Where Have All The References Gone?

Perl 5 programmers that start to learn Perl 6 often ask me how to take a reference to something, and my answers usually aren’t really helpful. In Perl 6, everything that can be held in a variable is an object, and objects are passed by reference everywhere (though you don’t always notice that, because objects like strings and numbers are immutable, so there’s no difference to passing by value). So, everything is already treated as a reference in some sense, and there’s no point in explicitly taking references.

But people aren’t happy with that answer, because it doesn’t explain how to get stuff done that involved references in Perl 5. So here are a few typical use cases of references, and how Perl 6 handles them.

Creating Objects

In Perl 5, an object is really just a reference to a blessed value (but people usually say "blessed reference", because you virtually never use the blessed value without going through a reference).

So, in Perl 5 you’d write

 package My::Class;
 # constructor
 sub new { bless {}, shift };
 # an accessor
 sub foo {
     my $self = shift; 
     # the ->{} dereferences $self as a hash
     $self->{foo} // 5;
 }

 # use the object:
 say My::Class->new->foo;

In Perl 6, you just don’t think about references; classes are much more declarative, and there’s no need for dereferencing anything anywhere:

 class My::Class {
     # attribute with accessor (indicated by the dot)
     # and default value
     has $.foo = 5;
 }

 # use it:
 say My::Class.new.foo

If you don’t like the default constructor, you can still use bless explicitly, but even then you don’t have to think about references:

 method new() {
     # the * specifies the storage, and means "default storage"
     self.bless(*);
 }

So, no explicit reference handling when dealing with OO. Great.

Nested Data Structures

In both Perl 5 and Perl 6, lists flatten automatically by default. So if you write

 my @a = (1, 2);
 my @b = (3, 4);
 push @a, @b

then @a ends up with the four elements 1, 2, 3, 4, not with three elements of which the third is an array.

In Perl 5, nesting the data structure happens by taking a reference to @b:

 push @a, \@b;

In Perl 6, item context replaces this use of references. It is best illustrated by a rather clumsy method to achieve the same thing:

 my $temp = @b;
 push @a, $temp;  # does not flatten the two items in $temp,
                 # because $temp is a scalar

Of course there are shortcuts; the following lines work too:

 push @a, $(@b);
 push @a, item @b;

(As a side note, push @a, $@b is currently not allowed, it tries to catch a p5ism; I will also try to persuade Larry and the other language designers to allow it, and have it mean the same thing as the other two).

On the flip side you need explicit dereferencing to get the values out of item context:

 my @a = 1, 2;
 my $scalar = @a;
 for @a { 
     # two iterations
 }
 for $scalar {
     # one iteration only
 }
 for @$scalar {
     # two iterations again
 }

This explicit use of scalar and list context is the closest analogy to Perl 5 references, because it requires explicit context annotations in the same places where referencing and dereferencing is used in Perl 5.

But it’s not really the same, because there are cases where Perl 5 needs references, but Perl 6 can deduce the item context all on its own:

 @a[3] = @b; # automatically puts @b in item context

Mutating Arguments

Another use references in Perl 5 is for passing data to routines that should be modified inside the routine:

 sub set_five; {
     my $x = shift;
     # explicit dereferencing with another $:
     $$x = 5;
 }
 my $var;
 # explicit taking of a reference
 set_five \$var;

In Perl 6, there is a separate mechanism for this use case:

 sub set_five($x is rw) {
     # no dereferencing
     $x = 5;
 }
 my $var;
 # no explicit reference taking
 set_five $var;

So again a use case of Perl 5 references is realized by another mechanism in Perl 6 (signature binding, or binding in general).

Summary

Nearly everything is a reference in Perl 6, but you still don’t see them, unless you take a very close look. The control of list flattening with item and list context is the one area where Perl 5’s referencing and dereferencing shines through the most.