5 elements are: $rotations.tail.head(5)";
    say "Time after getting last elem: {now - INIT now}";
}

# OUTPUT:
# Time after getting Bool: 0.0230339
# We got 4999 rotations!
# Time after getting count: 26.04481484
# Fetching last one...
# Last one's first 5 elements are: 4999 0 1 2 3
# Time after getting last elem: 26.0466234

First things first, let's take a look at what we're doing in our Iterator. We take an Iterable (in the sub call on line 37, we use a Range object out of which we can milk 5000 elements in this case), shallow-clone it (using [ ... ] operator) and keep that clone in @!stuff attribute of our Iterator. During object instantiation, we also save how many items @!stuff has in it into $!n attribute, inside the TWEAK submethod.

For each .pull-one of the Iterator, we .rotate our @!stuff attribute, storing the rotated result back in it, as well as making a shallow clone of it, which is what we return for the iteration.

We also already implemented the .skip-one and .skip-at-least optimization methods, where we use a private $!steps attribute to alter how many steps the next .pull-one will .rotate our @!stuff by. Whenever .pull-one is called, we simply reset $!steps to its default value of 1 using the LEAVE phaser.

Let's check out how this thing performs! We store our precious Seq in $rotations variable that we first check for truthiness, to see if it has any elements in it at all; then we tell the world how many rotations we can fish out of that Seq; lastly, we fetch the last element of the Seq and (for screen space reasons) print the first 5 elements of the last rotation.

All three steps—check .Bool, check .elems, and fetch last item with .tail are timed, and the results aren't that pretty. While .Bool took relatively quick to complete, the .elems call took ages (26s)! That's actually not all of the damage. Recall from PART I of this series that both .Bool and .elems cache the Seq unless special methods are implemented in the Iterator. This means that each of those rotations we made are still there in memory, using up space for nothing! What are we to do? Let's try implementing those special methods .Bool and .elems are looking for!

This only thing we need to change is to add two extra methods to our Iterator that determinine how many elements we can generate (.count-only) and whether we have any elements to generate (.bool-only):

method count-only { $!n     }
method bool-only  { $!n > 0 }

For the sake of completeness, here is our previous example, with these two methods added to our Iterator:

sub rotator (*@stuff) {
    Seq.new: class :: does Iterator {
        has int $!n;
        has int $!steps = 1;
        has     @.stuff is required;

        submethod TWEAK { $!n = @!stuff − 1 }

        method count-only { $!n     }
        method bool-only  { $!n > 0 }

        method pull-one {
            if $!n-- > 0 {
                LEAVE $!steps = 1;
                [@!stuff .= rotate: $!steps]
            }
            else {
                IterationEnd
            }
        }
        method skip-one {
            $!n > 0 or return False;
            $!n--; $!steps++;
            True
        }
        method skip-at-least (\n) {
            if $!n > all 0, n {
                $!steps += n;
                $!n     −= n;
                True
            }
            else {
                $!n = 0;
                False
            }
        }
    }.new: stuff => [@stuff]
}

my $rotations := rotator ^5000;

if $rotations {
    say "Time after getting Bool: {now - INIT now}";

    say "We got $rotations.elems() rotations!";
    say "Time after getting count: {now - INIT now}";

    say "Fetching last one...";
    say "Last one's first 5 elements are: $rotations.tail.head(5)";
    say "Time after getting last elem: {now - INIT now}";
}

# OUTPUT:
# Time after getting Bool: 0.0087576
# We got 4999 rotations!
# Time after getting count: 0.00993624
# Fetching last one...
# Last one's first 5 elements are: 4999 0 1 2 3
# Time after getting last elem: 0.0149863

The code is nearly identical, but look at those sweet, sweet timings! Our entire program runs about 1,733 times faster because our Seq can figure out if and how many elements it has without having to iterate or rotate anything. The .tail call sees our optimization (side note: that's actually very recent) and it too doesn't have to iterate over anything and can just use our .skip-at-least optimization to skip to the end. And last but not least, our Seq is no longer being cached, so the only things kept around in memory are the things we care about. It's a huge win-win-win for very little extra code.

But wait... there's more!

Push it real good...

The Seqs we looked at so far did heavy work: each generated value took a relatively long time to generate. However, Seqs are quite versatile and at times you'll find that generation of a value is cheaper than calling .pull-one and storing that value somewhere. For cases like that, there're a few more methods we can implement to make our Seq perform better.

For the next example, we'll stick with the basics. Our Iterator will generate a sequence of positive even numbers up to the wanted limit. Here's what the call to the sub that makes our Seq looks like:

say evens-up-to 20; # OUTPUT: (2 4 6 8 10 12 14 16 18)

And here's the all of the code for it. The particular operation we'll be doing is storing all the values in an Array, by assigning to it:

sub evens-up-to {
    Seq.new: class :: does Iterator {
        has int $!n = 0;
        has int $.limit is required;
        method pull-one { ($!n += 2) < $!limit ?? $!n !! IterationEnd }
    }.new: :$^limit
}

my @a = evens-up-to 1_700_000;

say now - INIT now; # OUTPUT: 1.00765440

For a limit of 1.7 million, the code takes around a second to run. However, all we do in our Iterator is add some numbers together, so a lot of the time is likely lost in .pull-oneing the values and adding them to the Array, one by one.

In cases like this, implementing a custom .push-all method in our Iterator can help. The method receives one argument that is a reification target. We're pretty close to bare "metal" now, so we can't do anything fancy with the reification target object other than call .push method on it with a single value to add to the target. The .push-all always returns IterationEnd, since it exhausts the Iterator, so we'll just pop that value right into the return value of the method's Signature:

sub evens-up-to {
    Seq.new: class :: does Iterator {
        has int $!n = 0;
        has int $.limit is required;
        method pull-one {
            ($!n += 2) < $!limit ?? $!n !! IterationEnd
        }
        method push-all (\target --> IterationEnd) {
            target.push: $!n while ($!n += 2) < $!limit;
        }
    }.new: :$^limit
}

my @a = evens-up-to 1_700_000;
say now - INIT now; # OUTPUT: 0.91364949

Our program is now 10% faster; not a lot. However, since we're doing all the work in .push-all now, we no longer need to deal with state inside the method's body, so we can shave off a bit of time by using lexical variables instead of accessing object's attributes all the time. We'll make them use native int types for even more speed. Also, (at least currently), the += meta operator is more expensive than a simple assignment and a regular +; since we're trying to squeeze every last bit of juice here, let's take advantage of that as well. So what we have now is this:

sub evens-up-to {
    Seq.new: class :: does Iterator {
        has int $!n = 0;
        has int $.limit is required;
        method pull-one {
            ($!n += 2) < $!limit ?? $!n !! IterationEnd
        }
        method push-all (\target --> IterationEnd) {
            my int $limit = $!limit;
            my int $n     = $!n;
            target.push: $n while ($n = $n + 2) < $limit;
            $!n = $n;
        }
    }.new: :$^limit
}

my @a = evens-up-to 1_700_000;
say now - INIT now; # OUTPUT: 0.6688109

There we go. Now our program is 1.5 times faster than the original, thanks to .push-all. The gain isn't as dramatic as we what saw with other methods, but can come in quite handy when you need it.

There are a few more .push-* methods you can implement to, for example, do something special when your Seq is used in codes like...

for $your-seq -> $a, $b, $c { ... }

...where the Iterator would be asked to .push-exactly three items. The idea behind them is similar to .push-all: you push stuff onto the reification target. Their utility and performance gains are ever smaller, useful only in particular situations, so I won't be covering them.

It's worth noting the .push-all can be used only with Iterators that are not lazy, since... well... it expects you to push all the items. And what exactly are lazy Iterators? I'm so glad you asked!

A quick brown fox jumped over the lazy Seq

Let's pare down our previous Seq that generates even numbers down to the basics. Let's make it generate an infinite list of even numbers, using an anonymous state variable:

sub evens {
    Seq.new: class :: does Iterator {
        method pull-one { $ += 2 }
    }.new
}

put evens

Since the list is infinite, it'd take us an infinite time to fetch them all. So what exactly happens when we run the code above? It... quite predictably hangs when the put routine is called; it sits and patiently waits for our infinite Seq to complete. The same issue occurs when trying to assign our seq to a @-sigiled variable:

my @evens = evens # hangs

Or even when trying to pass our Seq to a sub with a slurpy parameter_Parameters):

sub meows (*@evens) { say 'Got some evens!' }
meows evens # hangs

That's quite an annoying problem. Fortunately, there's a very easy solution for it. But first, a minor detour to the land of naming clarification!

A rose by any other name would laze as sweet

In Perl 6 some things are or can be made "lazy". While it evokes the concept of on-demand or "lazy" evaluation, which is ubiquitous in Perl 6, things that are lazy in Perl 6 aren't just about that. If something is-lazy, it means it always wants to be evaluated lazily, fetching only as many items as needed, even in "mostly lazy" Perl 6 constructs that would otherwise eagerly consume even from sources that do on-demand generation.

For example, a sequence of lines read from a file would want to be lazy, as reading them all in at once has the potential to use up all the RAM. An infinite sequence would also want to be is-lazy because an eager evaluation would cause it to hang, as the sequence never completes.

So a thing that is-lazy in Perl 6 can be thought of as being infinite. Sometimes it actually will be infinite, but even if it isn't, it being lazy means it has similar consequences if used eagerly (too much CPU time used, too much RAM, etc).


Now back to our infinite list of even numbers. It sounds like all we have to do is make our Seq lazy and we do that by implementing .is-lazy method on our Iterator that simply returns True:

sub evens {
    Seq.new: class :: does Iterator {
        method pull-one { $ += 2 }
        method is-lazy (--> True) {}
    }.new
}

sub meows (*@evens) { say 'Got some evens!' }

put         evens; # OUTPUT: ...
my @evens = evens; # doesn't hang
meows       evens; # OUTPUT: Got some evens!

The put routine now detects its dealing with something terribly long and just outputs some dots. Assignment to Array no longer hangs (and will instead reify on demand). And the call to a slurpy doesn't hang either and will also reify on demand.

There's one more Iterator optimization method left that we should discuss...

A Sinking Ship

Perl 6 has sink context, similar to "void" context in other languages, which means a value is being discarded:

42;

# OUTPUT:
# WARNINGS for ...:
# Useless use of constant integer 42 in sink context (line 1)

The constant 42 in the above program is in sink context—its value isn't used by anything—and since it's nearly pointless to have it like that, the compiler warns about it.

Not all sinkage is bad however and sometimes you may find that gorgeous Seq on which you worked so hard is ruthlessly being sunk by the user! Let's take a look at what happens when we sink one of our previous examples, the Seq that generates up to limit even numbers:

sub evens-up-to {
    Seq.new: class :: does Iterator {
        has int $!n = 0;
        has int $.limit is required;
        method pull-one {
            ($!n += 2) < $!limit ?? $!n !! IterationEnd
        }
    }.new: :$^limit
}

evens-up-to 5_000_000; # sink our Seq

say now - INIT now; # OUTPUT: 5.87409072

Ouch! Iterating our Seq has no side-effects outside of the Iterator that it uses, which means it took the program almost six seconds to do absolutely nothing.

We can remedy the situation by implementing our own .sink-all method. Its default implementation .pull-ones until the end of the Seq (since Seqs may have useful side effects), which is not what we want for our Seq. So let's implement a .sink-all that does nothing!

sub evens-up-to {
    Seq.new: class :: does Iterator {
        has int $!n = 0;
        has int $.limit is required;
        method pull-one {
            ($!n += 2) < $!limit ?? $!n !! IterationEnd
        }
        method sink-all(--> IterationEnd) {}
    }.new: :$^limit
}

evens-up-to 5_000_000; # sink our Seq

say now - INIT now; # OUTPUT: 0.0038638

We added a single line of code and made our program 1,520 times faster—the perfect speed up for a program that does nothing!

However, doing nothing is not the only thing .sink-all is good for. Use it for clean up that would usually be done at the end of iteration (e.g. closing a file handle the Iterator was using). Or simply set the state of the system to what it would be at the end of the iteration (e.g. .seek a file handle to the end, for sunk Seq that produces lines from it). Or, as an alternative idea, how about warning the user their code might contain an error:

sub evens-up-to {
    Seq.new: class :: does Iterator {
        has int $!n = 0;
        has int $.limit is required;
        method pull-one {
            ($!n += 2) < $!limit ?? $!n !! IterationEnd
        }
        method sink-all(--> IterationEnd) {
            warn "Oh noes! Looks like you sunk all the evens!\n"
                ~ 'Why did you make them in the first place?'
        }
    }.new: :$^limit
}

evens-up-to 5_000_000; # sink our Seq

# OUTPUT:
# Oh noes! Looks like you sunk all the evens!
# Why did you make them in the first place?
# ...

That concludes our discussion on optimizing your Iterators. Now, let's talk about using Iterators others have made.

It's a marathon, not a sprint

With all the juicy knowledge about Iterators and Seqs we now possess, we can probably see how this piece of code manages to work without hanging, despite being given an infinite Range of numbers:

.say for ^∞ .grep(*.is-prime).map(* ~ ' is a prime number').head: 5;

# OUTPUT:
# 2 is a prime number
# 3 is a prime number
# 5 is a prime number
# 7 is a prime number
# 11 is a prime number

The infinite Range probably is-lazy. That .grep probably .pull-ones until it finds a prime number. The .map .pull-ones each of the .grep's values and modifies them, and .head allows at most 5 values to be .pull-oned from it.

In short what we have here is a pipeline of Seqs and Iterators where the Iterator of the next Seq is based on the Iterator of the previous one. For our study purposes, let's cook up a Seq of our own that combines all of the steps above:

sub first-five-primes (*@numbers) {
    Seq.new: class :: does Iterator {
        has     $.iter;
        has int $!produced = 0;
        method pull-one {
            $!produced++ == 5 and return IterationEnd;
            loop {
                my $value := $!iter.pull-one;
                return IterationEnd if $value =:= IterationEnd;
                return "$value is a prime number" if $value.is-prime;
            }
        }
    }.new: iter => @numbers.iterator
}

.say for first-five-primes ^∞;

# OUTPUT:
# 2 is a prime number
# 3 is a prime number
# 5 is a prime number
# 7 is a prime number
# 11 is a prime number

Our sub slurps up_Parameters) its positional arguments and then calls .iterator method on the @numbers Iterable. This method is available on all Perl 6 objects and will let us interface with the object using Iterator methods directly.

We save the @numbers's Iterator in one of the attributes of our Iterator as well as create another attribute to keep track of how many items we produced. In the .pull-one method, we first check whether we already produced the 5 items we need to produce, and if not, we drop into a loop that calls .pull-one on the other Iterator, the one we got from @numbers Array.

We recently learned that if the Iterator does not have any more values for us, it will return the IterationEnd constant. A constant whose job is to signal the end of iteration is finicky to deal with, as you can imagine. To detect it, we need to ensure we use the binding (:=), not the assignment (=) operator, when storing the value we get from .pull-one in a variable. This is because pretty much only the container identity (=:=) operator will accept such a monstrosity, so we can't stuff the value we .pull-one into just any container we please.

In our example program, if we do find that we received IterationEnd from the source Iterator, we simply return it to indicate we're done. If not, we repeat the process until we find a prime number, which we then put into our desired string and that's what we return from our .pull-one.

All the rest of the Iterator methods we've learned about can be called on the source Iterator in a similar fashion as we called .pull-one in our example.

Conclusion

Today, we've learned a whole ton of stuff! We now know that Seqs are powered by Iterator objects and we can make custom iterators that generate any variety of values we can dream about.

The most basic Iterator has only .pull-one method that generates a single value and returns IterationEnd when it has no more values to produce. It's not permitted to call .pull-one again, once it generated IterationEnd and we can write our .pull-one methods with the expectation that will never happen.

There are plenty of optimization opportunities a custom Iterator can take advantage of. If it can cheaply skip through items, it can implement .skip-one or .skip-at-least methods. If it can know how many items it'll produce, it can implement .bool-only and .count-only methods that can avoid a ton of work and memory use when only certain values of a Seq are needed. And for squeezing the very last bit of performance, you can take advantage of .push-all and other .push-* methods that let you push values onto the target directly.

When your Iterator .is-lazy, things will treat it with extra care and won't try to fetch all of the items at once. And we can use the .sink-all method to avoid work or warn the user of potential mistakes in their code, when our Seq is being sunk.

Lastly, since we know how to make Iterators and what their methods do, we can make use of Iterators coming from other sources and call methods on them directly, manipulating them just how we want to.

We now have all the tools to work with Seq objects in Perl 6. In the PART III of this series, we'll learn how to compactify all of that knowledge and skillfully build Seqs with just a line or two of code, using the sequence operator.

Stay tuned!

-Ofun

Perl 6: Seqs, Drugs, And Rock'n'Roll

Read this article on Perl6.Party

I vividly recall my first steps in Perl 6 were just a couple of months before the first stable release of the language in December 2015. Around that time, Larry Wall was making a presentation and showed a neat feature—the sequence operator—and it got me amazed about just how powerful the language is:

# First 12 even numbers:
say (2, 4 … ∞)[^12];      # OUTPUT: (2 4 6 8 10 12 14 16 18 20 22 24)

# First 10 powers of 2:
say (2, 2², 2³ … ∞)[^10]; # OUTPUT: (2 4 8 16 32 64 128 256 512 1024)

# First 13 Fibonacci numbers:
say (1, 1, *+* … ∞)[^13]; # OUTPUT: (1 1 2 3 5 8 13 21 34 55 89 144 233)

The ellipsis () is the sequence operator and the stuff it makes is the Seq object. And now, a year and a half after Perl 6's first release, I hope to pass on my amazement to a new batch of future Perl 6 programmers.

This is a 3-part series. In PART I of this article we'll talk about what Seq s are and how to make them without the sequence operator. In PART II, we'll look at the thing-behind-the-curtain of Seq's: the Iterator type and how to make Seqs from our own Iterators. Lastly, in PART III, we'll examine the sequence operator in all of its glory.

Note: I will be using all sorts of fancy Unicode operators and symbols in this article. If you don't like them, consult with the Texas Equivalents page for the equivalent ASCII-only way to type those elements.

PART I: What the Seq is all this about?

The Seq stands for Sequence and the Seq object provides a one-shot way to iterate over a sequence of stuff. New values can be generated on demand—in fact, it's perfectly possible to create infinite sequences—and already-generated values are discarded, never to be seen again, although, there's a way to cache them, as we'll see.

Sequences are driven by Iterator objects that are responsible for generating values. However, in many cases you don't have to create Iterators directly or use their methods while iterating a Seq. There are several ways to make a Seq and in this section, we'll talk about gather/take construct.

I gather you'll take us to...

The gather statement and take routine are similar to "generators" and "yield" statement in some other languages:

my $seq-full-of-sunshine := gather {
    say  'And nobody cries';
    say  'there’s only butterflies';

    take 'me away';
    say  'A secret place';
    say  'A sweet escape';

    take 'meee awaaay';
    say  'To better days'    ;

    take 'MEEE AWAAAAYYYY';
    say  'A hiding place';
}

Above, we have a code block with lines of song lyrics, some of which we say (print to the screen) and others we take (to be gathered). Just like, .say can be used as either a method or a subroutine, so you can use .take as a method or subroutine, there's no real difference; merely convenience.

Now, let's iterate over $seq-full-of-sunshine and watch the output:

for $seq-full-of-sunshine {
    ENTER say '▬▬▶ Entering';
    LEAVE say '◀▬▬ Leaving';

    say "❚❚ $_";
}

# OUTPUT:
# And nobody cries
# there’s only butterflies
# ▬▬▶ Entering
# ❚❚ me away
# ◀▬▬ Leaving
# A secret place
# A sweet escape
# ▬▬▶ Entering
# ❚❚ meee awaaay
# ◀▬▬ Leaving
# To better days
# ▬▬▶ Entering
# ❚❚ MEEE AWAAAAYYYY
# ◀▬▬ Leaving
# A hiding place

Notice how the say statements we had inside the gather statement didn't actualy get executed until we needed to iterate over a value that take routines took after those particular say lines. The block got stopped and then continued only when more values from the Seq were requested. The last say call didn't have any more takes after it, and it got executed when the iterator was asked for more values after the last take.

That's exceptional!

The take routine works by throwing a CX::Take control exception that will percolate up the call stack until something takes care of it. This means you can feed a gather not just from an immediate block, but from a bunch of different sources, such as routine calls:

multi what's-that (42)                     { take 'The Answer'            }
multi what's-that (Int $ where *.is-prime) { take 'Tis a prime!'          }
multi what's-that (Numeric)                { take 'Some kind of a number' }

multi what's-that   { how-good-is $^it                   }
sub how-good-is ($) { take rand > ½ ?? 'Tis OK' !! 'Eww' }

my $seq := gather map &what's-that, 1, 31337, 42, 'meows';

.say for $seq;

# OUTPUT:
# Some kind of a number
# Tis a prime!
# The Answer
# Eww

Once again, we iterated over our new Seq with a for loop, and you can see that take called from different multies and even nested sub calls still delivered the value to our gather successfully:

The only limitation is you can't gather takes done in another Promise or in code manually cued in the scheduler:

gather await start take 42;
# OUTPUT:
# Tried to get the result of a broken Promise
#   in block <unit> at test.p6 line 2
#
# Original exception:
#     take without gather

gather $*SCHEDULER.cue: { take 42 }
await Promise.in: 2;
# OUTPUT: Unhandled exception: take without gather

However, nothing's stopping you from using a Channel to proxy your data to be taken in a react block.

my Channel $chan .= new;
my $promise = start gather react whenever $chan { .take }

say "Sending stuff to Channel to gather...";
await start {
    $chan.send: $_ for <a b c>;
    $chan.close;
}
dd await $promise;

# OUTPUT:
# Sending stuff to Channel to gather...
# ("a", "b", "c").Seq

Or gathering takes from within a Supply:

my $supply = supply {
    take 42;
    emit 'Took 42!';
}

my $x := gather react whenever $supply { .say }
say $x;

# OUTPUT: Took 42!
# (42)

Stash into the cache

I mentioned earlier that Seqs are one-shot Iterables that can be iterated only once. So what exactly happens when we try to iterate them the second time?

my $seq := gather take 42;
.say for $seq;
.say for $seq;

# OUTPUT:
# 42
# This Seq has already been iterated, and its values consumed
# (you might solve this by adding .cache on usages of the Seq, or
# by assigning the Seq into an array)

A X::Seq::Consumed exception gets thrown. In fact, Seqs do not even do the Positional role, which is why we didn't use the @ sigil that type- checks for Positional on the variables we stored Seqs in.

The Seq is deemed consumed whenever something asks it for its Iterator after another thing grabbed it, like the for loop would. For example, even if in the first for loop above we would've iterated over just 1 item, we wouldn't be able to resume taking more items in the next for loop, as it'd try to ask for the Seq's iterator that was already taken by the first for loop.

As you can imagine, having Seqs always be one-shot would be somewhat of a pain in the butt. A lot of times you can afford to keep the entire sequence around, which is the price for being able to access its values more than once, and that's precisely what the Seq.cachemethod does:

my $seq := gather { take 42; take 70 };
$seq.cache;

.say for $seq;
.say for $seq;

# OUTPUT:
# 42
# 70
# 42
# 70

As long as you call .cache before you fetch the first item of the Seq, you're good to go iterating over it until the heat death of the Universe (or until its cache noms all of your RAM). However, often you do not even need to call .cache yourself.

Many methods will automatically .cache the Seq for you:

There's one more nicety with Seqs losing their one-shotness that you may see refered to as PositionalBindFailover. It's a role that indicates to the parameter binder that the type can still be converted into a Positional, even when it doesn't do Positional role. In plain English, it means you can do this:

sub foo (@pos) { say @pos[1, 3, 5] }

my $seq := 2, 4 … ∞;
foo $seq; # OUTPUT: (4 8 12)

We have a sub that expects a Positional argument and we give it a Seq which isn't Positional, yet it all works out, because the binder .caches our Seq and uses the List the .cache method returns to be the Positional to be used, thanks to it doing the PositionalBindFailover role.

Last, but not least, if you don't care about all of your Seq's values being generated and cached right there and then, you can simply assign it to a @ sigiled variable, which will reify the Seq and store it as an Array:

my @stuff = gather {
    take 42;
    say "meow";
    take 70;
}

say "Starting to iterate:";
.say for @stuff;

# OUTPUT:
# meow
# Starting to iterate:
# 42
# 70

From the output, we can see say "meow" was executed on assignment to @stuff and not when we actually iterated over the value in the for loop.

Conclusion

In Perl 6, Seqs are one-shot Iterables that don't keep their values around, which makes them very useful for iterating over huge, or even infinite, sequences. However, it's perfectly possible to cache Seq values and re-use them, if that is needed. In fact, many of the Seq's methods will automatically cache the Seq for you.

There are several ways to create Seqs, one of which is to use the gather and take where a gather block will stop its execution and continue it only when more values are needed.

In parts II and III, we'll look at other, more exciting, ways of creating Seqs. Stay tuned!

-Ofun

About Zoffix Znet

user-pic I blog about Perl.