Perl 6 IO TPF Grant: Monthly Report (February, 2017)

This document is the February, 2017 progress report for TPF Standardization, Test Coverage, and Documentation of Perl 6 I/O Routines grant


I'm currently running slightly behind the schedule outlined in the grant. I expect to complete the Action Plan and have it ratified by other core members by March 18th, which is the date of the 2017.03 compiler release. Then, I'll implement all of the Action Plan (and complete the grant) by the 2017.04 compiler release on April 15th. This is also the release the next Rakudo Star distribution will be based on, and so the regular end users will receive better IO there and then.

Some members of the Core Team voiced concerns over implementing any changes that can break users' code, even if the changes do not break 6.c-errata specification tests. Once the full set of changes is known, they will be reviewed on a case-by-case basis, and some of them may be implemented under 6.d.PREVIEW pragma, to be included in 6.d language version, leaving 6.c language versions untouched. Note that changes that are decided to be 6.d material may delay the completion of this grant due to not fully-fleshed out infrastructure for supporting multiple language versions. The April 15th deadline stated above applies only to changes to 6.c language and new deadline will be ascertained for completion of the 6.d changes.

User Communications

I wrote and disseminated advanced notice of the changes to be made due to this grant, to prepare the users to expect some code to break (some routines were found to be documented, despite being absent entirely from the Specification and not officially part of the language).

The notice can be seen at:

It is possible the Core Team will decide to defer all breaking changes to 6.d language version, to be currently implemented under v6.d.PREVIEW pragma.

Bonus Deliverable

The bonus deliverable—The Map of Perl 6 Routines—is now usable. The code is available in perl6/routine-map repository, and the rendered version is available on Its current state is sufficient to serve the intended purpose for this grant, but I'll certainly add improvements to it sometime in the future, such as linking to docs, linking to routines' source code, having an IRC bot looking stuff up in it, etc.

It'll also be fairy easy to use the Map to detect undocumented routines or ones that are documented under the incorrect type.

Identified Issues/Deficiencies with IO Routines

These points, issues, and ideas were identified this month and will be included for consideration in the Action Plan.

  • Calling practically any method on a closed IO::Handle results in an LTA (Less Than Awesome) error message that reads <something> requires an object with REPR MVMOSHandle where <something> is sometimes the name of the method called by the user and others is some internal method invoked indirectly. We need better errors for closed file handles; and not something that would require a is-fh-closed() type of conditional called in all the methods, which would be a hefty performance hit.
  • Several routines have been identified which in other languages return useful information: number of bytes actually written or current file position, whereas in Perl 6 they just return a Bool (.print, .say, .write) or a Mu type object (.seek). Inconsistently, .printf does appear to return the number of bytes written. It should be possible to make other routines similarly useful, although I suspect some of it may have to wait until 6.d language release.
  • The .seek routine takes the seek location as one of three Enum values. Not only are they quite lengthy to type, they're globally available for no good reason and .seek is virtually unique in using this calling convention. I will seek to standardize this routine to take mutually-exclusive named arguments instead, preferably with much shorter names, but those are yet to be bikeshed.
  • IO.umask routine simply shells out to umask. This fails terribly on OSes that don't have that command, especially since the code still tries to decode the received input as an octal string, even after the failure. Needs improvement.
  • link's implementation and documentation confuses what a "target" is. Luckily (or sadly?) there are exactly zero tests for this routine in the Perl 6 Specification, so we can change it to match the behaviour of ln Linux command and the foo $existing-thing, $new-thing argument order of move, rename, and other similar routines.
  • When using run(:out, 'some-non-existant-command').out.slurp-rest it will silently succeed and return an empty string. If possible, this should be changed to return the failure or throw at some point.
  • chdir's :test parameter for directory permissions test is taken as a single string parameter. This makes it extremely easy to mistakenly write broken code: for example, "/".IO.chdir: "tmp", :test<r w> succeeds, while "/".IO.chdir: "tmp", :test<w r> fails with a misleading error message saying the directory is not readable/writable. I will propose for :test parameter to be deprecated in favour of using multiple named arguments to indicate desired tests. By extension, similar change will be applied to indir, tmpdir, and homedir routines (if they remain in the language).
  • Documentation: several inaccuracies in the documentation were found. I won't be identifying these in my reports/Action Plan, but will simply ensure the documentation matches the implementation once the Action Plan is fully implemented.

Discovered Bugs

The hunt for 6-legged friends has these finds so far:

Will (attempt to) fix as part of the grant

  • indir() has a race condition where the actual dir it runs in ends up being wrong. Using indir '/tmp/useless', { qx/rm -fr */ } in one thread and backing up your precious data in another has the potential to result in some spectacular failurage.
  • perl6 -ne '@ = lines' crashes after first iteration, crying about MVMOSHandle REPR. I suspect the code is failing to follow iterator protocol somewhere and is attempting to read on an already closed handle. I expect to be able to resolve this and the related RT#128047 as part of the grant.
  • .tell incorrectly always returns 0 on files opened in append mode
  • link mixes up target and link name in its error message

Don't think I will be able to fix these as part of the grant

  • seek() with SeekFromCurrent as location fails to seek correctly if called after .readchars, but only on MoarVM. This appears to occur due to some sort of buffering. I filed this as RT#130843.
  • On JVM, .readchars incorrectly assumes all chars are 2 bytes long. This appears to be just a naive substitute for nqp::readcharsfh op. I filed this as RT#130840.

Already Fixed

  • While making the Routine Map, I discovered .WHICH and .Str methods on IO::Special were only methods defined only for the :D subtype, resulting in a crash when using, say, infix:<eqv> operator on the type object, instead Mu.WHICH/.Str candidates getting invoked. This bug was easy and I already commited fix in radudo/dd4dfb14d3 and tests to cover it in roast/63370fe054

Auxiliary Bugs

While doing the work for this grant, I also discovered some non-IO related bugs (some of which I fixed):

I Botched a Perl 6 Release And Now a Robot Is Taking My Job

Deconfusion note: if you're just a regular Perl 6 user, you likely use and only ever heard of Rakudo Star, which is a distribution that includes the Rakudo Perl 6 Compiler, some modules, and the docs. This post details a release of that compiler only, which gets released more often than Rakudo Star. So please don't think there's a new release, if Star is all you use.

Part I: Humans Make Errors

Today is the third Saturday of the month, which is an awesome day! It's when the Rakudo Perl 6 Compiler sees its monthly release. I was at the helm today, so I chugged along through the Rakudo release guide and the NQP release guide, whose release is part of the process as well.

We're All Green To Go

As I was nearing the end of the process, the first hint of a problem surfaced when a user joined the #perl6-dev IRC channel:

<nwc10> the tag 2016.08 seems to be missing from the nqp github repository
<Zoffix> nwc10, it was created about 44 minutes ago. Ensure you got the
latest everything
<nwc10> very strange. because if I pull again in my nqp checkout I still don't see it
<Zoffix> Did you add --tags? it's git pull --tags or git fetch --tags or something like that
<nwc10> Zoffix: correct. no I didn't. thanks
<Zoffix> \o/
<nwc10> OK, why do I usually not need to do that?

Good question. I pulled in a fresh checkout and ran the full test suite again, just to be sure. Everything passed. Luckily, I got the answer rather quickly:

<nwc10> git log --graph --decorate origin/master 2016.08
<nwc10> how that it and master have diverged
<nwc10> the tag is for a commit which is not a parent of nqp master

Here's the story: I have a local NQP checkout. I bump its version and its MoarVM dependency. Tag the release. And start the full build and test suite run. That's a lengthly process, so while that is happening, I go to GitHub and use the online editor to make a minor tweak to the text of the NQP's release guide.

The tests finish. I try to push my tag and the version bump and get the usual error saying the online repo has diverged. Instinctively, I run my gr alias for git pull --rebase to bring in the new changes, and then push all of the work into the repo.

The problem is that --rebase doesn't move my tag, so it's still referencing that old commit. If you clone the repo, everything appears to work for that tag, but if you pull in the changes into the existing repo, you don't get the tag. Also, git describe (which we use when doing mid-release NQP/MoarVM version bumps) was now missing my tag, because the commit my tag tagged is not the parent of the master HEAD.

At this point, I don't yet know about the breakage for folks who are doing git pull in an existing branch, and so I continue with the release, since all of my tests are green. Finish up. Then relax by departing to fly in No Man's Sky. Awesome!

The Robocide

The first hint (wait, the second hint?) of trouble showed up when the latest commits did not appear in our eval bot that's supposed to update itself to HEAD:

<Zoffix> m: dd [ $*VM.version, $*PERL.compiler.version ]
<camelia> rakudo-moar c201a7: OUTPUT«[v2016.07.24.g.31.eccd.7,
<Zoffix> Oh. I guess camelia doesn't build every commit right away.

I did not create camelia, so was unfamiliar with her workings, but there was another bot whom I did create and knew for sure it was supposed to update to HEAD every hour... it didn't:

<Zoffix> s: dd $*VM.version, $*PERL.compiler.version
<SourceBaby> Zoffix, Something's wrong: ␤ERR: v2016.07.24.g.31.eccd.7
v2016​.07.1.243.gc.201.a.76␤Cannot resolve caller sourcery(Nil); none of
these signatures match:␤ ($thing, Str:D $method, Capture $c)␤
($thing, Str:D $method)␤ (&code)␤ (&code, Capture $c)␤
in block at -e line 6␤␤
* Zoffix gets a bad feeling

The Fallout

I didn't have to wonder about what the hell was going on for too long, since shortly thereafter a user in #perl6 channel turned up, saying they can't build:

<cuonglm> Hi, anyone got trouble when building 2016.08 rakudo
release from source I got `error: pathspec '2016.08' did not match
any file(s) know to git. Sounds like git history was overriden

After a short conversation with the person, it became obvious that while cloning and building the repo (or using the release I released) may have worked fine, simply git pulling ran into the issue with the tag.

It was obvious there was an issue and it needed fixing. The question was: how. The first suggestions seemed scary:

<mst> you can always burn the tag with fire and replace it with a fixed one.
<mst> the advantages may outweigh the risks

We already had users with checkouts with a broken tag asking for help in the chat channel and since I've sent the release announcement email, there may have been many users experiencing such a checkout issue. I didn't know what to do, so the first thing I did was panic and attempt to find someone else to fix my mess:

<Zoffix> jnthn, [Coke], TimToady are you around? I made a booboo with the tag

A lot of the core devs are in Europe, so there's a bit of timezone conflict with me, and with YAPC::Europe happening, some of them were not even in the chat at the time. Left to my own devices, I joined #git on Freenode, and the kind folks in there reaffirmed that replacing the tag might be a bad idea. Victory... I didn't have to learn how to do that:

<Zoffix> What if I delete the tag and create a new one.
What happens to people who have cloned the branch with the incorrect tag?
<_ikke_> Zoffix: A good question. Tags are kind of special as
in they're not expecting to change (and git somewhat ignores changing tags from the remote)
<_ikke_> Zoffix: man git tag has a section about it ("On retagging")
<gitinfo> Zoffix: the git-tag manpage is available at
<Zoffix> OK, then I think I'll leave it as it is, and wait for
someone smarter than me to figure out a fix :P
<Zoffix> Haha "Just admit you screwed up, and use
a different name." Yup, I'll go with that :P
<Zoffix> Thanks for the help!
<_ikke_> YW ;-)

With tag mangling out of the question and users having issues, the next answer was to make an emergency point release with a corrected tag, so I proceeded to do so:

<Zoffix> OK. I'm gonna do a 2016.08.1 'cause I'm not 100% sure
just changing the tag will fix everything and won't introduce new problems.

Making another NQP release, with a correct tag, seemed to resolve the issue on my box, but I wasn't sure there wasn't any more fallout about to happen with the current Rakudo release, so I made a point release of the compiler as well, just to be safe, and to sync up versions with NQP (so folks don't think a 2016.08.01 Rakudo release is missing, when they see a 2016.08.1 NQP release).

Jumping into the release guides at mid-point proved challenging, and considering I've just done a release, so did trying to keep track of which steps I've already done. Tired. Embarrassed. And too sober for the occasion. I headed to the pub, knowing this is the last time I'll make a release like this.

Part II: I, Robot

If you've heard of The Joel Test, you may've noticed Perl 6's release process described above currently fails item #2:

2. Can you make a build in one step?

Painfully aware of the issue, I was thinking about improving the situation ever since the first time I stepped up to cut a release. If you follow the #perl6-dev channel, you may have even seen me show off a couple of prototypes. Such as my Rele6sr web app that lets you keep track of release-blocking bug tickets:

Or my Rele6sr bot that reminds you about an upcoming release, giving a list of new tickets since last release, as well as the ChangeLog entries to review:

<Zoffix> Rele6sr: reminder
<Rele6sr> 🎺🎺🎺 Friends! I bear good news! Rakudo's release will
happen in just 1 days! Please update the ChangeLog with anything you worked
on that should be known to our users. 🎺🎺🎺
<Rele6sr> 🎺🎺🎺 Here are still-open new RT tickets since last release: And here is the git log output for commits
since last release: 🎺🎺🎺

Today's incident indicated to me that it's time to stop messing around with prototypes and put out a complete plan of action.

Spread It Out

There are two things the Release Guide contains that may make it seem like automating releases is a tough nut to crack: reviewing bug tickets for blockers and populating the ChangeLog with needed items. We can't make a robot reliably do those things (yet?), so the largest part of the release process requests a human to do so. But... what if we spread that job throughout the entire month between the releases?

This month, I will create a web app that will keep track of all the reported tickets and will let the release manager log in and mark the tickets they reviewed as either release blockers or non-blockers. As a bonus, the prototype buggable bug-queue interfacing bot that some of you may have seen will also use the app to do its thing more correctly, and the app will also serve as a nicer interface to tickets for people who don't like our original RT website.

The same principle will apply to ChangeLog entries too, where the site will keep track of the commits the release manager has reviewed and added to the ChangeLog. As such, by "keeping state" around, the web app will let the release manager spend only a couple of minutes every few days reviewing changes and bugs, instead of cramming all of the work into a single session on the release day.

Build In One Step

Providing the release manager keeps up with the above steps throughout the month, on the release day they will have a single thing to do: issue the release command to the Rele6sr bot:

<Zoffix> Rele6sr: release
<Rele6sr> Yey! It's that time! Starting the release.
You can watch the progress on
Tell me to abort at any time to abort the process.

Since the issue review process has been subtracted from the release instructions, all that's left is grunt work that can be entirely automated. The bot will use Google Compute Engine API to fire up my 24-core dev box. It will then ssh into it and clone the repos, update versions, run the builds, run the tests, tag stuff, generate tarballs, test the tarballs, scp the tarballs to Rakudo's website, email the release announcement, and update the Wikipedia page (if there's an API for that). The human can watch all of that happen on a page that feeds a websocket with the output from Proc::Async and abort if anything goes wrong. The bot will automatically abort if any of the tests in the test suite fail. As a bonus, I'm hoping to make the bot fire up instances of VMs with several different OSes, to add an extra layer of testing in different environments.

Granted, at least with the first several releases, there will be safety stop-measures and verification steps where a human can ensure the robot is doing the right thing, and abort the process, if needed. But there's no technical limitation to the bot correctly cutting a release after being issued a single command.

Exterminate All Humans

Automating complex build jobs eliminates mistakes. A robot doesn't care if it has to rebuild from scratch a hundred times while you're trying to debug a release-blocker (OK, I think it doesn't care... any robots' rights activists in the audience to tell me I'm wrong?).

Both my today's mistake and the difficulty of fixing it could've been avoided if the release process were entirely automated. The fewer things you have to do manually, the fewer places a mistake can creep into.

The Joel Test, while an amusing read, has a lot of truth and simplicity to it. I suggest you re-evaluate any of the points your project fails the test on, and I believe you'll save yourself a lot of headache if you do so.


Today's events were a great learning experience and a catalyst for improvement. Humans are squishy and fallable and they make mistakes, especially out of habit.

Trying to follow a regular list of instructions during an irregular situation can prove difficult and introduce more mistakes, making a bad situation worse.

If you haven't yet, try to automate your release process as much as you can. I know Perl 6 will follow that advice. This was the last time I cut a release. My job was taken by a robot.

The Awesome Errors of Perl 6

If you're following tech stuff, you probably know by now about the folks at Rust land working on some totally awesome error reporting capabilities. Since Perl 6 is also known for its Awesome Errors, mst inquired for some examples to show off to the rustaceans, and unfortunately I drew a blank...

Errors are something I try to avoid and rarely read in full. So I figured I'll hunt down some cool examples of Awesome Errors and write about them. While I could just bash my head on the keyboard and paste the output, that'd be quite boring to read, so I'll talk about some of the tricky errors that might not be obvious to beginners, and how to fix them.

Let the head bashing begin!

The Basics

Here's some code with an error in it;

say "Hello world!;
say "Local time is {}";

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Two terms in a row (runaway multi-line "" quote starting at line 1 maybe?)
# at /home/zoffix/test.p6:2
# ------> say "⏏Local time is {}";
#     expecting any of:
#         infix
#         infix stopper
#         postfix
#         statement end
#         statement modifier
#         statement modifier loop

The first line is missing the closing quote on the string, so everything until the opening quote on the second line is still considered part of the string. Once the supposedly closing quote is found, Perl 6 sees word "Local," which it identifies as a term. Since two terms in a row are not allowed in Perl 6, the compiler throws an error, offering some suggestions on what it was expecting, and it detects we're in a string and suggests we check we didn't forget a closing quote on line 1.

The ===SORRY!=== part doesn't mean you're running the Canadian version of the compiler, but rather that the error is a compile-time (as compared to a run-time) error.


Here's an amusing error. We have a subroutine that returns things, so we call it and use a for loop to iterate over the values:

sub things { 1 … ∞ }

for things {
    say "Current stuff is $_";

# ===SORRY!===
# Function 'things' needs parens to avoid gobbling block
# at /home/zoffix/test.p6:5
# ------> }⏏<EOL>
# Missing block (apparently claimed by 'things')
# at /home/zoffix/test.p6:5
# ------> }⏏<EOL>

Perl 6 lets you omit parentheses when calling subroutines. The error talks about gobbling blocks. What happens is the block we were hoping to give to the for loop is actually being passed to the subroutine as an argument instead. The second error in the output corroborates by saying the for loop is missing its block (and makes a suggestion it was taken by the our things subroutine).

The first error tells us how to fix the issue: Function 'things' needs parens, so our loop needs to be:

for things() {
    say "Current stuff is $_";

However, were our subroutine actually expecting a block to be passed, no parentheses would be necessary. Two code blocks side by side would result in "two terms in a row" error we've seen above, so Perl 6 knows to pass the first block to the subroutine and use the second block as the body of the for loop:

sub things (&code) { code }

for things { 1 … ∞ } {
    say "Current stuff is $_";

Did You Mean Levestein?

Here's a cool feature that will not only tell you you're wrong, but also point out what you might've meant:

sub levenshtein {}

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Undeclared routine:
#     levestein used at line 2. Did you mean 'levenshtein'?

When Perl 6 encounters names it doesn't recognize it computes Levenshtein distance for the things it does know to try to offer a useful suggestion. In the instance above it encountered an invocation of a subroutine it didn't know about. It noticed we do have a similar subroutine, so it offered it as an alternative. No more staring at the screen, trying to figure out where you made the typo!

The feature doesn't consider everything under the moon each time it's triggered, however. Were we to capitalize the sub's name to Levenshtein, we would no longer get the suggestion, because for things that start with a capital letter, the compiler figures it's likely a type name and not a subroutine name, so it checks for those instead:

class Levenshtein {};

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Undeclared name:
#    Lvnshtein used at line 2. Did you mean 'Levenshtein'?

Once You Go Seq, You Never Go Back

Let's say you make a short sequence of Fibonacci numbers. You print it and then you'd like to print it again, but this time square each member. What happens?

my $seq = (1, 1, * + * … * > 100);
$seq             .join(', ').say;
${ $_² }).join(', ').say;

# 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144
# 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)
#   in block <unit> at test.p6 line 3

Ouch! A run-time error. What's happening is the Seq type we get from the the sequence operator doesn't keep stuff around. When you iterate over it, each time it gives you a value, it discards it, so once you've iterated over the entire Seq, you're done.

Above, we're attempting to iterate over it again, and so the Rakudo runtime cries and complains, because it can't do it. The error message does offer two possible solutions. We can either use the .cache method to obtain a List we'll iterate over:

my $seq = (1, 1, * + * … * > 100).cache;
$seq             .join(', ').say;
${ $_² }).join(', ').say;

Or we can use an Array from the get go:

my @seq = 1, 1, * + * … * > 100;
@seq             .join(', ').say;{ $_² }).join(', ').say;

And even though we're storing the Seq in an Array, it won't get reified until it's actually needed:

my @a = 1 … ∞;
say @a[^10];

# (1 2 3 4 5 6 7 8 9 10)

These Aren't The Attributes You're Looking For

Imagine you have a class. In it, you have some private attributes and you've got a method that does a regex match using the value of that attribute as part of it:

class {
    has $!prefix = 'foo';
    method has-prefix ($text) {
        so $text ~~ /^ $!prefix/;

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Attribute $!prefix not available inside of a regex, since regexes are methods on Cursor.
# Consider storing the attribute in a lexical, and using that in the regex.
# at /home/zoffix/test.p6:4
# ------>         so $text ~~ /^ $!prefix⏏/;
#     expecting any of:
#         infix stopper

Oops! What happened?

It's useful to understand that as far as the parser is concerned, Perl 6 is actually braided from several languages: Perl 6, Quote, and Regex languages are parts of that braiding. This is why stuff like this Just Works™:

say "foo { "bar" ~ "meow" } ber ";

# foo barmeow ber

Despite the interpolated code block using the same " quotes to delimit the strings within it as the quotes on our original string, there's no conflict. However, the same mechanism presents us with a limitation in regexes, because in them, the looked up attributes belong to the Cursor object responsible for the regex.

To avoid the error, simply use a temporary variable to store the $!prefix in—as suggested by the error message—or use the given block:

class {
    has $!prefix = 'foo';
    method has-prefix ($text) {
        given $!prefix { so $text ~~ /^ $_/ }


Ever tried to access an element of a list that's out of range?

my @a = <foo bar ber>;
say @a[*-42];

# Effective index out of range. Is: -39, should be in 0..Inf
#  in block <unit> at test.p6 line 2

In Perl 6, to index an item from the end of a list, you use funky syntax: [*-42]. That's actually a closure that takes an argument (which is the number of elements in the list), subtracts 42 from it, and the return value is used as an actual index. You could use @a[sub ($total) { $total - 42 }] instead, if you were particularly bored.

In the error above, that index ends up being 3 - 42, or -39, which is the value we see in the error message. Since indexes cannot be negative, we receive the error, which also tells us the index must be 0 to positive infinity (with any indexes above what the list contains returning Any when looked up).

A Rose By Any Other Name, Would Code As Sweet

If you're an active user of Perl 6's sister language, the Perl 5, you may sometimes write Perl-5-isms in your Perl 6 code:

say "foo" . "bar";

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Unsupported use of . to concatenate strings; in Perl 6 please use ~
# at /home/zoffix/test.p6:1
# ------> say "foo" .⏏ "bar";

Above, we're attempting to use Perl 5's concatenation operator to concatenate two strings. The error mechanism is smart enough to detect such usage and to recommend the use of the correct ~ operator instead.

This is not the only case of such detection. There are many. Here's another example, detecting accidental use of Perl 5's diamond operator, with several suggestions of what the programmer may have meant:

while <> {}

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Unsupported use of <>; in Perl 6 please use lines() to read input, ('') to
# represent a null string or () to represent an empty list
# at /home/zoffix/test.p6:1
# ------> while <⏏> {}

Heredoc, Theredoc, Everywheredoc

Here's an evil error and there's nothing awesome about it, but I figured I'd mention it, since it's easy to debug if you know about it, and quite annoying if you don't. The error is evil enough that it may have been already improved if you're reading this far enough in the future from when I'm writing this.

Try to spot what the problem is... read the error at the bottom first, as if you were the one who wrote (and so, are familiar with) the code:

my $stuff = qq:to/END/;
Blah blah blah

for ^10 {
    say 'things';

for ^20 {
    say 'moar things';

sub foo ($wtf) {
    say 'oh my!';

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# Variable '$wtf' is not declared
# at /home/zoffix/test.p6:13
# ------> sub foo (⏏$wtf) {

Huh? It's crying about an undeclared variable, but it's pointing to a signature of a subroutine. Of course it won't be declared. What sort of e-Crack is the compiler smoking?

For those who didn't spot the problem: it's the spurious semicolon after the closing END of the heredoc. The heredoc ends where the closing delimiter appears on a line all by itself. As far as the compiler is concerned, we've not seen the delimiter at END;, so it continues parsing as if it were still parsing the heredoc. A qq heredoc lets you interpolate variables, so when the parser gets to the $wtf variable in the signature, it has no idea it's in a signature of an actual code and not just some random text, so it cries about the variable being undeclared.

Won't Someone Think of The Reader?

Here's a great error that prevents you from writing horrid code:

my $a;
sub {

# ===SORRY!=== Error while compiling /home/zoffix/test.p6
# $a has already been used as a non-placeholder in the surrounding sub block,
#   so you will confuse the reader if you suddenly declare $^a here
# at /home/zoffix/test.p6:4
# ------>         $^a⏏.say;

Here's a bit of a background: you can use the $^ twigil on variables to create an implicit signature. To make it possible to use such variables in nested blocks, this syntax actually creates the same variable without the twigil, so $^a and $a are the same thing, and the signature of the sub above is ($a).

In our code, we also have an $a in outer scope and supposedly we print it first, before using the $^ twigil to create another $a in the same scope, but one that contains the argument to the sub... complete brain-screw! To avoid this, just rename your variables to something that doesn't clash. How about some Thai?

my $ความสงบ = 'peace';
sub {
}('to your variables');

# peace
# to your variables

Well, Colour Me Errpressed!

If your terminal supports it, the compiler will emit ANSI codes to colourize the output a bit:

for ^5 {
    say meow";

That's all nice and spiffy, but if you're, say, capturing output from the compiler to display it elsewhere, you may get the ANSI code as is, like 31m===[0mSORRY![31m===[0m.

That's awful, but luckily, it's easy to disable the colours: just set RAKUDO_ERROR_COLOR environmental variable to 0:

You can set it from within the program too. You just have to do it early enough, so put it somewhere at the start of the program and use the BEGIN phaser to set it as soon as the assignment is compiled:


for ^5 {
    say meow";

An Exceptional Failure

Perl 6 has a special exception—Failure—that doesn't explode until you try to use it as a value, and you can even defuse it entirely by using it in boolean context. You can produce your own Failures by calling the fail subroutine and Perl 6 uses them in core whenever it can.

Here's a piece of code where we define a prefix operator for calculating the circumference of an object, given its radius. If the radius is negative, it calls fail, returning a Failure object:

sub prefix:<⟳> (\𝐫) {
    𝐫 < 0 and fail 'Your object warps the Universe a new one';
    τ × 𝐫;

say 'Calculating the circumference of the mystery object';
my $cₘ = ⟳ −𝑒;

say 'Calculating the circumference of the Earth';
my $cₑ = ⟳ 6.3781 × 10⁶;

say 'Calculating the circumference of the Sun';
my $cₛ = ⟳ 6.957 × 10⁸;

say "The circumference of the largest object is {max $cₘ, $cₑ, $cₛ} metres";

# Calculating the circumference of the mystery object
# Calculating the circumference of the Earth
# Calculating the circumference of the Sun
# Your object warps the Universe a new one
#   in sub prefix:<⟳> at test.p6 line 2
#   in block <unit> at test.p6 line 7
# Actually thrown at:
#   in block <unit> at test.p6 line 15

We're calculating the circumference for a negative radius on line 7, so if it were just a regular exception, our code would die there and then. Instead, by the output we can see that we continue to calculate the circumference of the Earth and the Sun, until we get to the last line.

There we try to use the Failure in $cₘ variable as one of the arguments to the max routine. Since we're asking for the actual value, the Failure explodes and gives us a nice backtrace. The error message includes the point where our Failure blew up (line 15), where we received it (line 7) as well as where it came from (line 2). Pretty sweet!


Useful, descriptive errors are becoming the industry standard and Perl 6 and Rust languages are leading that effort. The errors must go beyond merely telling you the line number to look at. They should point to a piece of code you wrote. They should make a guess at what you meant. They should be referencing your code, even if they originate in some third party library you're using.

Most of Perl 6 errors display the piece of code containing the error. They use algorithms to offer valid suggestions when you mistyped a subroutine name. If you're used to other languages, Perl 6 will detect your "accent," and offer the correct way to pronounce your code in Perl 6. And instead of immediately blowing up, Perl 6 offers a mechanism to propagate errors right to the code the programmer is writing.

Perl 6 has Awesome Errors.

Perl 6 Core Hacking: Where's Da Sauce, Boss?

Read this article on Perl6.Party

Imagine you were playing with Perl 6 and you came across a buglet or you were having some fun with the Perl 6 bug queue—you'd like to debug a particular core subroutine or method, so where's the source for it at?

Asked such a question, you might be told it's in Rakudo compiler's GitHub repository. Depending on how deep down the rabbit hole you wish to go, you may also stop by NQP's repo, which is a subset of Perl 6 that's used in Rakudo, or the MoarVM's repo, which is the leading virtual machine Perl 6 runs on.

The answer is fine, but we can do better. We'd like to know exactly where da sauce is.

Stick to The Basics

The most obvious way is to just use grep command in the source repository. The code is likely in src/ directory, or src/core more specifically.

We'll use a regex that catches sub, method, and multi keywords. For example, here's our search for path sub or method:

$ grep -nER '^\s*(multi|sub|method|multi sub|multi method)\s+path' src/core

src/core/    method path() { self.Stringy.IO }
src/core/CompUnit/Repository/    method path-spec(CompUnit::Repository::Locally:D:) {
src/core/CompUnit/Repository/    method path-spec() {
src/core/CompUnit/Repository/    method path-spec() {
src/core/CompUnit/Repository/    method path-spec() {
src/core/CompUnit/PrecompilationStore/    method path(CompUnit::PrecompilationId $compiler-id,
src/core/CompUnit/    method path(--> IO::Path) { ... }
src/core/IO/Spec/    method path {
src/core/IO/Spec/    method path {
src/core/IO/    method path(IO::Handle:D:)            { $!path.IO }

It's not too terrible, but it's a rather blunt tool. We have these problems:

  • There are false positives; we have several path-spec methods found
  • It doesn't tell us which of the results is for the actual method we have in our code. There's Cool, IO::Spec::Unix, and IO::Handle all with method path in them. If I call "foo".IO.path, which of those get called?

The last one is particularly irksome, but luckily Perl 6 can tell us where the source is from. Let's ask it!

But here's line number... So code me maybe

The Code class from which all subs and methods inherit provides .file and .line methods that tell which file that particular Code is defined in, including the line number:

say "The code is in {.file} on line {.line}" given &foo;

sub foo {
    say 'Hello world!';

# The code is in test.p6 on line 3

That looks nice and simple, but it gets more awkward with methods:

class Kitty {
    method meow {
        say 'Meow world!';

say "The code is in {.file} on line {.line}" given Kitty.^can('meow')[0];

# The code is in test.p6 on line 2

We got extra cruft of the .^can metamodel call, which returns a list of Method objects. Above we use the first one to get the .file and .line number from, but is it really the method we were looking for? Take a look at this example:

class Cuddly {
    method meow ('meow', 'meow') {
        say 'Meow meow meow!';

class Kitty is Cuddly {
    multi method meow ('world') {
        say 'Meow world!';

    multi method meow ('meow') {
        say 'Meow meow';

We have a method meow in one class and in another class we have two multi methods meow. How can we print the location of the last method, the one that takes a single 'meow' as an argument?

First, let's take a gander at all the items .^can returns:

say Kitty.^can('meow');
# (meow meow)

Wait a minute, we have three methods in our code, so how come we only have two meows in the output? Let's print the .file and .line for both meows:

for 0, 1 {
    say "The code is in {.file} on line {.line}"
        given Kitty.^can('meow')[$_];
# The code is in gen/moar/m-CORE.setting on line 587
# The code is in test.p6 on line 2

The second meow gives us a sane result; it's our method defined in class Cuddly. The first one, however, gives us some weird file.

What's happening here is the line is referencing the proto for the multies. Since in this case instead of providing our own proto we use the autogenerated one, the referenced file has nothing to do with our code. We can, of course, add a proto into the code, but then the line number would still reference the proto, not the last meow method. Is there anything that we can do?

You .cando It!

The Routine class, from which both Method and Sub classes inherit, provides the .cando method. Given a Capture, it returns a list of candidates that can handle it, with the narrowest candidate first in the list, and since the returned object is a Code, we can query its specific .file and .line:

class Cuddly {
    method meow ('meow', 'meow') {
        say 'Meow meow meow!';

class Kitty is Cuddly {
    multi method meow ('world') {
        say 'Meow world!';

    multi method meow ('meow') {
        say 'Meow meow';

my $code = gather {
    for Kitty.^can('meow') -> $meth {
        .take for $meth.cando: \(Kitty, 'meow');

say "The code is in {.file} on line {.line}" with $code[0];

# The code is in test.p6 on line 12

Hooray! We got the correct location of the multi we wanted. We still have our two classes with three meow methods total. On line 17–21 we loop over the two meow Methods the .^can metamodel call gives us. For each of them we call the .cando method with the Capture that matches the multi we want (note that we do need to provide the needed object as the first argument of the Capture). We then .take all found candidates to gather them into the $code variable.

The first value we get is the narrowest candidate and is good 'nuf for us, so we call the .file and .line on it, which gives us the location we were looking for. Sounds like we nailed this .file and .line business down rather well. Let's dive into the core, shall we?

Can't see the core files for the setting

If this is the first time you're to see the print out of the .file/.line for some core stuff, you're in for a surprise. Actually, we've already seen the surprise, but you may have thought it to be a fluke:

say "{.file}:{.line}" given &say;
# gen/moar/m-CORE.setting:29038

All of the nice, good looking files you see in src/core in the repo actually get compiled into one giant file called the "setting." My current setting is 40,952 lines long and the .line of core subs and methods refers to one of those thousands of lines.

Now sure, we could pop the setting open and watch our editor grind to a stuttering halt (I'm looking at you, Atom!). However, that doesn't help us find the right repo file to edit if we want to make changes to how it works. So what do we do?

A keen eye will look at the contents of the setting or at the file that generates it and notice that for each of the separate files in the repo, the setting has this type of comment before the contents of the file are inserted into the setting:

#line 1 src/core/

This means if we're clever enough, we can write a sub that translates a line number in the setting to the separate file we can locate in the repo. Here's a plan of action: we pop open the setting file and read it line by line. When we encounter one of the above comments, we make a note of which file we're in as well as how many lines deep in the setting we're currently at.

The location of the setting file may differ, depending on how you installed Perl 6, but on my system (I use rakudobrew), it's in $*EXECUTABLE.parent.parent.parent.child('gen/moar/m-CORE.setting'), so the code for finding the actual file that defines our core sub or method is this:

sub real-location-for ($wanted) {
    state $setting = $*EXECUTABLE.parent.parent.parent.child: 'gen/moar/m-CORE.setting';
    my ($cur-line-num, $offset) = 0, 0;
    my $file;
    for $setting.IO.lines -> $line {
        return %( :$file, :line($cur-line-num - $offset), )
            if ++$cur-line-num == $wanted;

        if $line ~~ /^ '#line 1 ' $<file>=\S+/ {
            $file   = $<file>;
            $offset = $cur-line-num + 1;
    fail 'Were not able to find location in setting.';

say "{.<file>}:{.<line>}" given real-location-for &say.line;

# src/core/

The $wanted contains the setting line number given to us by .line call and the $cur-line-num contains the number of the current line we're examining. We loop until the $cur-line-num reaches $wanted and return a Hash with the results. For each line that matches our special comment, we store the real name of the file the code is from into $file and store the $offset of the first line of the code in that file. Once done, we simply subtract the $offset from the setting $cur-line-num and we get the line number in the source file.

This is pretty awesome and useful, but it's still not what I had in mind when I said we wanted to know exactly where da sauce is. I don't want to clone the repo and go to the repo and open my editor. I want to just look at code.

If it's worth doing, it's worth overdoing

There's one place where we can stare at Rakudo's source code until it blushes and looks away: GitHub. Since our handy sub gives us a filename and a line number, we can construct a URL that points to a specific file and line in the source code, like this one, for example:

There's an obvious problem with such an approach: the URL points to the master branch (called nom, for "New Object Model," in Rakudo). Commits go into the repo daily, and unless we rebuild our Perl 6 several times a day, there's a good chance the location our GitHub URL points to is wrong.

Not only do we have to point to a specific file and line number, we have to point to the right commit too. On GitHub's end, it's easy: we just replace nom in the URL with the appropriate commit number—we just need Rakudo to tell us what that number is.

The two dynamic variables $*VM and $*PERL contain some juicy information. By introspecting them, we can locate some useful info and what looks like commit prefix parts in version numbers:

say $*VM.^methods;
# (BUILD platform-library-name Str gist config prefix precomp-ext
# precomp-target precomp-dir name auth version signature desc)

say $*VM.version;
# v2016.06

say $*PERL.^methods;
# (BUILD VMnames DISTROnames KERNELnames Str gist compiler name auth version
# signature desc)

say $*PERL.compiler.^methods;
# (BUILD build-date Str gist id release codename name auth version
# signature desc)

say $*PERL.compiler.version;
# v2016.06.10.g.7.cff.429

Rakudo is a compiler and so we're interested in the value of $*PERL.compiler.version. It contains the major release version, followed by g, followed by the commit prefix of this particular build. The prefix is split up on number-letter boundaries, so we'll need to join up all the bits and split on g. But, take a look at $*VM.version, which is the version of the virtual machine we're running the code on. There aren't any gs and commits in it and for a good reason: it's a tagged major release, and the name of the tag is the version. The same will occur for Rakudo on release builds, like the ones shipped with Rakudo Star. So we'll need to check for such edge cases and this is the code:

my $where = .Str ~~ /g/
    ?? .parts.join.split("g")[*-1]
    !! .Str
given $*PERL.compiler.version;

given a $*PERL .compiler .version, if it contains letter g, join up version bits, split on g, and the last portion will be our commit prefix; if it doesn't contain letter g, then we're dealing with a release tag, so we'll take it as-is. All said and done, our code for locating source becomes this:

my $where = .Str ~~ /g/
    ?? .parts.join.split("g")[*-1]
    !! .Str
given $*PERL.compiler.version;

say [~] '',
        $where, '/', .<file>, '#L', .<line>
given real-location-for &say.line;


Hey! Awesome! We got a link that points to the correct commit and file! Let celebrations begin! Wait. What? You followed the link and noticed the line number is not quite right? What gives? Did we mess up our algorithm?

Crank Up The Insanity

If you take a look again at the script that generates the setting file, you'll notice it strips things: comments and special backend-specific chunks of code.

There are two ways to fix this. The sane approach would be to commit a change that would make that script insert an empty line for each line it skips and then pretend that we didn't commit that just to make our personal project work. Then, there's the Zoffix Way to fix this: we got the GitHub link, so why don't we fetch that code and figure out what the right line number is. Hey! That second way sounds much more fun! Let's do just that!

The one link we've seen so far is this: It's not quite what we want, since it's got HTML and bells and whistles in it. We want raw code and GitHub does offer that at a slightly different URL: The plan of action then becomes:

  • Get the line number in the setting
  • Use our real-location-for sub to get the filename and sorta-right line number in a source file
  • Get the commit our compiler was built with
  • Generate a GitHub URL for raw code for that file on that commit and fetch that code
  • Use the same algorithm as in the setting generating script to convert the code we fetched into the version that lives in our setting, while keeping track of the number of lines we strip
  • When we reach the correct line number in the converted file, we adjust the original line number we had by the number of lines we stripped
  • Generate a regular GitHub URL to the commit, file, and corrected line number
  • ???
  • Profit!

I could go over the code, but it's just a dumb, unfun algorithm, and most importantly, you don't need to know it. Because... there's a module that does just that!

What Sorcery Is This?

The module is called CoreHackers::Sourcery and when you use it, it'll augment the Code class and all core classes that inherit from it with .sourcery method, as well as provide a sourcery subroutine.

So, to get the location of the code for say sub, just run:

use CoreHackers::Sourcery;

# src/core/

That gives us the correct location of the proto. We can either pop open a file in a repo checkout or view the code at the provided GitHub URL.

Want to get the location of a specific multi? There's no need to mess with .cando! The arguments you give to the .sourcery method will be used to select the best matching multi, so to find the location of the say multi that will handle say "foo" call, just run:


# src/core/

That covers the subs. For methods, you can go with the whole .^can meta dance, but we like simple things, and so we'll use the subroutine form of sourcery:

put sourcery Int, 'abs';         # method of a type object
put sourcery 42,  'split';       # method of an Int object
put sourcery 42,  'base', \(16); # best candidate for `base` method called with 16 as arg

This is pretty handy. And the whole hitting the GitHub thing? The module will cache the code fetched from GitHub, so things like this won't take forever:

put "Int.{.name} is at {.sourcery}" for Int.^methods;

However, if you do actually run that code, after some output you'll be greeted with this error:

# Method 'sourcery' not found for invocant of class 'Method+{Callable[Bool:D]}'
#   in block  at test.p6 line 1
#   in block <unit> at test.p6 line 1

The class it mentions is not a pure Method object, but has a mixin in it. While CoreHackers::Sourcery recomposes all core subclasses of Code class after augmenting it, it doesn't do that for such mixes, so you'd have to recompose them yourself:

for Int.^methods {
    put "Int.{.name} is at {.sourcery}" ;

Or better still, just use the subroutine form of sourcery:

put "Int.{.name} is at {sourcery $_}" for Int.^methods;

Do It For Me

For most stuff, we wouldn't want to do a whole bunch of typing to use a module and call subs and then copy/paste URLs or filenames. You'll notice sourcery returns a list of two items: the filename and the URL. This means we can make some nice and short aliases to call it and automatically pop open either our editor or web browser:

$ alias sourcery='perl6 -MCoreHackers::Sourcery -MMONKEY-SEE-NO-EVAL \
    -e '\''run "atom", "/home/zoffix/rakudo/" \
        ~ EVAL "sourcery(@*ARGS[0])[0]" '\'''

$ alias sourcery-web='perl6 -MCoreHackers::Sourcery -MMONKEY-SEE-NO-EVAL \
    -e '\''run "firefox", EVAL "sourcery(@*ARGS[0])[1]" '\'''

# opens Atom editor at the spot to edit code for Int.base
$  sourcery 'Int, "base"'

# opens Firefox, showing code for Int.base
$  sourcery 'Int, "base"'

We EVAL the argument we give to these aliases, so be careful with them. For sourcery alias, we run the Atom editor and give it the file to open. I prepended the location of my local Rakudo checkout, but you'd use yours. Most editors support opening file:line-number format to open files at a particular spot; if yours doesn't, modify the command.

For sourcery-web we use the URL returned by sourcery and open Firefox browser at this location. And just like that, with a few keystrokes, we can jump in to view or edit the code for a particular core sub or method in Rakudo!


We've learned where Rakudo's source lives, how to find the commit the current compiler is built off, and how to locate the source code for a particular sub or method in a giant file called the setting. We then further hacked away the inconveniences by getting to the actual place in the source code we can edit, culminating with a shiny module and a couple of handy command line aliases.

Happy hacking!

UPDATE 2016.08.05

Inspired by this blog post, lizmat++ has changed the setting generation script to not skip any lines, so making adjustments to line numbers by fetching source from GitHub is no longer necessary, as the line numbers match up with the original source.

Hacking on The Rakudo Perl 6 Compiler: Mix Your Fix

Read this article on Perl6.Party

While testing a fix for one of the Less Than Awesome behaviours in standalone Signature objects, I came across a bugglet. Smartmatching two Signatures throws, while spilling a bit of the guts:

<Zoffix> m: my $m = method ($a: $b) { }; say $m.signature ~~ :($a, $b);
<camelia> rakudo-moar 46838d: OUTPUT«Method 'type' not found for invocant of class 'Any'␤ in block at line 1␤␤»

So I figured I'll write about fixing it, 'cause hacking on internals is lots of fun. Let's roll!

Golf It Down

The less code there is to reproduces the bug, the fewer places there are for that bug to hide. We have a detached method and then we smartmatch its signature against something else. Let's try to golf it down a bit and smartmatch two Signatures, without involving a method:

<Zoffix> m: :($a, $b) ~~ :($a, $b);
<camelia> rakudo-moar 46838d: ( no output )

The bug disappeared, so perhaps out Signature on the left doesn't contain the stuff that triggers the bug. Let's dump the signature of the method to see what we should match against:

<Zoffix> m: my $m = method ($a: $b) { }; say $m.signature <camelia> rakudo-moar 46838d: OUTPUT«($a: $b, *%_)␤»

Aha! It has a slurpy hash: *%_. Let's try matching a Signature with a slurpy in it:

<Zoffix> m: :(*%) ~~ :();
<camelia> rakudo-moar 46838d: OUTPUT«Method 'type' not found for invocant of class 'Any'␤ in block at line 1␤␤»

And there we go: hole in three. Let's proceed.

Roast It

There's an official Perl 6 test suite that Rakudo must pass to be called a Perl 6 compiler. Since we got a bug on our hands, we should add a test for it to the test suite to ensure it doesn't rear its ugly head again.

The copy of the repo gets automatically cloned into t/spec when you run make spectest in Rakudo's checkout. If you don't have a commit bit, you can just change the remote/branch of that checkout to your fork:

cd t/spec
git remote rm origin
git remote add origin
git checkout your-branch
cd ../..

It may be tricky to figure out which file to put the test in, if you're new. You can always ask the good folks on for advice. In this case, I'll place the test into S06-signature/outside-subroutine.t

While not required, I find it helpful to open a ticket for the bug. This way I can reference it in my fix in the compiler repo, I can reference it in the commit to the test repo, and people get a place where to tell me why I'm being stupid when I am. I opened this bug as RT#128795.

Now, for the code of the test itself. I'll adjust the plan at the top of the file to include however many tests I'm writing—in this case one. I'll use the lives-ok test sub and stick our buggy golfed code into it. Here's the diff of the changes to the file; note the reference to the ticket number in the comment before the test:

@@ -1,7 +1,7 @@
  use v6;
  use Test;

 -plan 3;
 +plan 4;

  # RT #82946
  subtest 'signature binding outside of routine calls' => {
 @@ -25,4 +25,7 @@ subtest 'smartmatch on signatures with literal strings' => {
  # RT #128783
  lives-ok { EVAL ’:($:)‘ }, ’signature marker is allowed in bare signature‘;

 +# RT #128795
 +lives-ok { :(*%)~~ :() }, 'smartmatch with no slurpy on right side';
  # vim: ft=perl6

Run the file now to ensure the test fails. Hint: some files have fudging; explaining it is out of the scope of this article, but if you notice failures you're not expecting, look it up.

$ make t/spec/S06-signature/outside-subroutine.t
Test Summary Report
t/spec/S06-signature/outside-subroutine.t (Wstat: 256 Tests: 4 Failed: 1)
  Failed test:  4
  Non-zero exit status: 1

With the test in place, it's time to look at some source code. Let the bug hunt begin!

Make it Saucy

Our bug involves a Smartmatch operator, which aliases the left side to the topic variable $_ and calls .ACCEPTS method on the right side with it. Both of our sides are Signature objects, so let's pop open Rakudo's sauce code for that class.

In the Rakudo's repo, directory src/core/ contains most of the built in types in separate files named after those types, so we'll just pop open src/core/ in the editor and locate the definition of method ACCEPTS.

There are actually four multis for ACCEPTS. Here's the full code. Don't try to understand all of it, just note its size.

``` multi method ACCEPTS(Signature:D: Capture $topic) { nqp::p6bool(nqp::p6isbindable(self, nqp::decont($topic))); }

multi method ACCEPTS(Signature:D: @topic) {

multi method ACCEPTS(Signature:D: %topic) {

multi method ACCEPTS(Signature:D: Signature:D $topic) {
    my $sclass = self.params.classify({.named});
    my $tclass = $topic.params.classify({.named});
    my @spos := $sclass{False} // ();
    my @tpos := $tclass{False} // ();

    while @spos {
        my $s;
        my $t;
        last unless @tpos && ($t = @tpos.shift);
        if $s.slurpy or $s.capture {
        if $t.slurpy or $t.capture {
            return False unless any(@spos) ~~ {.slurpy or .capture};
        if not $s.optional {
            return False if $t.optional
        return False unless $t ~~ $s;
    return False if @tpos;
    if @spos {
        return False unless @spos[0].optional or @spos[0].slurpy or @spos[0].capture;

    for flat ($sclass{True} // ()).grep({!.optional and !.slurpy}) -> $this {
        my $other;
        return False unless $other=($tclass{True} // ()).grep(
            {!.optional and $_ ~~ $this });
        return False unless +$other == 1;

    my $here=($sclass{True}:v).SetHash;
    my $hasslurpy=($sclass{True} // ()).grep({.slurpy});
    $here{@$hasslurpy} :delete;
    $hasslurpy .= Bool;
    for flat @($tclass{True} // ()) -> $other {
        my $this;

        if $other.slurpy {
            return False if any($here.keys) ~~ -> Any $_ { !(.type =:= Mu) };
            return $hasslurpy;
        if $this=$here.keys.grep( -> $t { $other ~~ $t }) {
            $here{$this[0]} :delete;
        else {
            return False unless $hasslurpy;
    return False unless self.returns =:= $topic.returns;


The error we get from the bug mentions .type method call and there is one such method call in the code above (close to the end of it). In this case, there's quite a bit of code to sort through. It would be nice to be able to play around with it, stick a couple of dd or say calls to dump out variables, right?

That approach, however, is somewhat annoying because after each change we have to recompile the entire Rakudo. On the meatiest box I got, it takes about 60 seconds. Not the end of the world, but there's a way to make things lightning fast!

Mix Your Fix

We need to fix a bug in a method of a class. Another way to think of it is: we need to replace a broken method with a working one. Signature class is just like any other class, so if we want to replace one of its methods, we can just mix in a role!

The broken ACCEPTS will continue to live in the compiler, and we'll pop open a separate playground file and define a role—let's calls it FixedSignature—in it. To get our new-and-improved ACCEPTS method in standalone signature objects, we'll use the but operator to mix the FixedSignature in.

Here's the role, the mixing in, and the code that triggers the bug. I'll leave out method bodies for brieviety, but there's they are the same as in the code above.

role FixedSignature {
    multi method ACCEPTS(Signature:D: Capture $topic)     { #`(redacted for brevity) }
    multi method ACCEPTS(Signature:D: @topic)             { #`(redacted for brevity) }
    multi method ACCEPTS(Signature:D: %topic)             { #`(redacted for brevity) }
    multi method ACCEPTS(Signature:D: Signature:D $topic) { #`(redacted for brevity) }

my $a = :(*%) but FixedSignature;
my $b = :()   but FixedSignature;

say $a ~~ $b;

There are two more things we need to do for our role to work properly. First, we're dealing with multis and right now the multis in our role are creating ambiguities with the multis in the original Signature class. To avoid that, we'll define a proto:

proto method ACCEPTS (|) { * }

Since the code is using some NQP, we also need to bring in those features into our playground file with the role. Just add the appropriate pragma at the top of the file:


With these modifications, our final test file becomes the following:


role FixedSignature {
    proto method ACCEPTS (|) { * }

    multi method ACCEPTS(Signature:D: Capture $topic)     { #`(redacted for brevity) }
    multi method ACCEPTS(Signature:D: @topic)             { #`(redacted for brevity) }
    multi method ACCEPTS(Signature:D: %topic)             { #`(redacted for brevity) }
    multi method ACCEPTS(Signature:D: Signature:D $topic) { #`(redacted for brevity) }

my $a = :(*%) but FixedSignature;
my $b = :()   but FixedSignature;

say $a ~~ $b;

And with this trick in place, we now have a rapid-fire weapon to hunt down the bug with—the changes we make compile instantly.

Pull The Trigger

Now, we can debug the code just like any other. I prefer applying liberal amounts of dd (or say) calls and dumping out the variables to ensure their contents match expectations.

The .type method call our error message mentions is in this line:

return False if any($here.keys) ~~ -> Any $_ { !(.type =:= Mu) };

It calls it on the keys of $here, so let's dump the $here before that statement:

dd $here
return False if any($here.keys) ~~ -> Any $_ { !(.type =:= Mu) };
# SetHash $here =

Here's our offending Any, let's go up a bit and dump the $here right where it's defined:

my $here=$sclass{True}.SetHash;
dd $here;
# SetHash $here =

It's still there, and for a good reason. If we trace the creation of $sclass, we'll see it's this:

my $sclass = self.params.classify({.named});

The params of the Signature on the right of the smartmatch get classified based on whether they are named or not. The named parameters will be inside a list under the True key of $sclass. Since we do not have any named params, there won't be such a key, and we can verify that with this bit of code:

# {}

When we go to define $here, we get an Any from $sclass{True}, since that key doesn't exist, and when we call .SetHash on it, we get our problematic Sethash object with an Any in it. And so, we have our fix for the bug: ensure the True key in $sclass is actually there before creating a SetHash out of its value:

my $here=($sclass{True}:v).SetHash;

Add that to our playground file with the FixedSignature role in it, run it, and verify the fix works. Now, simply transplant the fix back into src/core/ and then compile the compiler.

perl --gen-moar --gen-nqp --backends=moar
make test
make install

Verify our fix worked before we proceed onto the final stages:

$ make t/spec/S06-signature/outside-subroutine.t
All tests successful.
Files=1, Tests=4,  1 wallclock secs ( 0.03 usr  0.00 sys +  0.32 cusr  0.02 csys =  0.37 CPU)
Result: PASS

A Clean Kill

So far, all we know is the bug we found was fixed and the tests we wrote for it pass. However, before we ship our fix, we must ensure we didn't break anything else. There are other devs working from the same repo and you'll be interfering with their work if you break stuff.

Run the full Roast test suite with make spectest command. You can use the TEST_JOBS environmental variable to specify the number of simultaneous tests. Generally a value slightly higher than the available cores works the fastest... and cores make all the difference. On my 24-core VM I cut releases on, the spectest completes in about 1 minute and 15 seconds. On my 2-core web server, it takes about 25 minutes. You get the idea.

TEST_JOBS=28 make spectest
All tests successful.
Files=1111, Tests=52510, 82 wallclock secs (13.09 usr 2.44 sys + 1517.34 cusr 97.67 csys = 1630.54 CPU)
Result: PASS

Once the spectest completes and we have the clean bill of health, we're ready to ship our fix. Commit the Rakudo fix, then go into t/spec and commit the Roast fix:

git commit -m 'Fix Smartmatch with two signatures, only one of which has slurpy hash' \
           -m 'Fixes RT#128795' src/core/
git push

cd t/spec
git commit -m 'smartmatch on signature with no slurpy on right side does not crash' \
           -m 'RT#128795' S06-signature/outside-subroutine.t
git push

If you're pushing to your fork of these projects, you have to go the extra step and submit a Pull Request (just go to your fork and GitHub should display a button just for that).

And we're done! Celebrate with the appropriate amount of fun.


Rakudo bugs can be easy to fix, requiring not much more than knowledge of Perl 6. To fix them, you don't need to re-compile the entire compiler, but can instead define a small role with a method you're trying to fix and modify and recompile just that.

It's important to add tests for the bug into the official test suite and it's also important to run the full spectest after you fix the bug. But most important of all, is to have fun fixing it.