class Attribute { }

In Perl 6 lingo, an attribute refers to a per-instance/object storage slot. An Attribute is used to talk about classes' and roles' attributes at the metalevel.

Normal usage of attributes does not require the user to use this class explicitly.

Traits

Trait is default

An attribute that is assigned Nil will revert to its default value set with the trait is default. In the case of arrays or associatives, the argument of is default will set the default item value or hash value.

class C {
    has $.a is default(42) is rw = 666
}
my $c = C.new;
say $c;
$c.a = Nil;
say $c;
# OUTPUT: «C.new(a => 666)␤C.new(a => 42)␤» 
class Foo {
    has @.bar is default(42) is rw
};
my $foo = Foo.new( bar => <a b c> );
$foo.bar =Nil;
say $foo; # OUTPUT: «Foo.new(bar => [42])␤»

Trait is required

Defined as:

multi sub trait_mod:<is> (Attribute $attr, :$required!)

The trait is required will mark the attribute as to be filled with a value when the object is instantiated. Failing to do so will result in a runtime error.

class C {
    has $.a is required
}
my $c = C.new;
CATCH{ default { say .^name, ': ', .Str } }
# OUTPUT: «X::Attribute::Required: The attribute '$!a' is required, but you did not provide a value for it.␤»

Available as of 6.d language version (early implementation exists in Rakudo compiler 2018.08+): You can specify a reason why the attribute is required:

class D {
    has $.a is required("it is a good idea");
}
my $d = D.new;
CATCH{ default { say .^name, ': ', .Str } }
# OUTPUT: «X::Attribute::Required: The attribute '$!a' is required because it is a good idea,␤but you did not provide a value for it.␤» 

is required doesn't just affect the default constructor, it checks for the attribute at a lower level, so it will work for custom constructors written using bless.

trait is DEPRECATED

multi sub trait_mod:<is>(Attribute:D $r, :$DEPRECATED!)

Marks an attribute as deprecated, optionally with a message what to use instead.

class C {
    has $.foo is DEPRECATED("'bar'");
}
my $c = C.new( foo => 42 );  # doesn't trigger with initialization (yet) 
say $c.foo;                  # does trigger on usage

After the program is finished, this will show something like this on STDERR:

# Saw 1 occurrence of deprecated code. 
# ===================================== 
# Method foo (from C) seen at: 
# script.p6, line 5 
# Please use 'bar' instead.

trait is rw

Defined as:

multi sub trait_mod:<is> (Attribute:D $attr, :$rw!)

Marks an attribute as read/write as opposed to the default readonly. The default accessor for the attribute will return a writable value.

class Boo {
   has $.bar is rw;
   has $.baz;
};
 
my $boo = Boo.new;
$boo.bar = 42; # works 
$boo.baz = 42;
CATCH { default { put .^name, ': ', .Str } };
# OUTPUT: «X::Assignment::RO: Cannot modify an immutable Any␤»

Methods

The usual way to obtain an object of type Attribute is by introspection:

class Useless {
    has @!things;
}
my $a = Useless.^attributes(:local)[0];
say $a.perl;            # OUTPUT: «Attribute.new␤» 
say $a.name;            # OUTPUT: «@!things␤» 
say $a.package;         # OUTPUT: «(Useless)␤» 
say $a.has_accessor;    # OUTPUT: «False␤»

Modifying a private attribute from the outside is usually not possible, but since Attribute is at the level of the metaclass, all is fair game.

method name

Defined as:

method name(Attribute:D: --> Str:D)

Returns the name of the attribute. Note that this is always the private name, so if an attribute is declared as has $.a, the name returned is $!a.

class Foo {
    has @!bar;
}
my $a = Foo.^attributes(:local)[0];
say $a.name;            # OUTPUT: «@!bar␤»

method package

Defined as:

method package()

Returns the package (class/grammar/role) to which this attribute belongs.

class Boo {
    has @!baz;
}
my $a = Boo.^attributes(:local)[0];
say $a.package;         # OUTPUT: «(Boo)␤»

method has_accessor

Defined as:

method has_accessor(Attribute:D: --> Bool:D)

Returns True if the attribute has a public accessor method.

class Container {
    has $!private;
    has $.public;
}
my $private = Container.^attributes(:local)[0];
my $public = Container.^attributes(:local)[1];
say $private.has_accessor; # OUTPUT: «False␤» 
say $public.has_accessor;  # OUTPUT: «True␤»

method rw

Defined as:

method rw(Attribute:D: --> Bool:D)

Returns True for attributes that have the "is rw" trait applied to them.

class Library {
    has $.address; # Read-only value 
    has @.new-books is rw;
}
my $addr = Library.^attributes(:local)[0];
my $new-books = Library.^attributes(:local)[1];
say $addr.rw;      # OUTPUT: «False␤» 
say $new-books.rw; # OUTPUT: «True␤»

method readonly

Defined as:

method readonly(Attribute:D: --> Bool:D)

Returns True for readonly attributes, which is the default, or False for attributes marked as is rw.

class Library {
    has $.address; # Read-only value 
    has @.new-books is rw;
}
my $addr = Library.^attributes(:local)[0];
my $new-books = Library.^attributes(:local)[1];
say $addr.readonly;      # OUTPUT: «True␤» 
say $new-books.readonly; # OUTPUT: «False␤»

method required

Defined as:

method required(Attribute:D: --> Any:D)

Returns 1 for attributes that have the "is required" trait applied, or Mu if the attribute did not have that trait applied. If the "is required" trait is applied with a string, then that string will be returned instead of 1.

class Library {
    has $.address is required;
    has @.new-books is required("we always need more books");
}
my $addr = Library.^attributes(:local)[0];
my $new-books = Library.^attributes(:local)[1];
say $addr.required;      # OUTPUT: «1␤» 
say $new-books.readonly; # OUTPUT: «"we always need more books"␤»

method type

Defined as:

method type(Attribute:D: --> Mu)

Returns the type constraint of the attribute.

class TypeHouse {
    has Int @.array;
    has $!scalar;
    has @.mystery;
}
my @types = TypeHouse.^attributes(:local)[0..2];
for 0..2 { say @types[$_].type }
# OUTPUT: «(Positional[Int]) 
# (Mu) 
# (Positional)␤»

method get_value

Defined as:

method get_value(Mu $obj)

Returns the value stored in this attribute of object $obj.

class Violated {
    has $!private-thing = 5;
}
my $private = Violated.^attributes(:local)[0];
say $private.get_value(Violated.new); # OUTPUT: «5␤»

Note that this method violates encapsulation of the object, and should be used with care. Here be dragons.

method set_value

Defined as:

method set_value(Mu $obj, Mu \new_val)

Binds the value new_val to this attribute of object $obj.

class A {
    has $!a = 5;
    method speak() { say $!a; }
}
my $attr = A.^attributes(:local)[0];
my $a = A.new;
$a.speak; # OUTPUT: «5␤» 
$attr.set_value($a, 42);
$a.speak; # OUTPUT: «42␤»

Note that this method violates encapsulation of the object, and should be used with care. Here be dragons.

method gist

Defined as

multi method gist(Attribute:D:)

Returns the name of the type followed by the name of the attribute.

class Hero {
    has @!inventory;
    has Str $.name;
    submethod BUILD( :$name, :@inventory ) {
        $!name = $name;
        @!inventory = @inventory
    }
}
say Hero.^attributes(:local)[0]; # OUTPUT: «Positional @!inventory» 

Since say implicitly calls .gist, that is what produces the output here.

Optional introspection

DEPRECATED

If an attribute is marked as DEPRECATED, then calling the DEPRECATED method is possible and will return "something else" (if no specific reason was specified) or the string that was specified with the DEPRECATED trait.

If an attribute is not marked as DEPRECATED, one cannot not call the DEPRECATED method. Therefore, the .?method syntax should be used.

class Hangout {
    has $.table;
    has $.bar is DEPRECATED("the patio");
}
my $attr-table = Hangout.^attributes(:local)[0];
my $attr-bar = Hangout.^attributes(:local)[1];
with $attr-table.?DEPRECATED -> $text {     # does not trigger 
    say "Table is deprecated with '$text'";
    # OUTPUT: 
}
with $attr-table.?DEPRECATED -> $text {
    say "Bar is deprecated with '$text'";
    # OUTPUT: «Bar is deprecated with 'the patio'"␤» 
}

Type Graph

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