Variables

Variables in Perl 6

Variable names can start with or without a special character called a sigil, followed optionally by a second special character named twigil and then an identifier. Variables are symbolic names for values or containers. Variable declarations or assignment of values may create a container on the fly.

Sigils

The sigil serves as a variable indicator and type constraint.

Sigil Type constraint Default type Assignment Examples
$ Mu (no type constraint) Any item Int, Str, Array, Hash
@ Positional Array list List, Array, Range, Buf
% Associative Hash list Hash, Map, Pair
& Callable Callable item Sub, Method, Block, Routine

Examples:

my $square = 9 ** 2;
my @array  = 123;   # Array variable with three elements 
my %hash   = London => 'UK'Berlin => 'Germany';

The default type can be set with is.

class FailHash is Hash {
    has Bool $!final = False;
    multi method AT-KEY ( ::?CLASS:D: Str:D \key ){
        fail X::OutOfRange.new(:what("Hash key"), :got(key), :range(self.keys)) if $!final && !self.EXISTS-KEY(key);
        callsame
    }
 
    method finalize() {
        $!final = True
    }
}
 
my %h is FailHash = oranges => "round"bananas => "bendy";
say %h<oranges>;
# OUTPUT: «round␤» 
%h.finalize;
say %h<cherry>;
CATCH { default { put .^name''.Str } }
# OUTPUT: «X::OutOfRange: Hash key out of range. Is: cherry, should be in (oranges bananas)» 

For information on variables without sigils, see sigilless variables.

Item and List Assignment

There are two types of variable assignment, item assignment and list assignment. Both use the equal sign = as operator. The syntax of the left-hand side determines whether an = means item or list assignment.

Item assignment places the value from the right-hand side into the variable (container) on the left.

List assignment leaves the choice of what to do to the variable on the left.

For example, Array variables (@ sigil) empty themselves on list assignment and then put all the values from the right-hand side into themselves.

The type of assignment (item or list) is decided by the first context seen in the current expression or declarator:

my $foo = 5;            # item assignment 
say $foo.perl;          # OUTPUT: «5␤» 
 
my @bar = 79;         # list assignment 
say @bar.^name;         # OUTPUT: «Array␤» 
say @bar.perl;          # OUTPUT: «[7, 9]␤» 
 
(my $baz= 1113;     # list assignment 
say $baz.^name;         # OUTPUT: «List␤» 
say $baz.perl;          # OUTPUT: «$(11, 13)␤» 

Thus, the behavior of an assignment contained within a list assignment depends on the expression or declarator that contains it.

For instance, if the internal assignment is a declarator, item assignment is used, which has tighter precedence than both the comma and the list assignment:

my @array;
@array = my $num = 42"str";   # item assignment: uses declarator 
say @array.perl;                # OUTPUT: «[42, "str"]␤» (an Array) 
say $num.perl;                  # OUTPUT: «42␤» (a Num) 

Similarly, if the internal assignment is an expression that is being used as an initializer for a declarator, the context of the internal expression determines the assignment type:

my $num;
my @array = $num = 42"str";    # item assignment: uses expression 
say @array.perl;                 # OUTPUT: «[42, "str"]␤» (an Array) 
say $num.perl;                   # OUTPUT: «42␤» (a Num) 
 
my ( @foo$bar );
@foo = ($bar= 42"str";       # list assignment: uses parentheses 
say @foo.perl;                   # OUTPUT: «[(42, "str"),]␤» (an Array) 
say $bar.perl;                   # OUTPUT: «$(42, "str")␤» (a List)# 

However, if the internal assignment is neither a declarator nor an expression, but is part of a larger expression, the context of the larger expression determines the assignment type:

my ( @array$num );
@array = $num = 42"str";    # list assignment 
say @array.perl;              # OUTPUT: «[42, "str"]␤» 
say $num.perl;                # OUTPUT: «42␤» 

The assignment expression is parsed as @array = (($num = 42), "str"), because item assignment has tighter precedence than the comma.

See operators for more details on precedence.

Sigilless variables

It's possible to create "variables" in Perl 6 that do not have sigils:

my \degrees = pi / 180;
my \θ       = 15 * degrees;

Note that these do not create containers. This means degrees and θ, above, actually directly represent Nums. To illustrate, try assigning to one after you've defined it:

θ = 3# Dies with the error "Cannot modify an immutable Num" 

Sigilless variables do not enforce context, so they can be used to pass something on as-is:

sub logged(&f|args{
    say('Calling ' ~ &f.name ~ ' with arguments ' ~ args.perl);
    my \result = f(|args);
    #  ^^^^^^^ not enforcing any context here 
    say(&f.name ~ ' returned ' ~ result.perl);
    return |result;
}

Twigils

Twigils influence the scoping of a variable; however, they have no influence over whether the primary sigil interpolates. That is, if $a interpolates, so do $^a, $*a, $=a, $?a, $.a, etc. It only depends on the $.

Twigil Scope
none Based only on declarator
* Dynamic
! Attribute (class member)
? Compile-time variable
. Method (not really a variable)
< Index into match object (not really a variable)
^ Self-declared formal positional parameter
: Self-declared formal named parameter
= Pod variables
~ The sublanguage seen by the parser at this lexical spot

The * Twigil

Dynamic variables are looked up through the caller, not through the outer scope. For example:

my $lexical   = 1;
my $*dynamic1 = 10;
my $*dynamic2 = 100;
 
sub say-all() {
    say "$lexical$*dynamic1$*dynamic2";
}
 
# prints 1, 10, 100 
say-all();
 
{
    my $lexical   = 2;
    my $*dynamic1 = 11;
    $*dynamic2    = 101;
 
    # prints 1, 11, 101 
    say-all();
}
 
# prints 1, 10, 101 
say-all();

The first time &say-all is called, it prints "1, 10, 100" just as one would expect. The second time though, it prints "1, 11, 101". This is because $lexical isn't looked up in the caller's scope but in the scope &say-all was defined in. The two dynamic variables are looked up in the caller's scope and therefore have the values 11 and 101. The third time &say-all is called $*dynamic1 isn't 11 anymore, but $*dynamic2 is still 101. This stems from the fact that we declared a new dynamic variable $*dynamic1 in the block and did not assign to the old variable as we did with $*dynamic2.

The dynamic variables differ from other variable types in that referring to an undeclared dynamic variable is not a compile time error but a runtime failure, so a dynamic variable can be used undeclared as long as it's checked for definedness or used in a boolean context before using it for anything else:

sub foo() {
    $*FOO // 'foo';
}
 
say foo# OUTPUT: «foo␤» 
 
my $*FOO = 'bar';
 
say foo# OUTPUT: «bar␤» 

The ! Twigil

Attributes are variables that exist per instance of a class. They may be directly accessed from within the class via !:

my class Point {
    has $.x;
    has $.y;
 
    method Str() {
        "($!x$!y)"
    }
}

Note how the attributes are declared as $.x and $.y but are still accessed via $!x and $!y. This is because in Perl 6 all attributes are private and can be directly accessed within the class by using $!attribute-name. Perl 6 may automatically generate accessor methods for you though. For more details on objects, classes and their attributes see object orientation.

The ? Twigil

Compile-time variables may be addressed via the ? twigil. They are known to the compiler and may not be modified after being compiled in. A popular example for this is:

say "$?FILE$?LINE"# prints "hello.pl: 23" if this is the 23 line of a 
                      # file named "hello.pl". 

For a list of these special variables, see compile-time variables.

The . Twigil

The . twigil isn't really for variables at all. In fact, something along the lines of

my class Point {
    has $.x;
    has $.y;
 
    method Str() {
        "($.x$.y)" # note that we use the . instead of ! this time 
    }
}

just calls the methods x and y on self, which are automatically generated for you because you used the . twigil when the attributes were declared. Note, however, that subclasses may override those methods. If you don't want this to happen, use $!x and $!y instead.

The fact that the . twigil does a method call implies that the following is also possible:

class SaySomething {
    method a() { say "a"}
    method b() { $.a}
}
 
SaySomething.b# OUTPUT: «a␤» 

For more details on objects, classes and their attributes and methods see object orientation.

The ^ Twigil

The ^ twigil declares a formal positional parameter to blocks or subroutines. Variables of the form $^variable are a type of placeholder variable. They may be used in bare blocks to declare formal parameters to that block. So the block in the code

my @powers-of-three = 1,3,9100;
say reduce { $^b - $^a }0|@powers-of-three;
# OUTPUT: «61␤» 

has two formal parameters, namely $a and $b. Note that even though $^b appears before $^a in the code, $^a is still the first formal parameter to that block. This is because the placeholder variables are sorted in Unicode order. If you have self-declared a parameter using $^a once, you may refer to it using only $a thereafter.

Although it is possible to use nearly any valid identifier as a placeholder variable, it is recommended to use short names or ones that can be trivially understood in the correct order, to avoid surprise on behalf of the reader.

Normal blocks and subroutines may also make use of placeholder variables but only if they do not have an explicit parameter list.

sub say-it    { say $^a} # valid 
sub say-it()  { say $^a} # invalid 
              { say $^a} # valid 
-> $x$y$x { say $^a} # invalid 

Placeholder variables cannot have type constraints or a variable name with a single upper-case letter (this is disallowed to enable catching some Perl5-isms).

The : Twigil

The : twigil declares a formal named parameter to a block or subroutine. Variables declared using this form are a type of placeholder variable too. Therefore the same things that apply to variables declared using the ^ twigil also apply here (with the exception that they are not positional and therefore not ordered using Unicode order, of course). So this:

say { $:add ?? $^a + $^b !! $^a - $^b }45 ) :!add
# OUTPUT: «-1␤» 

See ^ for more details about placeholder variables.

The = Twigil

The = twigil is used to access Pod variables. Every Pod block in the current file can be accessed via a Pod object, such as $=data, $=SYNOPSIS or =UserBlock. That is: a variable with the same name of the desired block and a = twigil.

  =begin code
  =begin Foo
  ...
  =end Foo
 
  #after that, $=Foo gives you all Foo-Pod-blocks
  =end code

You may access the Pod tree which contains all Pod structures as a hierarchical data structure through $=pod.

Note that all those $=someBlockName support the Positional and the Associative roles.

The ~ Twigil

Note: Slangs are NYI in Rakudo.

The ~ twigil is for referring to sublanguages (called slangs). The following are useful:

$~MAIN the current main language (e.g. Perl statements)
$~Quote the current root of quoting language
$~Quasi the current root of quasiquoting language
$~Regex the current root of regex language
$~Trans the current root of transliteration language
$~P5Regex the current root of the Perl 5 regex language

You augment these languages in your current lexical scope.

use MONKEY-TYPING;
augment slang Regex {  # derive from $~Regex and then modify $~Regex 
    token backslash:std<\Y> { YY };
}

Variable Declarators and Scope

Most of the time it's enough to create a new variable using the my keyword:

my $amazing-variable = "World";
say "Hello $amazing-variable!"# OUTPUT: «Hello World!␤» 

However, there are many declarators that change the details of scoping beyond what Twigils can do.

Declarator Effect
my Introduces lexically scoped names
our Introduces package-scoped names
has Introduces attribute names
anon Introduces names that are private to the construct
state Introduces lexically scoped but persistent names
augment Adds definitions to an existing name
supersede Replaces definitions of an existing name

There are also two prefixes that resemble declarators but act on predefined variables:

Prefix Effect
temp Restores a variable's value at the end of scope
let Restores a variable's value at the end of scope if the block exits unsuccessfully

The my Declarator

Declaring a variable with my gives it lexical scope. This means it only exists within the current block. For example:

{
    my $foo = "bar";
    say $foo# OUTPUT: «"bar"␤» 
}
say $foo# Exception! "Variable '$foo' is not declared" 

This dies because $foo is only defined as long as we are in the same scope.

Additionally, lexical scoping means that variables can be temporarily redefined in a new scope:

my $location = "outside";
 
sub outer-location {
    # Not redefined: 
    say $location;
}
 
outer-location# OUTPUT: «outside␤» 
 
sub in-building {
    my $location = "inside";
    say $location;
}
 
in-building;    # OUTPUT: «inside␤» 
 
outer-location# OUTPUT: «outside␤» 

If a variable has been redefined, any code that referenced the outer variable will continue to reference the outer variable. So here, &outer-location still prints the outer $location:

sub new-location {
    my $location = "nowhere";
    outer-location;
}
 
new-location# OUTPUT: «outside␤» 

To make new-location() print nowhere, make $location a dynamic variable using the * twigil.

my is the default scope for subroutines, so my sub x() {} and sub x() {} do exactly the same thing.

The our Declarator

our variables work just like my variables, except that they also introduce an alias into the symbol table.

module M {
    our $Var;
    # $Var available here 
}
 
# Available as $M::Var here. 

Declaring a list of variables

The declarators my and our take a list of variables in parentheses as argument to declare more then one variable at a time.

my (@a$s%h);

This can be used in conjunction with destructuring assignment. Any assignment to such a list will take the number of elements provided in the left list and assign corresponding values from the right list to them. Any missing elements are left will result in undefined values according to the type of the variables.

my (Str $aStr $bInt $c= <a b>;
say [$a$b$c].perl;
# OUTPUT: «["a", "b", Int]␤» 

To destructure a list into a single value, create a list literal with one element by using ($var,). When used with a variable declarator, providing parentheses around a single variable is sufficient.

sub f { 1,2,3 };
my ($a= f;
say $a.perl;
# OUTPUT: «1␤» 

To skip elements in the list use the anonymous state variable $.

my ($,$a,$,%h= ('a''b', [1,2,3], {:1th});
say [$a%h].perl;
# OUTPUT: «["b", {:th(1)}]␤» 

The has Declarator

has scopes attributes to instances of a class or role, and methods to classes or roles. has is implied for methods, so has method x() {} and method x() {} do the same thing.

See object orientation for more documentation and some examples.

The anon Declarator

The anon declarator prevents a symbol from getting installed in the lexical scope, the method table and everywhere else.

For example, you can use it to declare subroutines which know their own name, but still aren't installed in a scope:

my %operations =
    half   => anon sub half($x{ $x / 2 },
    square => anon sub square($x{ $x * $x },
    ;
say %operations<square>.name;       # square 
say %operations<square>(8);         # 64 

The state Declarator

state declares lexically scoped variables, just like my. However, initialization happens exactly once the first time the initialization is encountered in the normal flow of execution. Thus, state variables will retain their value across multiple executions of the enclosing block or routine.

Therefore, the subroutine

sub a {
    state @x;
    state $l = 'A';
    @x.push($l++);
};
 
say a for 1..6;

will continue to increment $l and append it to @x each time it is called. So it will output:

[A]
[A B]
[A B C]
[A B C D]
[A B C D E]
[A B C D E F]
 

This works per "clone" of the containing code object, as in this example:

({ state $i = 1$i++.say} xx 3).map: {$_(), $_()}# says 1 then 2 thrice 

Note that this is not a thread-safe construct when the same clone of the same block is run by multiple threads. Also remember that methods only have one clone per class, not per object.

As with my, declaring multiple state variables must be placed in parentheses and for declaring a single variable, parentheses may be omitted.

Many operators come with implicit binding which can lead to actions at a distance.

Use .clone or coercion to create a new container that can be bound to.

my @a;
sub f() {
    state $i;
    $i++;
    @a.push: "k$i" => $i # <-- .clone goes here 
};
f for 1..3;
dd @a# OUTPUT: «Array @a = [:k1(3), :k2(3), :k3(3)]␤» 

State variables are shared between all threads. The result can be unexpected.

sub code(){ state $i = 0say ++$i$i };
await
    start { loop { last if code() >= 5 } },
    start { loop { last if code() >= 5 } };
 
# OUTPUT: «1␤2␤3␤4␤4␤3␤5␤» 
# OUTPUT: «2␤1␤3␤4␤5␤» 
# many other more or less odd variations can be produced 

The $ Variable

In addition to explicitly declared named state variables, $ can be used as an anonymous state variable without an explicit state declaration.

say "1-a 2-b 3-c".subst(:g, /\d/{<one two three>[$++]});
# OUTPUT: «one-a two-b three-c␤» 

Furthermore, state variables can be used outside of subroutines. You could, for example, use $ in a one-liner to number the lines in a file.

perl6 -ne 'say ++$ ~ " $_"' example.txt

Each reference to $ within a lexical scope is in effect a separate variable.

perl6 -e '{ say ++$; say $++  } for ^5'
# OUTPUT: «1␤0␤2␤1␤3␤2␤4␤3␤5␤4␤» 

If you need to use the value of $ more than once in a scope, it should be copied to a new variable.

sub foo() {
    given ++$ {
        when 1 {
            say "one";
        }
        when 2 {
            say "two";
        }
        when 3 {
            say "three";
        }
        default {
            say "many";
        }
    }
}
 
foo() for ^3;
# OUTPUT: «one␤two␤three␤» 

Note that the implicit state declarator is only applied to the variable itself, not the expression that may contain an initializer. If the initializer has to be called exactly once, the state declarator has to be provided.

subset DynInt where $ = ::('Int'); # the initializer will be called for each type check 
subset DynInt where state $ = ::('Int'); # the initializer is called once, this is a proper cache 

The @ Variable

Similar to the $ variable, there is also a Positional anonymous state variable @.

sub foo($x{
    say (@).push($x);
}
 
foo($_for ^3;
 
# OUTPUT: «[0] 
#          [0 1] 
#          [0 1 2]␤» 

The @ here is parenthesized in order to disambiguate the expression from a class member variable named @.push. Indexed access doesn't require this disambiguation but you will need to copy the value in order to do anything useful with it.

sub foo($x{
    my $v = @;
    $v[$x= $x;
    say $v;
}
 
foo($_for ^3;
 
# OUTPUT: «[0] 
#          [0 1] 
#          [0 1 2]␤» 

As with $, each mention of @ in a scope introduces a new anonymous array.

The % Variable

In addition, there's an Associative anonymous state variable %.

sub foo($x{
    say (%).push($x => $x);
}
 
foo($_for ^3;
 
# OUTPUT: «{0 => 0} 
#          {0 => 0, 1 => 1} 
#          {0 => 0, 1 => 1, 2 => 2}␤» 

The same caveat about disambiguation applies. As you may expect, indexed access is also possible (with copying to make it useful).

sub foo($x{
    my $v = %;
    $v{$x} = $x;
    say $v;
}
 
foo($_for ^3;
 
# OUTPUT: «{0 => 0} 
#          {0 => 0, 1 => 1} 
#          {0 => 0, 1 => 1, 2 => 2}␤» 

As with the other anonymous state variables, each mention of % within a given scope will effectively introduce a separate variable.

The augment Declarator

With augment, you can add attributes and methods to existing classes and grammars, provided you activated the MONKEY-TYPING pragma first.

Since classes are usually our scoped, and thus global, this means modifying global state, which is strongly discouraged. For almost all situations, there are better solutions.

# don't do this 
use MONKEY-TYPING;
augment class Int {
    method is-answer { self == 42 }
}
say 42.is-answer;       # OUTPUT: «True␤» 

(In this case, the better solution would be to use a function).

The temp Prefix

Like my, temp restores the old value of a variable at the end of its scope. However, temp does not create a new variable.

my $in = 0# temp will "entangle" the global variable with the call stack 
            # that keeps the calls at the bottom in order. 
sub f(*@c{
    (temp $in)++;
     "<f>\n"
     ~ @c».indent($in).join("\n")
     ~ (+@c ?? "\n" !! "")
     ~ '</f>'
};
sub g(*@c{
    (temp $in)++;
    "<g>\n"
    ~ @c».indent($in).join("\n")
    ~ (+@c ?? "\n" !! "")
    ~ "</g>"
};
print g(g(f(g()), g(), f()));
 
# OUTPUT: «<g> 
#           <g> 
#            <f> 
#             <g> 
#             </g> 
#            </f> 
#            <g> 
#            </g> 
#            <f> 
#            </f> 
#           </g> 
#          </g>␤» 

The let Prefix

Restores the previous value if the block exits unsuccessfully. A successful exit means the block returned a defined value or a list.

my $answer = 42;
 
{
    let $answer = 84;
    die if not Bool.pick;
    CATCH {
        default { say "it's been reset :(" }
    }
    say "we made it 84 sticks!";
}
 
say $answer;

In the above case, if the Bool.pick returns true, the answer will stay as 84 because the block returns a defined value (say returns true). Otherwise the die statement will cause the block to exit unsuccessfully, resetting the answer to 42.

Type Constraints and Initialization

Variables have a type constraint via the container they are bound to, which goes between the declarator and the variable name. The default type constraint is Mu. You can also use the trait of to set a type constraint.

my Int $x = 42;
$x = 'a string';
CATCH { default { put .^name''.Str } }
# OUTPUT: «X::TypeCheck::Assignment: Type check failed in assignment to $x; expected Int but got Str ("a string")␤» 

If a scalar variable has a type constraint but no initial value, it's initialized with the type object of the default value of the container it's bound to.

my Int $x;
say $x.^name;       # OUTPUT: «Int␤» 
say $x.defined;     # OUTPUT: «False␤» 

Scalar variables without an explicit type constraint are typed as Mu but default to the Any type object.

Variables with the @ sigil are initialized with an empty Array; variables with the % sigil are initialized with an empty Hash.

The default value of a variable can be set with the is default trait, and re-applied by assigning Nil to it:

my Real $product is default(1);
say $product;                       # OUTPUT: «1␤» 
$product *= 5;
say $product;                       # OUTPUT: «5␤» 
$product = Nil;
say $product;                       # OUTPUT: «1␤» 

Default Defined Variables Pragma

To force all variables to have a definedness constraint, use the pragma use variables :D. The pragma is lexically scoped and can be switched off with use variables :_.

use variables :D;
my Int $i;
# OUTPUT: «===SORRY!=== Error while compiling <tmp>␤Variable definition of type Int:D (implicit :D by pragma) requires an initializer ... 
my Int $i = 1# that works 
{ use variables :_my Int $i} # switch it off in this block 

Note that assigning Nil will revert the variable to its default value. The default value of a defined constraint type is the type appended with :D (e.g. Int:D). That means a definedness constraint is no guarantee of definedness. This only applies to variable initializers, not to Signatures. or subsequent assignments to a variable.

Special Variables

Perl 5 is infamous for its many obscure special variables. Perl 6 also has special variables (but only three are extra short).

Other special variables have longer, more descriptive names.

Pre-defined lexical variables

There are three special variables that are available in every block:

Variable Meaning
$_ topic variable
$/ regex match
$! exceptions

The $_ Variable

$_ is the topic variable. It's the default parameter for blocks that do not have an explicit signature, so constructs like for @array { ... } and given $var { ... } bind to $_ by invoking the block.

for <a b c> { say $_ }  # sets $_ to 'a', 'b' and 'c' in turn 
say $_ for <a b c>;     # same, even though it's not a block 
given 'a'   { say $_ }  # sets $_ to 'a' 
say $_ given 'a';       # same, even though it's not a block 

CATCH blocks set $_ to the exception that was caught. The ~~ smart-match operator sets $_ on the right-hand side expression to the value of the left-hand side.

Calling a method on $_ can be shortened by leaving off the variable name:

.say;                   # same as $_.say 

m/regex/ and /regex/ regex matches and s/regex/subst/ substitutions work on $_:

say "Looking for strings with non-alphabetic characters...";
for <ab:c d$e fgh ij*> {
    .say if m/<-alpha>/;
}
 
# OUTPUT: «Looking for strings with non-alphabetic characters... 
#          ab:c 
#          d$e 
#          ij*␤» 

The $/ Variable

$/ is the match variable. It stores the result of the last Regex match and so usually contains objects of type Match.

'abc 12' ~~ /\w+/;  # sets $/ to a Match object 
say $/.Str;         # OUTPUT: «abc␤» 

The Grammar.parse method also sets the caller's $/ to the resulting Match object. For the following code:

use XML::Grammar# zef install XML 
XML::Grammar.parse("<p>some text</p>");
say $/;
 
# OUTPUT: «「<p>some text</p>」 
#           root => 「<p>some text</p>」 
#            name => 「p」 
#            child => 「some text」 
#             text => 「some text」 
#             textnode => 「some text」 
#           element => 「<p>some text</p>」 
#            name => 「p」 
#            child => 「some text」 
#             text => 「some text」 
#             textnode => 「some text」␤» 

Positional Attributes

$/ can have positional attributes if the Regex had capture-groups in it, which are just formed with parentheses.

'abbbbbcdddddeffg' ~~ / a (b+) c (d+ef+) g /;
say $/[0]; # OUTPUT: «「bbbbb」␤» 
say $/[1]; # OUTPUT: «「dddddeff」␤» 

These can also be accessed by the shortcuts $0, $1, $2, etc.

say $0# OUTPUT: «「bbbbb」␤» 
say $1# OUTPUT: «「dddddeff」␤» 

To get all of the positional attributes, any of $/.list, @$/, or @() can be used.

say @().join# OUTPUT: «bbbbbdddddeff␤» 

Named Attributes

$/ can have named attributes if the Regex had named capture-groups in it, or if the Regex called out to another Regex.

'I.... see?' ~~ / \w+ $<punctuation>=[ <-[\w\s]>+ ] \s* $<final-word> = [ \w+ . ] /;
say $/<punctuation># OUTPUT: «「....」␤» 
say $/<final-word>;  # OUTPUT: «「see?」␤» 

These can also be accessed by the shortcut $<named>.

say $<punctuation># OUTPUT: «「....」␤» 
say $<final-word>;  # OUTPUT: «「see?」␤» 

To get all of the named attributes, any of $/.hash, %$/, or %() can be used.

say %().join;       # OUTPUT: «"punctuation     ....final-word  see?"␤» 

The $! Variable

$! is the error variable. If a try block or statement prefix catches an exception, that exception is stored in $!. If no exception was caught, $! is set to the Any type object.

Note that CATCH blocks do not set $!. Rather they set $_ inside the block to the caught exception.

Compile-time variables

$?FILE Which file am I in?
$?LINE Which line am I at?
::?CLASS Which class am I in?
&?BLOCK Which block am I in?
%?LANG What is the current set of interwoven languages?
%?RESOURCES The files associated with the "Distribution" of the current compilation unit.

&?ROUTINE

Which routine am I in? Has method .name

sub awesome-sub { say &?ROUTINE.name }
awesome-sub # OUTPUT: awesome-sub 
for '.' {
    .Str.say when !.IO.d;
    .IO.dir()».&?BLOCK when .IO.d # lets recurse a little! 
}

Other compile-time variables:

$?PACKAGE Which package am I in?
$?MODULE Which module am I in?
$?CLASS Which class am I in? (as variable)
$?ROLE Which role am I in? (as variable)
$?GRAMMAR Which grammar am I in?
$?TABSTOP How many spaces is a tab in a heredoc or virtual margin?
$?USAGE The usage message generated from the signatures of MAIN subs.
$?ENC Default encoding of Str.encode/Buf.decode/various IO methods.

Dynamic variables

$*ARGFILES An IO::CatHandle that uses @*ARGS as source files, if it contains any files, or $*IN otherwise
@*ARGS Arguments from the command line.
$*IN Standard input filehandle, AKA stdin
$*OUT Standard output filehandle, AKA stdout
$*ERR Standard error filehandle, AKA stderr
%*ENV Environment variables
$*REPO A variable holding information about modules installed/loaded
$*TZ The system's local timezone.
$*CWD The Current Working Directory.
$*KERNEL Which kernel am I running under?
$*DISTRO Which OS distribution am I running under? (e.g. C<say "Some sort of Windows" if $*DISTRO.is-win>)
$*VM Which virtual machine am I running under?
$*PERL Which Perl am I running under?
$*PID Process ID of the current process.
$*PROGRAM-NAME Path to the current executable as it was entered on the command line, or C<-e> if perl was invoked with the -e flag.
$*PROGRAM Location (in the form of an C<IO::Path> object) of the Perl program being executed.
$*EXECUTABLE Absolute path of the perl executable that is currently running.
$*EXECUTABLE-NAME The name of the perl executable that is currently running. (e.g. perl6-p, perl6-m, Niecza.exe) Favor $*EXECUTABLE because it's not guaranteed that the perl executable is in PATH.
$*USER The user that is running the program. It's an object that evaluates to "username (uid)". It will evaluate to the username only if treated as a string and the numeric user id if treated as a number.
$*GROUP The primary group of the user who is running the program. It's an object that evaluates to "groupname (gid)". It will evaluate to the groupname only if treated as a string and the numeric group id if treated as a number.
$*HOME An L<IO::Path> object representing the "home directory" of the user that is running the program. Uses C«%*ENV<HOME>» if set. On Windows, uses C«%*ENV<HOMEDRIVE> ~ %*ENV<HOMEPATH>». If the home directory cannot be determined, it will be L<Any>
$*SPEC The appropriate L<IO::Spec> sub-class for the platform that the program is running on.
$*TMPDIR An L<IO::Path> object representing the "system temporary directory" as determined by L«C<.tmpdir IO::Spec::* method|/routine/tmpdir»
$*TOLERANCE Used by the C<=~=> operator, and any operations that depend on it, to decide if two values are approximately equal. Defaults to 1e-15.
$*THREAD A L<Thread> object representing the currently executing thread.
$*SCHEDULER A L<ThreadPoolScheduler> object representing the current default scheduler. (see note below)

Note on usage of $*SCHEDULER:

For the current Rakudo, by default this imposes a maximum of 16 threads on the methods .hyper and .race. To change the maximum number of threads, either set the environment variable RAKUDO_MAX_THREADS before running perl6 or create a scoped copy with the default changed before using .hyper or .race:

my $*SCHEDULER = ThreadPoolScheduler.newmax_threads => 64 );

This behavior is not tested in the spec tests and is subject to change.