Day 2 – Perl 6: Sigils, Variables, and Containers

Having a rudimentary understanding of containers is vital for enjoyable programming in Perl 6. They're ubiquitous and not only do they affect the kind of variables you get, they also dictate how Lists and Maps behave when iterated.

Today, we'll learn what containers are and how to work with them, but first, I'd like you to temporarily forget everything you might know or suspect about Perl 6's sigils and variables, especially if you're coming from Perl 5's background. Everything.

Show Me The Money

In Perl 6, a variable is prefixed with a $ sigil and is given a value with a binding operator (:=). Like so:

my $foo := 42;
say "The value is $foo"; # OUTPUT: «The value is 42␤»

If you've followed my suggestion to forget everything you know, it won't shock you to learn the same applies to List and Hash types:

my $ordered-things := <foo bar ber>;
my $named-things := %(:42foo, :bar<ber>);
say "$named-things<foo> bottles of $ordered-things[2] on the wall";
# OUTPUT: «42 bottles of ber on the wall␤»
.say for $ordered-things; # OUTPUT: «foo␤bar␤ber␤»
.say for $named-things; # OUTPUT: «bar => ber␤foo => 42␤»

Knowing just this, you can write a great variety of programs, so if you ever start to feel like there's just too much to learn, remember you don't have to learn everything at once.

We Wish You a Merry Listmas

Let's try doing more things with our variables. It's not uncommon to want to change a value in a list. How well do we fare with what we have so far?

my $list := (1, 2, 3);
$list[0] := 100;
# OUTPUT: «Cannot use bind operator with this left-hand side […] »

Although we can bind to variables, if we attempt to bind to some value, we get an error, regardless of whether the value comes from a List or just, say, a literal:

1 := 100;
# OUTPUT: «Cannot use bind operator with this left-hand side […] »

This is how Lists manage to be immutable. However, 'Tis The Season and wishes do come true, so let's wish for a mutable List!

What we need to get a hold of is a Scalar object because the binding operator can work with it. As the name suggests, a Scalar holds one thing. You can't instantiate a Scalar via the .new method, but we can get them by just declaring some lexical variables; don't need to bother giving them names:

my $list := (my $, my $, my $);
$list[0] := 100;
say $list; # OUTPUT: «(100 (Any) (Any))␤»

The (Any) in the output are the default values of the containers (on that, a bit later). Above, it seems we managed to bind a value to a list's element after List's creation, did we not? Indeed we did, but…

my $list := (my $, my $, my $);
$list[0] := 100;
$list[0] := 200;
# OUTPUT: «Cannot use bind operator with this left-hand side […] »

The binding operation replaces the Scalar container with a new value (100), so if we try to bind again, we're back to square one, trying to bind to a value instead of a container again.

We need a better tool for the job.

That's Your Assignment

The binding operator has a cousin: the assignment operator (=). Instead of replacing our Scalar containers with a binding operator, we'll use the assignment operator to assign, or "store", our values in the containers:

my $list := (my $ = 1, my $ = 2, my $ = 3);
$list[0] = 100;
$list[0] = 200;
say $list;
# OUTPUT: «(200 2 3)␤»

Now, we can assign our original values right from the start, as well as replace them with other values whenever we want to. We can even get funky and put different type constraints on each of the containers:

my $list := (my Int $ = 1, my Str $ = '2', my Rat $ = 3.0);
$list[0] = 100; # OK!
$list[1] = 42; # Typecheck failure!
# OUTPUT: «Type check failed in assignment;
# expected Str but got Int (42) […] »

That's somewhat indulgent, but there is one thing that could use a type constraint: the $list variable. We'll constrain it to the Positional role to ensure it can only hold Positional types, like List and Array:

my Positional $list := (my $ = 1, my $ = '2', my $ = 3.0);

Don't know about you, but that looks awfully verbose to me. Luckily, Perl 6 has syntax to simplify it!

Position@lly

First, let's get rid of the explicit type constraint on the variable. In Perl 6, you can use @ instead of $ as a sigil to say that you want the variable to be type-constrained with role Positional:

my @list := 42;
# OUTPUT: «Type check failed in binding;
# expected Positional but got Int (42) […] »

Second, instead of parentheses to hold our List, we'll use square brackets. This tells the compiler to create an Array instead of a List. Arrays are mutable and they stick each of their elements into a Scalar container automatically, just like we did manually in the previous section:

my @list := [1, '2', 3.0];
@list[0] = 100;
@list[0] = 200;
say @list;
# OUTPUT: «[200 2 3]␤»

Our code became a lot shorter, but we can toss out a couple more characters. Just like assigning, instead of binding, to a $-sigiled variable gives you a Scalar container for free, you can assign to @-sigiled variable to get a free Array. If we switch to assignment, we can get rid of the square brackets altogether:

my @list = 1, '2', 3.0;

Nice and concise.

Similar ideas are behind %– and &-sigiled variables. The % sigil implies a type-constraint on Associative role and offers the same shortcuts for assignment (giving you a Hash) and creates Scalar containers for the values. The &-sigiled variables type-constrain on role Callable and assignment behaves similar to $ sigils, giving a free Scalar container whose value you can modify:

my %hash = :42foo, :bar<ber>;
say %hash; # OUTPUT: «{bar => ber, foo => 42}␤»
my &reversay = sub { $^text.flip.say }
reversay '6 lreP ♥ I'; # OUTPUT: «I ♥ Perl 6␤»
# store a different Callable in the same variable
&reversay = *.uc.say; # a WhateverCode object
reversay 'I ♥ Perl 6'; # OUTPUT: «I ♥ PERL 6␤»

The One and Only

Earlier we learned that assignment to $-sigiled variables gives you a free Scalar container. Since scalars, as the name suggests, contain just one thing… what exactly happens if you put a List into a Scalar? After all, the Universe remains unimploded when you try to do that:

my $listish = (1, 2, 3);
say $listish; # OUTPUT: «(1 2 3)␤»

Such behaviour may make it seem that Scalar is a misnomer, but it does actually treat the entire list as a single thing. We can observe the difference in a couple of ways. Let's compare a List bound to a $-sigiled variable (so no Scalar is involved) with one that is assigned into a $-sigiled variable (automatic Scalar container):

# Binding:
my $list := (1, 2, 3);
say $list.perl;
say "Item: $_" for $list;
# OUTPUT:
# (1, 2, 3)
# Item: 1
# Item: 2
# Item: 3
# Assignment:
my $listish = (1, 2, 3);
say $listish.perl;
say "Item: $_" for $listish;
# OUTPUT:
# $(1, 2, 3)
# Item: 1 2 3

The .perl method gave us an extra insight and showed the second List with a $ before it, to indicate it's containerized in a Scalar. More importantly, when we iterated over our Lists with the for loop, the second List resulted in just a single iteration: the entire List as one item! The Scalar lives up to its name.

This behaviour isn't merely of academic interest. Recall that Arrays (and Hashes) create Scalar containers for their values. This means that if we nest things, even if we select an individual list or hash stored inside the Array (or Hash) and try to iterate over it, it'd be treated as just a single item:

my @stuff = (1, 2, 3), %(:42foo, :70bar);
say "List Item: $_" for @stuff[0];
say "Hash Item: $_" for @stuff[1];
# OUTPUT:
# List Item: 1 2 3
# Hash Item: bar 70
# foo 42

The same reasoning—that lists and hashes in Scalar containers are a single item—applies when you try to flatten an Array's elements or pass them as an argument to a slurpy parameter:

my @stuff = (1, 2, 3), %(:42foo, :70bar);
say flat @stuff;
# OUTPUT: «((1 2 3) {bar => 70, foo => 42})␤»
-> *@args { @args.say }(@stuff)
# OUTPUT: «[(1 2 3) {bar => 70, foo => 42}]␤»

It's this behaviour that can drive Perl 6 beginners up the wall, especially those who come from auto-flattening languages, such as Perl 5. However, now that we know why this behaviour is observed, we can change it!

Decont

If the Scalar container is the culprit, all we need to do is remove it. We need to de-containerize our list and hash, or "decont" for short. In your Perl 6 travels, you'll find several ways to accomplish that, but one way that's designed precisely for that is the decont methodop (<>):

my @stuff = (1, 2, 3), %(:42foo, :70bar);
say "Item: $_" for @stuff[0]<>;
say "Item: $_" for @stuff[1]<>;
# OUTPUT:
# Item: 1
# Item: 2
# Item: 3
# Item: bar 70
# Item: foo 42

It's easy to remember: it looks like a squished box (a trampled container). After retrieving our containerized items by indexing into the Array, we appended the decont and removed the contents from their Scalar containers, causing our loop to iterate over each item in them individually.

If you wish to decont every element of an Array in one go, simply use the hyper operator (», or >> if you prefer ASCII) along with the decont:

my @stuff = (1, 2, 3), %(:42foo, :70bar);
say flat @stuff»<>;
# OUTPUT: «(1 2 3 bar => 70 foo => 42)␤»
-> *@args { @args.say }(@stuff»<>)
# OUTPUT: «[1 2 3 bar => 70 foo => 42]␤»

With the containers removed, our list and hash flattened just like we wanted. And of course, we could have avoided the Array and bound our original List to the variable instead. Since Lists don't put their elements into containers, there's nothing to decont:

my @stuff := (1, 2, 3), %(:42foo, :70bar);
say flat @stuff;
# OUTPUT: «(1 2 3 bar => 70 foo => 42)␤»
-> *@args { @args.say }(@stuff)
# OUTPUT: «[1 2 3 bar => 70 foo => 42]␤»

Don't Let It Slip Away

While we're here, it's worth noting that many people use the slip operator (|), when they want to do the decont (we're not talking about using it when passing arguments to Callables):

my @stuff = (1, 2, 3), (4, 5);
say "Item: $_" for |@stuff[0];
# OUTPUT:
# Item: 1
# Item: 2
# Item: 3

Although it gets the job done as far as deconting goes, it can introduce subtle bugs that could be very difficult to track down. Try to spot one here, in a program that iterates over an infinite list of non-negative integers and prints those that are prime:

my $primes = ^.grep: *.is-prime;
say "$_ is a prime number" for |$primes;

Give up? This program leaks memory… very slowly. Even though, we're iterating over an infinite list of items, that's not an issue because .grep method returns a Seq object that doesn't keep already-iterated items around and so memory usage never grows there.

The problematic part is our | slip operator. It converts our Seq into a Slip, which is a type of a List and keeps around all of the values we already consumed. Here's a modified version of the program that grows faster, if you wanted to see that growth in htop:

# CAREFUL! Don't consume all of your resources!
my $primes = ^.map: *.self;
Nil for |$primes;

Let's try it again, but this time using the decont method op:

my $primes = ^.map: *.self;
Nil for $primes<>;

The memory usage is stable now and the program can sit there and iterate until the end of times. Of course, since we know it's the Scalar container that causes containerization and we wish to avoid it here, we can simply bind the Seq to the variable instead:

my $primes := ^.map: *.self;
Nil for $primes;

I Want Less

If you detest sigils, Perl 6 got something you can smile about: sigil-less variables. Just prefix the name with a backslash during declaration, to indicate you don't want no stinkin' sigils:

my= 42;
say Δ²; # OUTPUT: «1764␤»

You don't get any free Scalars with such variables and so, during declaration, it makes no difference between binding or assignment to them. They behave similar to how binding a value to a $-sigiled variable behaves, including the ability to bind Scalars and make the variable mutable:

my= my $ = 42;
Δ = 11;
say Δ²; # OUTPUT: «121␤»

A more common place where you might see such variables is as parameters of routines, here, these mean you want is raw trait applied to the parameter. The meaning exists for the + positional slurpy parameter as well (no backslash is needed), where having it is raw means you won't get unwanted Scalar containers due to the slurpy being an Array as it has the @ the sigil:

sub sigiled ($x is raw, +@y) {
$x = 100;
say flat @y
}
sub sigil-less (\x, +y) {
x = 200;
say flat y
}
my $x = 42;
sigiled $x, (1, 2), (3, 4); # OUTPUT: «((1 2) (3 4))␤»
say $x; # OUTPUT: «100␤»
sigil-less $x, (1, 2), (3, 4); # OUTPUT: «(1 2 3 4)␤»
say $x; # OUTPUT: «200␤»

Defaulting on Default Defaults

One awesome feature offered by containers is default values. You may have heard that in Perl 6 Nil signals the absence of a value and not a value in itself. Container defaults is where it comes into play:

my $x is default(42);
say $x; # OUTPUT: «42␤»
$x = 10;
say $x; # OUTPUT: «10␤»
$x = Nil;
say $x; # OUTPUT: «42␤»

A container's default value is given to it using the is default trait. Its argument is evaluated at compile time and the resultant value is used whenever the container lacks a value. Since Nil's job is to signal just that, assigning a Nil into a container will result in the container containing its default value, not a Nil.

Defaults can be given to Array and Hash containers just the same and if you wish your containers to contain a Nil literally, when no value is present, just specify Nil as a default:

my @a is default<meow> = 1, 2, 3;
say @a[0, 2, 42]; # OUTPUT: «(1 3 meow)␤»
@a[0]:delete;
say @a[0]; # OUTPUT: «meow␤»
my %h is default(Nil) = :bar<ber>;
say %h<bar foos>; # OUTPUT: «(ber Nil)␤»
%h<bar>:delete;
say %h<bar> # OUTPUT: «Nil␤»

The container's default has a default default: the explicit type constraint that's present on the container:

say my Int $y; # OUTPUT: «(Int)␤»
say my Mu $z; # OUTPUT: «(Mu)␤»
say my Int $i where *.is-prime; # OUTPUT: «(<anon>)␤»
$i.new; # OUTPUT: (exception) «You cannot create […]»

If no explicit type constraint is present, the default default is an Any type object:

say my $x; # OUTPUT: «(Any)␤»
say $x = Nil; # OUTPUT: «(Any)␤»

Note that the default values you may use in routine signatures for optional parameters are not the container defaults and assigning Nil to subroutine arguments or into parameters will not utilize the defaults from the signature.

Customizing

If the standard behaviour of containers doesn't suit your needs, you can make your own container, using the Proxy type:

my $collector := do {
my @stuff;
Proxy.new: :STORE{ @stuff.push: @_[1] },
:FETCH{ @stuff.join: "|" }
}
$collector = 42;
$collector = 'meows';
say $collector; # OUTPUT: «42|meows␤»
$collector = 'foos';
say $collector; # OUTPUT: «42|meows|foos␤»

The interface is somewhat clunky, but it gets the job done. We create the Proxy object using method .new that takes two required named arguments: STORE and FETCH, each taking a Callable.

The FETCH Callable gets called whenever a value is read from the container, which can happen more times than is immediately apparent: in the code above, the FETCH Callable is called 10 times as the container percolates through dispatch and routines of the two say calls. The Callable is called with a single positional argument: the Proxy object itself.

The STORE Callable gets called whenever a value is stored into our container, for example, with an assignment operator (=). The first positional argument to the Callable is the Proxy object itself, and the second argument is the value that was given to be stored.

We'd like STORE and FETCH Callables to share the @stuff variable, so we use the do statement prefix with a code block to contain it all nicely inside.

We bind our Proxy to a variable and the rest is just normal variable usage. The output shows the altered behaviour our custom container provides.

Proxies are also handy as a return value from methods to provide extra behaviour with mutable attributes. For example, here's an attribute that from the outside appears to be just a normal mutable attribute, but actually coerces its value from an Any type to an Int

class Foo {
has $!foo;
method foo {
Proxy.new: :STORE(-> $, Int() $!foo { $!foo }),
:FETCH{ $!foo }
}
}
my $o = Foo.new;
$o.foo = ' 42.1e0 ';
say $o.foo; # OUTPUT: «42␤»

Quite sweet! And if you want a Proxy with a better interface with a few more features under its belt, check out the Proxee module.

That's All, Folks

That about covers it all. The remaining beasts you'll see in the land of Perl 6 are "twigils": variables with TWO symbols before the name, but as far as containers go, they behave the same as the variables we've covered. The second symbol simply indicates additional information, such as whether the variable is an implied positional or named parameter…

sub test { say "$^implied @:parameters[]" }
test 'meow', :parameters<says the cat>;
# OUTPUT: «meow says the cat␤»

…or whether the variable is a private or public attribute:

with class Foo {
has $!foo = 42;
has @.bar = 100;
method what's-foo { $!foo }
}.new {
say .bar; # OUTPUT: «[100]␤»
say .what's-foo # OUTPUT: «42␤»
}

That's a journey for another day, however.

Conclusion

Perl 6 has a rich system of variables and containers that differs vastly from Perl 5. It's important to understand the way it works, as it affects the way iteration and flattening of lists and hashes behaves.

Assignment to variables offers valuable shortcuts, such as providing Scalar, Array, or Hash containers, depending on the sigil. Binding to variables allows you to bypass such shortcuts, if you so require.

Sigil-less variables exist in Perl 6 and they have similar behaviour to how $-sigiled variables with binding work. When used as parameters, these variables behave like is raw trait was applied to them.

Lastly, containers can have default values and it's possible to create your own custom containers that can either be bound to a variable or returned from a routine.

Happy Holidays!

7 thoughts on “Day 2 – Perl 6: Sigils, Variables, and Containers

  1. Thanks for the article, this is good stuff. I knew the `@foo[]` zen slice operator was a thing but never understood when or why to use it, and the `@:` twigil is news to me.

  2. P.S.: Since Lists are meant to be immutable, as of Rakudo v2017.12.152.geed.54.caef you can’t bind to their elements even if they’re Scalars, so a couple of examples in the article don’t work any more. Just pretend… :)

  3. Thanks Zoffix. I was just re-reading this, and realized it must be one of the most insightful P6 posts that I’ve come across.

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