Day 19 – Fixing Flow

Finding flow while coding is one of the joys of programming.

Encountering simple syntactic bugs, however, can sometimes interrupt flow. A single missing semicolon, for example, can result in a “WAT!?” followed by a “DOH!”

Perl 6 helps you around the code -> run -> fix cycle by identifying the cause and location of a bug and often suggesting a solution. Take this program:

say "hello"
say "world";

When you run it, Perl 6 will suggest what’s wrong, where the problem is, and what to do next …

===SORRY!=== Error while compiling /home/nige/
Two terms in a row across lines (missing semicolon or comma?)
at /home/nige/
------> say "hello"⏏

That helps to keep things flowing.

Normally, at this point, it’s off to your $EDITOR to manually add the semicolon and the cycle repeats.

What if Perl 6 could suggest a fix and apply it for you?

Introducing perl6fix

Here is the beginnings of a command-line utility called, perl6fix, that takes the hint from Perl 6 and tries to speed up the code -> run -> fix cycle by applying the fix for you.


Let’s look at the code.

It needs a handle on bug descriptions found in Perl 6 output.

class Bug {
   has Int        $.line-number;
   has SourceFile $.source-file;
   has Str        $.description;
   has Str        $.pre-context;

And a way to describe fixes:

class Fix {

    has $.prompt;
    has $.pattern;
    has $.action;

    method apply ($bug) {
    method applies-to ($bug) {
        return True without $.pattern;
        return $bug.description ~~ $.pattern;

And a way to update the source file:

class SourceFile is IO::Path {

    has @!content-lines = self.IO.slurp.lines;

    method append-to-first-matching-line ($append-char, $text-to-match) {
        return unless my $first-matching-index = @!content-lines.first(rx/$text-to-match/, :k);
        @!content-lines[$first-matching-index] ~= $append-char;;
    method swap-characters-on-line ($from-chars, $to-chars, $line-number) {
        @!content-lines[$line-number - 1].subst-mutate(/$from-chars/, $to-chars);;
    method save {

Here is just some of the fixes I encountered while writing this program:

my @fixes = (
        prompt  => 'add semicolon',
        pattern => rx/'missing semicolon or comma?'/,
        action  => sub ($bug) {
            $bug.source-file.append-to-first-matching-line(';', $bug.pre-context);
         prompt  => 'add comma',
         pattern => rx/'missing semicolon or comma?'/,
         action  => sub ($bug) {
             $bug.source-file.append-to-first-matching-line(',', $bug.pre-context);
         prompt  => 'convert . to ~',
         pattern => rx/'Unsupported use of . to concatenate strings; in Perl 6 please use ~'/,
         action  => sub ($bug) {
              $bug.source-file.swap-characters-on-line('.', '~', $bug.line-number);
         prompt  => 'convert qr to rx',
         pattern => rx/'Unsupported use of qr for regex quoting; in Perl 6 please use rx'/,
         action  => sub ($bug) {
              $bug.source-file.swap-characters-on-line('qr', 'rx', $bug.line-number);

There’s many more potential fixes (I’m just starting).

The perl6fix script wraps perl6, captures STDERR (if there is any), and then looks for a bug report in the output:

sub find-bug ($perl6-command) {

    return unless my $error-output = capture-stderr($perl6-command);

    # show the error

    # re-run the command again - this time grabbing a JSON version of the bug
    return unless my $error-as-json = capture-stderr('RAKUDO_EXCEPTIONS_HANDLER=JSON ' ~ $perl6-command);
    return unless my $bug-description = from-json($error-as-json);

    # just handle these exception types to start with 
    for 'X::Syntax::Confused', 'X::Obsolete' -> $bug-type {
        next unless my $bug = $bug-description{$bug-type}; 
            description => $bug<message>,
            source-file =>$bug<filename>),
            line-number => $bug<line>,
            pre-context => $bug<pre>

The next step is to see if there are any fixes for this type of bug:

sub fix-bugs ($perl6-command-line) {

    my $bug = find-bug($perl6-command-line);
    unless $bug {
        say 'No bugs found to fix.'; 

    # find a potential list of fixes for this type of bug
    my @found-fixes = @fixes.grep(*.applies-to($bug));

    say $bug.description;
    say $bug.source-file.path ~ ' ' ~ $bug.line-number;
    say 'Suggested fixes found: ';

    my $fix-count = 0;

    for @found-fixes -> $fix {
        my $option = ($fix-count == 1)
                   ?? "*[1] " ~ $fix.prompt
                   !! " [$fix-count] " ~ $fix.prompt;
        say ' ' ~ $option;

     my $answer = prompt('apply fix [1]? ') || 1;
     my $fix = @found-fixes[$answer - 1];


     # look for more bugs! until we're done

With the help of a shell alias you can even run it to fix the previous perl6 command. Like so:


Just add an alias to your bash or zsh profile. For example:

alias fix='/home/nige/perl6/perl6fix prev $(fc -ln -1)'

Now it’s your turn

As you can see this is just a start.

You’re welcome to take the full script and evolve your own set of automatic Perl 6 fixes.

You could even adapt the script to apply fixes for Perl 5 or other languages? What about a version using grammars? Or a macro-powered version integrated directly into Perl 6?

Well maybe that last one is something to look forward to next Christmas!

Hope you have a happy Christmas and a flowing, Perl 6 powered 2017!

Perl – have an appropriate amount of fun!

A decade ago Audrey Tang started the Pugs project to implement Perl 6 in Haskell and declared that Perl 6 was optimized for fun: -Ofun.

Game designers know what -Ofun looks like – it’s when players find themselves engrossed in their game, time flies and they experience flow. The art of great game design is balancing the player’s increasing ability with the challenge of the game. Sometimes it’s a bumpy ride!


Flow can happen when programming too: when you’re free from distraction, and the challenge you’re facing is not too easy, not too hard and you’re making positive progress.

Audrey’s invitation to new Perl 6 contributors to have ‘the appropriate amount of fun’ encouraged them to step forward and pick their place on this challenge/ability curve.

The challenge of designing and implementing Perl 6 has presented some scary technical dragons! Fortunately some of the most able programmers stepped forward to hack them down to size.

Over the past decade, it has been a joy to regularly read the #perl6 IRC logs and watch this process unfold. Despite the naysayers, #perl6 has remained a positive, productive place – the propensity for -Ofun is part of the DNA of Perl 6.


Camelia, the Perl 6 butterfly logo, helps sum this up. Camelia includes a reference to a ‘camel’ in her name, however, unlike a camel she doesn’t smell or spit. Flow requires a positive mindset. Camelia is a licence to have -Ofun – not the frivolous, Christmas cracker type, but the deeply productive, enjoyable and enduring type.

The Christmas release is a turning point in the whirlpool of Perl 6 development. The -Ofun will now freely flow outwards to new programmers.

Perl 6, with its emphasis on whipupitude and expressivity, makes it ideally suited to finding flow while programming.

If you haven’t unwrapped Perl 6 yet, this Christmas is the perfect time. You can download Perl 6 here and remember to have an appropriate amount of fun!

Day 20 – Helping Santa with Roles

On Day 14 of the Advent calendar I stopped lurking and started learning Perl 6 by converting a Perl 5 Timer object into its Perl 6 equivalent. Day 14 ends with a ‘challenge’ to convert the Perl 6 Timer class into a role.

Perl 6 roles encapsulate functionality for ready reuse by classes and objects.

The Timer class is a good candidate for ‘role-ification’ – it’s simple and is potentially useful to all sorts of objects.

Santa wants to go Lean this year, don’t worry he’s still rotund – he just wants to apply Lean principles to the process of present giving. The Timer role will help identify bottlenecks so Santa Inc. can improve.

To turn the Timer class into a role we just swap class for role (i.e., s/class/role/):

role Timer {

    has $!start-time;   # private scalar storing the starting time
    has @!event-log;    # private list of events
                        # dashes-in-identifiers save keystrokes too

    method record ($event_description) {
        # initialise the start-time if this is the first time through
        $!start-time //= now;

            timestamp   => now - $!start-time, 
            description => $event_description

    method report {
        my @report_lines;

        for @!event-log -> %event {
            @report_lines.push("[%event<timestamp>.fmt("%.3f")] %event<description>");
        return @report_lines.join("\n");

So that’s the role declared. Now I need a class or object to compose it into – something that does the role. How about a Timer for Santa’s Helpers?

use Timer;

class SantasHelper does Timer {
    has @!presents = qw<train lego doll> ;

    method wrap-present($child's_name) {
        return @!presents.pick ~ " for " ~ $child's_name;            

A quick check in the REPL shows:

> use SantasHelper;
> my $bruce =;    # this helper has an Australian accent
> $bruce.^methods
record report wrap-present

Pointing to the Higher Order Workings (HOW) of the $bruce object via .^ shows the Timer role’s methods (record, report) are now combined with the methods for SantasHelper (wrap-present). Let’s start wrapping presents:

use SantasHelper;

my $helper =;

my @children = qw<Hugo Honey Willow>;

# using {@children} to interpolate the list
$helper.record("started wrapping presents for {@children}"); 

for @children -> $child {
    my $present = $helper.wrap-present($child);
    $helper.record("wrapped $present");
say $; 

The report shows:

[0.006] started wrapping presents for Hugo Honey Willow
[0.013] wrapped train for Hugo
[0.016] wrapped doll for Honey
[0.020] wrapped doll for Willow


But now there’s a problem! Santa’s Lean approach has exposed a bottleneck. To wrap all the presents in time Santa will need to roll up his sleeves and start wrapping too. We need to refactor the wrap-present method into its own role so Santa can wrap-presents too:

role WrappingPresents {
    has @!presents = qw<doll train lego> ;

    method wrap-present($child's_name) {
        return @!presents.pick ~ " for " ~ $child's_name;            

It turns out we can dynamically add a role to an existing object with does:

> use Santa
> my $santa =
> use WrappingPresents
> $santa does WrappingPresents
Santa+{WrappingPresents}.new(name => Any)
> $santa.wrap-present('Emma')
lego for Emma
> $santa.wrap-present('Claire')
lego for Claire

Santa Inc is now back on track.

It’s been fun playing with Perl 6 roles. Coding in Perl 6 feels malleable and expressive – and I’ve only just started!

jnthn++ has more details about parameterised roles and what happens when you compose multiple roles with the same method signature.

For now I’ll leave the last word to Perl 6’s Santa:

> $santa.greeting();
> Happy Christmas!

Day 14 – A Perl 6 Timer: Ready, Set, Go!

Christmas is coming!

With Perl 6 promising to be ready for production next year – now is the time to stop lurking and start learning. It’s exciting! But what to try first?

OK – so I’ve downloaded Rakudo and tinkered with the REPL. Time to try translating a simple module that I often use in Perl 5 projects. It’s a simple timer that records events and renders a timed report:

my $timer = Timer->new;
$timer->record('first event');
$timer->record('second event');
say $timer->report;

Here’s the output:

[0.000] first event
[0.007] second event

It helps trace code and identify bottlenecks. Here’s a Perl 5.20 version:

package Timer;

use Time::HiRes qw(gettimeofday tv_interval);
use Mojo::Base -base;
use experimental 'signatures', 'postderef';

has '_event_log'  => sub { [] };
has '_start_time' => sub { [ gettimeofday ] };

sub record ($self, $event_description) {
    # record the event and timestamp
    my $event = {
        timestamp   => sprintf("%.3f", tv_interval($self->_start_time)),
        description => $event_description,
    push($self->_event_log->@*, $event);

sub report ($self) {
    # render a full report
    my $report = '';
    foreach my $event ($self->_event_log->@*) {
         $report .= '[' . $event->{timestamp} . '] ' . $event->{description} . "\n";
    return $report;

Now how would this look in Perl 6?

Perl 6 does full flavour, duck quacking OO so we can replace Perl 5’s package and declare a Timer class:

class Timer {

No need for that spurious 1; at the end of the file any more either. We need to store an initial start time and a list of event records. In Perl 6, object attributes are defined with the help of ‘twigils’ – that is, secondary sigils …

class Timer {
    has $!start-time;  # private scalar storing the starting time
    has @!event-log;   # private list of events 
                       # dashes-in-identifiers save keystrokes too

The exclamation mark here in $!start-time is a secondary sigil and declares the attribute as private. The @!event-log list is also marked as private.

You can imagine the ! character is like a portcullis dropping down at the entrance of a castle – blocking external access to the attribute behind. The . twigil is used for public attributes (e.g., $.this-attribute-is-public-no-portcullis-here).

The twigil tells you at first glance what an attribute’s contract is with the class. No more context-switching and scrolling back to the top of the file to find out. Thanks Larry for saving some brain-space!

So the attributes are defined, what about the methods, record() and report()?

The record() method starts the internal timer the first time it’s called so it needs an equivalent to Perl 5’s Time::HiRes. A quick Google search brings up an example on Like the ancient Rosetta Stone this site is useful for translating between languages and is a hangout for early adopting polyglot programmers. Perl 5 and Perl 6 are well represented there so it’s a great place for converting Perl 5-isms into Perl 6. It seems now is the new gettimeofday!

I had a little chat with the Perl6 REPL about this:

> now # returns an Instant object
> now.HOW # access the Higher Order Workings
> now.^methods # use ^ to access the HOW
new from-posix to-posix Bridge Num Int narrow Rat abs sign conj sqrt rand sin asin cos acos tan atan atan2 sec asec cosec acosec cotan acotan sinh asinh cosh acosh tanh atanh sech asech cosech acosech cotanh acotanh floor ceiling round unpolar cis Complex log exp truncate isNaN base Real log10 roots succ pred sleep Str perl Numeric ACCEPTS Bool gist DUMP <anon>
> now.perl # Data::Dumper in Perl 6 => <1127461994944/795>)
> now.gist # Human readable explanation
> my $time = now
> now - $time # can we get the difference in time?

So this will do the trick! The record() method works like a stopwatch snapshotting the difference since the start:

method record ($event_description) {
    # initialise the start-time if this is the first time through
    $!start-time //= now;

        timestamp   => now - $!start-time, # take the difference
        description => $event_description

The report() method pretty prints the @!event-log:

method report {
    my @report_lines;
    for @!event-log -> %event {
         @report_lines.push("[%event<timestamp>.fmt("%.3f")] %event<description>");
    return @report_lines.join("\n");

Metaphorically speaking the for loop unrolls the @!event-log carpet (@) shooting values (->) into individual hash entries (%event)!

Sigils are invariant in Perl 6 so a %hash means %hash and the sigil doesn’t change while accessing a hash value: %hash<key>. Interpolation into a string just works.The full Perl 6 version looks like:

class Timer {
    has $!start-time;
    has @!event-log;

    method record ($event_description) {
        # initialise the start-time if this is the first time through
        $!start-time //= now;
            timestamp   => now - $!start-time,
            description => $event_description
    method report {
        my @report_lines;
        for @!event-log -> %event {
             @report_lines.push("[%event<timestamp>.fmt("%.3f")] %event<description>");
        return @report_lines.join("\n");

I’m happy with that!

Now … what can I learn next?

How about converting the class into a Perl 6 role instead?

There’s a learning challenge for you too. Can you turn it into a Perl 6 role?

Let me start the timer … Ready, Set, Go!