class Variable {}
Variables have a wealth of compile-time information, but at runtime, accesses to a variable usually act on the value stored inside it, not the variable itself. The runtime class of a variable is Scalar.
Class Variable holds the compile-time information that traits can use to introspect and manipulate variables.
Traits§
trait is default§
Sets the default value with which a variable is initialized, and to which it is reset when Nil is assigned to it. Trait arguments are evaluated at compile time. Closures won't do what you expect: they are stored as is and need to be called by hand.
my Int $x is default(42); say $x; # OUTPUT: «42» $x = 5; say $x; # OUTPUT: «5» # explicit reset: $x = Nil; say $x; # OUTPUT: «42»
The trait is default can be used also with subscripting things like arrays and hashes:
my @array is default( 'N/A' ); @array[22].say; # OUTPUT: N/A @array = Nil; @array.say; # OUTPUT: [N/A] @array[4].say; # OUTPUT: N/A my %hash is default( 'no-value-here' ); %hash<non-existent-key>.say; # OUTPUT: no-value-here %hash<foo> = 'bar'; %hash<>.say; # OUTPUT: {foo => bar} %hash<wrong-key>.say; # OUTPUT: no-value-here
trait is dynamic§
multi trait_mod:<is>(Variable:D, :$dynamic)
Marks a variable as dynamic, that is, accessible from inner dynamic scopes without being in an inner lexical scope.
sub introspect() { say $CALLER::x; } my $x is dynamic = 23; introspect; # OUTPUT: «23» { # not dynamic my $x; introspect() # dies with an exception of X::Caller::NotDynamic }
The is dynamic trait is a rather cumbersome way of creating and accessing dynamic variables. A much easier way is to use the * twigil:
sub introspect() { say $*x; } my $*x = 23; introspect; # OUTPUT: «23» { # not dynamic my $x; introspect() # dies with an exception of X::Dynamic::NotFound }
trait of§
multi trait_mod:<of>(Mu:U $target, Mu:U $type)
Sets the type constraint of a container bound to a variable.
my $i of Int = 42; $i = "forty plus two"; CATCH { default { say .^name, ' ', .Str } } # OUTPUT: «X::TypeCheck::Assignment Type check failed in assignment to $i; expected Int but got Str ("forty plus two")»
You can use any value defined in compile time as a type constraint, including constants:
constant \T = Int; my $i of T = 42;
which would be equivalent to the previous definition.
Methods§
method name§
method name(Variable:D: str)
Returns the name of the variable, including the sigil.
Typegraph§
Variable