Perl 6 variables

Variable names start with a special character called a sigil, followed optionally by a second special character named twigil, and then an identifier.

Sigils

The sigil serves both as rough type constraint, and as an indicator as to whether the contents of the variable flatten in list context. See also the documentation in List.

Sigil Type constraint Default type Flattens Assignment
$ Mu (no type constraint) Any No item
& Callable Callable No item
@ Positional Array Yes list
% Associative Hash Yes list

Examples:

my $square = 9 ** 2;
my @array  = 1, 2, 3;   # Array variable with three elements
my %hash   = London => 'UK', Berlin => 'Germany';

There are two types of assignment, item assignment and list assignment. Both use the equal sign = as operator. The distinction whether an = means item or list assignment is based on the syntax of the left-hand side. (TODO: explain in detail, or do that in operators).

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. Contrary to item assignment, it means that the type of the variable on the left always stays Array, regardless of the type of the right-hand side.

Note that the item assignment has tighter precedence than list assignment, and also tighter than the comma. See operators for more details.

Twigils

Twigils influence the scoping of a variable. Please be aware that twigils 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
* ! ? . < ^ : dynamic attribute (class member) compile-time "constant" method (not really a variable) index into match object (not really a variable) self-declared formal positional parameter self-declared formal named parameter
~ the sublanguage seen by the parser at this lexical spot

*

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 callers 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.

!

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

    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 objects.

?

Compile-time "constants" 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".

Although they may not be changed at runtime, the user is allowed to (re)define such constants.

constant $?TABSTOP = 4; # this causes leading tabs in a heredoc or in a POD
                        # block's virtual margin to be counted as 4 spaces.

For a list of those special variables see Compile-time "constants".

.

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

    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 as you declared your attributes. 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 just does a method call also implies that the following is possible too.

    class SaySomething {
        method a() { say "a"; }
        method b() { $.a; }
    }

    SaySomething.a; # prints "a"

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

<

The < twigil is just an alias for $/<...> where $/ is the match variable. For more information on the match variable see $/.

^

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

for ^4 {
    say "$^seconds follows $^first";
}

which prints

1 follows 0
3 follows 2

has two formal parameters, namely $first and $second. Note that even though $^second appears before $^first in the code, $^first 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.

Subroutines may also make use of placeholder variables but only if they do not have an explicit parameter list. This is true for normal blocks too.

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

Placeholder variables syntactically cannot have any type constraints. Be also aware that one can not have placeholder variables with a single upper-case letter. This is disallowed in favor of being to able to catch some Perl 5-isms.

:

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

See ^ for more details about placeholder variables.

=

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 Foo
...
=end Foo

#after that, $=Foo gives you all Foo-Pod-blocks

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 role.

~

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 may supersede or augment those languages in your current lexical scope by doing

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

or

supersede slang Regex { # completely substitute $~Regex
    ...
}

Special Variables

Often-Used Variables

# TODO: find a better heading

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

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

$_

$_ is the topic variable. It is the default parameter for blocks that do not have an explicit signature, so constructs like for @array { ... } and given $var { ... } binds to $_ simply 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 $_.

$/

$/ is the match variable. It stores the result of each regex match, and usually contains objects of type Match.

'abc 12' ~~ /\w+/;  # sets $/ to a Match object
say $/.Str;         # abc

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

Other match variables are aliases to elements of $/:

$0          # same as $/[0]
$1          # same as $/[1]
$<named>    # same as $/<named>

$!

$! 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 "constants"

$?FILE      Which file am I in?
$?LINE      Which line am I at?
&?ROUTINE   Which routine am I in?
&?BLOCK     Which block am I in?
%?LANG      What is the current set of interwoven languages?

Other compile-time constants:

$?KERNEL    Which kernel am I compiled for?
$?DISTRO    Which OS distribution am I compiling under
$?VM        Which virtual machine am I compiling under
$?XVM       Which virtual machine am I cross-compiling for
$?PERL      Which Perl am I compiled for?
$?SCOPE     Which lexical scope am I in?
$?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  Magic command-line input handle.
@*ARGS      Arguments from the command line.
$*IN        Standard input filehandle.
$*OUT       Standard output filehandle.
$*ERR       Standard error filehandle.
$*TZ        The system's local timezone.
$*CWD       The Current Working Directory.

Other variables

$*PROGRAM_NAME     Path to the current executable as it was typed in on the
                   command line, or C<-e> if perl was invoked with the -e flag.
$*PID              Process ID of the current process.
$*OS               Which Operating System am I compiling under (e.g. Linux).
$*OSVER            Version of the current Operating System.
$*EXECUTABLE_NAME  The name of the perl executable that is currently running.