Day 1 – The Grinch of Perl 6: A Practical Guide to Ruining Christmas

Look at them! All smiling and happy. Coworkers, friends, and close family members. All enjoying programming in Perl 6 version 6.c "Christmas". Great concurrency primitives, core grammars, and a fantastic object model. It sickens me!

But wait a second… wait just a second. I got an idea. An awful idea. I got a wonderful, awful idea! We can ruin their "Christmas". All we need is a few tricks up our sleeves. Muahuahahaha!!

Welcome to the 2017th Perl 6 Advent Calendar! Each day, from today until Christmas, we'll have an awesome blog post about Perl 6 lined up for you.

Today, we'll show our naughty side and purposefully do naughty things. Sure, these have good uses, but being naughty is a lot more fun. Let's begin!

But True does False

Have you heard of the but operator? A fun little thing:

say True but False ?? 'Tis true' !! 'Tis false';
# OUTPUT: «Tis false␤»
my $n = 42 but 'forty two';
say $n; # OUTPUT: «forty two␤»
say $n + 7; # OUTPUT: «49␤»

It's an infix operator that first clones the object on the left hand side and then mixes in a role provided on the right hand side into the clone:

my $n = 42 but role Evener {
method is-even { self %% 2 }
say $; # OUTPUT: «True␤»
say $n.^name; # OUTPUT: «Int+{Evener}␤»

Those aren't roles in the first two examples above. The but operator has a handy shortcut: if the thing on the right isn't a role, it creates one for you! The role will have a single method, named after the .^name of the object on the right hand side, and the method will simply return the given object. Thus, this…

put True but 'some boolean'; # OUTPUT: «some boolean␤»

…is equivalent to:

put True but role {
method ::(BEGIN 'some boolean'.^name) {
'some boolean'
} # OUTPUT: «some boolean␤»

The .^name on our string returns Str, since it's a Str object:

say 'some boolean'.^name; # OUTPUT: «Str␤»

And so the role provides a method named Str, which put calls on non-Str objects to obtain a stringy value to output, causing our boolean to have an altered stringy representation.

As an example, string '0' is True in Rakudo Perl 6 but is False in Pumpkin Perl 5. Using the but operator, we can alter a string to behave like Perl 5's version:

role Perl5Str {
method Bool {
nextsame unless self eq '0';
sub perlify { $^v but Perl5Str };
say so perlify 'meows'; # OUTPUT: «True␤»
say so perlify '0'; # OUTPUT: «False␤»
say so perlify ''; # OUTPUT: «False␤»

The role provides the .Bool method that the so routine calls. Inside the method, we re-dispatch to the original .Bool method using nextsame routine unless the string is a '0', in which case we simply return False.

The but operator has a brother: an infix does operator. It behaves very similarly, except it does not clone. (N.B.: the shortcut for automatically making roles from non-roles is available in does only on bleeding edge Rakudo, version 2017.11-1-g47ebc4a and up)

my $o = class { method stuff { 'original' } }.new;
say $o.stuff; # OUTPUT: «original␤»
$o does role { method stuff { 'modded' } };
say $o.stuff; # OUTPUT: «modded␤»

Some of the things in a program are globally accessible and in some implementations (e.g. Rakudo), certain constants are cached. This means we can get quite naughty in a separate part of a program and those Christmas celebrators won't even know what hit 'em!

How about, we override what the prompt routine reads? They like Christmas? We'll give them some Christmas trees:

$*IN does role { method get { "🎄 {callsame} 🎄" } }
my $name = prompt "Enter your name: ";
say "You entered your name as: $name";
# Enter your name: (typed by user:) Zoffix Znet
# You entered your name as: 🎄 Zoffix Znet 🎄

That override will work even if we stick it into a module. We can also kick it up a notch and mess with enums and cached constants, though this naughtiness likely won't be able to cross the module boundary and other implementation-specific cache invalidation:

True does False;
say 42 ?? "tis true" !! "tis false";
# OUTPUT: «tis true␤»

So far, that didn't quite have the wanted impact, but let's try coercing our number to a Bool:

True does False;
say 42.Bool ?? "tis true" !! "tis false";
# OUTPUT: «tis false␤»

There we go! And now, for the final Grinch-worthy touch, we'll mess with numerical results of computations on numbers. Rakudo caches Int constants. Infix + operator also uses the internal-ish-ish .Bridge method when computing with numerics of different types. So, let's override the .Bridge on our constant to return something funky:

BEGIN 42 does role { method Bridge { 12e0 } }
say 42 + 15; # OUTPUT: «57␤»
say 42 + 15e0; # OUTPUT: «27␤»

That's proper evil, sure to ruin any Christmas, but we're only getting started…

Wrapping It Up

What kind of Christmas would it be without wrapped presents?! Oh, for presents we shall have and Perl 6's .wrap method provided by Routine type will let us wrap 'em up, oh so good.

use soft;
sub foo { say 'in foo' }
&foo.wrap: -> | {
say 'in the wrap';
say 'back in the wrap';
# in the wrap
# in foo
# back in the wrap

We enable use soft pragma to prevent unwanted inlining of routines that would otherwise interfere with our wrap. Then, we use a routine we want to wrap as a noun by using it with its & sigil and call the .wrap method that takes a Callable.

The given Callable's signature must be compatible with the one on the wrapped routine (or its proto if it's a multi); otherwise we'd not be able to both dispatch to the routine correctly and call the wrapper with the args. In the example above, we simply use an anonymous Capture (|) to accept all possible arguments.

Inside the Callable we have two say calls and make use of callsame routine to call the next available dispatch candidate, which happens to be our original routine. This comes in handy, since were we to attempt to call foo by its name inside the wrapper, we'd start the dispatch over from scratch, resulting in an infinite dispatch loop.

Since methods are Routines, we can wrap them as well. We can get a hold of the Method object using the .^lookup meta method:

IO::Handle.^lookup('print').wrap: my method (|c) {
my &wrapee = nextcallee;
wrapee self, "🎄 Ho-ho-ho! 🎄\n";
wrapee self, |c
print "Hello, World!\n";
# 🎄 Ho-ho-ho! 🎄
# Hello, World!

Here, we grab the .print method from IO::Handle type and .wrap it. We wish to make use of self inside the method, so we're wrapping using a standalone method (my method …) instead of a block or a subroutine. The reason we want to have self is to be able to call the very method we're wrapping to print our Christmassy message. Because our method is detached, the callwith and related routines will need self fed to them along with the rest of the args, to ensure we continue dispatch to the right object.

Inside the wrap, we use the nextcallee routine to obtain the original method.If it's a multi, we'll get the proto, not a specific candidate that best matches the original arguments, so the next candidate ordering is slightly different inside the wrap, compared to traditional routines. We grab the nextcallee in to a variable, because we want to call it more than once and calling it shifts the routine off the dispatch stack. In the first call, we print our Christmassy message and in the second call, we merely slip our Capture (|c) of original args, performing the call like it were originally meant to happen.

Thanks to the .wrap, we can alter or even completely redefine behaviour of subroutines and methods, which is sure to be jolly fun when your friends try to use them. Ho-ho-ho!

Invisibility Cloak

The tricks we've played so far are wonderfully terrible, but they're just too obvious and too… visible. Since Perl 6 has superb Unicode support, I think we should search the mass of Unicode characters for some fun mischief. In particular, we're looking for invisible characters that are NOT whitespace. Just one is sufficient for our purpose, but these four are fairly invisible on my computer:

[⁠] U+2060 WORD JOINER [Cf]

Perl 6 supports custom terms and operators that can consist of any characters, except whitespace. For example, here's my patented Shrug Operator:

sub infix:<¯\(°_o)/¯> {
($^a, $^b).pick
say 'Coke' ¯\(°_o)/¯ 'Pepsi';
# OUTPUT: «Pepsi␤»

And here's a term, made out of non-identifier characters (we could've used the actual characters in the definition as well):

sub term:«\c[family: woman woman boy boy]» {
'♫ We— are— ♪ faaaamillyyy ♬'
say 👩‍👩‍👦‍👦;
# OUTPUT: «♫ We— are— ♪ faaaamillyyy ♬»

With our invisible, non-whitespace characters in hand, we can make invisible operators and terms!

sub infix:«\c[INVISIBLE TIMES]» { $^a × $^b }
my \r = 42;
say "Area of the circle is " ~ π⁢r²;
# OUTPUT: «Area of the circle is 5541.76944093239␤»

Let's make a Jolly module that will export some invisible terms and operators. We'll then sprinkle them into our Christmassy friends' code:

unit module Jolly;
sub term:«\c[INVISIBLE TIMES]» is export { 42 }
sub infix:«\c[INVISIBLE TIMES]» is export {
$^a × $^b
sub prefix:«\c[INVISIBLE SEPARATOR]» (|)
is looser(&[,]) is export
say "Ho-ho-ho!";

We've used the same character for the term and the infix operator. That's fine, as Perl 6 has fairly strict expectation of terms being followed by operators and vice versa, so it'll know when we meant to use the term or when to use the infix operator. Here's the resultant Grinch code, along with the output it produces:

say 42⁢⁢;
# 1764
# Ho-ho-ho!

That'll sure be fun to debug! Here's a list of characters in that line of code, for you to see where we've used our invisible goodies:

.say for '⁣say 42⁢⁢;'.uninames;


Productivity at Christmas time drops to a standstill. People have the Holidays and the New Year on their minds. Wouldn't surprise me to see a whole bunch of TODO comments in all the codes. But what if we were able to detect and complain about them? There's nothing more Grinch-like than aborting program compilation whenever someone is feeling lazy!

Perl 6 has Slangs. It's an experimental feature that currently does not have an officially supported interface, however, for our purpose, it'll do just fine.

Using Slangs, it's possible to lexically mutate Perl 6's grammar and introduce language features and behaviour, just like a Perl 6 core developer would:

BEGIN $*LANG.refine_slang: 'MAIN',
role SomeExtraGrammar {
token term:sym<meow> {
'This is not a syntax error'
role SomeExtraActions {
method EXPR (Mu $/) {
say "Parsed expression: " ~ $/;
This is not a syntax error;
say 'hehe'
# Parsed expression: This is not a syntax error
# Parsed expression: 'hehe'
# Parsed expression: say 'hehe'
# hehe

The "experimental" part of the Slangs feature largely lies in having to rely on the structure of core Grammar and core Actions; currently there's no official guarantee those will remain unchanged, which makes Slangs fragile.

For our naughty, Grinchy trick, we'll be modifying behaviour of comments and if we read the code to trace what calls the comment token, we'll find it's actually part of the redefined ws token, which, as you may know from everyday Perl 6 grammars, is responsible for whitespace matching in, among other things, grammar rules.

This complicates the matter slightly, as ws is such a cornerstone token that, along with comp_unit, statementlist, and statement, it can't be modified in the mainline (code outside routines and blocks). The reason is the Slang is loaded after the mainline is already being parsed using the stock version of these tokens. The tokens inside statement token can be changed even in the mainline, because statement token reblesses the grammar, but ws does not get such luxury.

Since we're starting to tread far into the deep end… enough talk! Let's code:

BEGIN $*LANG.refine_slang: 'MAIN', role {
token comment:sym<todo> {
'#' \s* 'TODO' ':'? \s+ <( \N*
{ die "Ho-ho-ho! I think you were"
~ " meant to finish " ~ $/ }
sub business-stuff {
# TODO: business stuff
# ===SORRY!===
# Ho-ho-ho! I think you were meant to finish business stuff

We use the BEGIN phaser to make the Slang modification happen at compile time, since we're trying to affect how further compilation is performed.

We added a new proto token comment:sym<todo> to core Perl 6 grammar that matches content similar to what a regular comment would match, except it also looks for the TODO our Christmassy friends decided to leave around. The \N* atom captures whatever string the user typed after the TODO and the <( match capture marker tells the compiler to exclude the previously matched stuff in the token from the captured text inside the Match object stored in the $/ variable.

At the end of the token, we simply use a code block to die with a message that tells the user to finish up their TODO. Quite crafty!

Since we'd rather the user not notice our jolly tricks, let's stick the Slang into a module that's to be loaded by the target code. We'll just make a slight tweak to the original code:

# File: ./Jolly.pm6
sub EXPORT {
$*LANG.refine_slang: 'MAIN', role {
token comment:sym<todo> {
'#' \s* 'TODO' ':'? \s+ <( \N*
{ die "Ho-ho-ho! I think you were"
~ " meant to finish " ~ $/ }
# File: ./script.p6
use lib <.>;
use Jolly;
sub business-stuff {
# TODO: business stuff
# ===SORRY!===
# Ho-ho-ho! I think you were meant to finish business stuff

We want the slang to run at the compilation time of the script, not the module, so we removed the BEGIN phaser and instead stuck the code to be inside sub EXPORT, which will run when the module is used during script's compilation. The is just how I prefer to write {} in EXPORT sub, to indicate we do not wish to export any symbols. In our script, we now merely have to use the module and the Slang gets activated. Awesome!


Today, we started off the 2017 Perl 6 Advent Calendar by being naughty Grinches and messing with users' programs. We mutated objects using but and does operators. Wrapped methods and subroutines with our custom routines that implemented extra features. Made invisible terms and operators. And even mutated the language itself to do our bidding.

Over the next 23 days, we'll see more Perl 6 Advent articles, so be sure to check back. And maybe, by the end of it all, our Grinchy hearts will grow three sizes…