RFC 159, by Nathan Wiger: True Polymorphic Objects

Proposed on 25 August 2000, frozen on 16 September 2000

On polymorphism

RFC159 introduces the concept of true polymorphic object.

Objects that can morph into numbers, strings, booleans and much more on-demand. As such, objects can be freely passed around and manipulated without having to care what they contain (or even that they’re objects).

When one looks at how 42, "foo", now work in Raku nowadays, one can only see that that vision has pretty much been implemented. Because most of the time, one doesn’t really care about the fact that 42 is really an Int object, "foo" is really a Str object and that now represents a new Instant object every time it is called. The only thing one cares about, is that they can be used in expressions:

say "foo" ~ "bar";  # foobar
say 42 + 666;       # 708
say now - INIT now; # 0.0005243

RFC159 lists a number of method names to be used to indicate how an object should behave under certain circumstances, with a fallback provided by the system if the class of the object does not provide that method. In most cases these methods did not make it into Raku, but some of them did with a different name:

Name in RFC Name in Raku When
STRING Str Called in a string context
NUMBER Numeric Called in a numeric context
BOOLEAN Bool Called in a boolean context

And some of them even retained their name:

Name in RFC When
BUILD Called in object blessing
STORE Called in an lvalue = context
FETCH Called in an rvalue = context
DESTROY Called in object destruction

but with sometimes subtly different semantics from the RFC.

Only a few made it

In the end, only a limited set of special methods was decided on for Raku. All of the other methods in RFC159 have been implemented by polymorphic operators that coerce when needed. For instance the proposed PLUS method has been implemented as an infix + operator that has a “default” candidate that coerces its operands to a number.

So, effectively, if you have an object of class Foo and you want that to act as a number, one only needs to add a Numeric method to that class. An expression such as:

my $foo = Foo.new;
say $foo + 42;

is effectively executing:

say infix:<+>( $foo, 42 );

and the infix:<+> candidate that takes Any objects, does:

return infix:<+>( $foo.Numeric, 42.Numeric );

And if such a class Foo does not provide a Numeric method, then it will throw an exception.

The DESTROY method

In Raku, object destruction is non-deterministic. If an object is no longer in use, it will probably get garbage collected. The probable part is because Raku does not know a global destruction phase, unlike Perl. So when a program is done, it just does an exit (although that logic does honour any END blocks).

An object is marked “ready for removal” when it can no longer be “reached”. It then has its DESTROY method called when the garbage collection logic kicks in. Which can be any amount of time after it became unreachable.

If you need deterministic calling of the DESTROY method, you can use a LEAVE phaser. Or if that doesn’t allow you to scratch your itch, you can possibly use the FINALIZER module.

STORE / FETCH on scalar values

Conceptually, you can think of a container in Raku as an object with STORE and FETCH methods. Whenever you set a value in a container, it conceptually calls the STORE method. And whenever the value inside the container is needed, it conceptually calls the FETCH method. In pseudo-code:

my $foo = 42;  # Scalar.new(:name<$foo>).STORE(42)

But what if you want to control access to a scalar value, similar to Perl’s tie? Well, in Raku you can, with a special type of container class called Proxy. An example of its usage:

sub proxier($value? is copy) {
    return-rw Proxy.new(
        FETCH => method { $value },
        STORE => method ($new) {
            say "storing";
            $value = $new
        }
    )
}

my $a := proxier(42);
say $a;    # 42
$a = 666;  # storing
say $a;    # 666

Subroutines return their result values de-containerized by default. There are basically two ways of making sure the actual container is returned: using return-rw (like in this example), or by marking the subroutine with the is rw trait.

STORE on compound values

Since FETCH only makes sense on scalar values, there is no support for FETCH on compound values, such as hashes and arrays, in Raku. I guess one could consider calling FETCH in such a case to be the Zen slice, but it was decided that that would just return the compound value itself.

The STORE method on compound values however, allows for some interesting functionality. The STORE method is called whenever there is an initialization of the entire compound value. For instance:

@a = 1,2,3;

basically executes:

@a := @a.STORE( (1,2,3) );

But what if you don’t have an initialized @a yet? Then the STORE method is supposed to actually create a new object and initialize this with the given values. And the STORE method can tell, because then it also receives a INITIALIZE named argument with a True value. So when you write this:

my @b = 1,2,3;

what basically gets executed is:

@b := Array.new.STORE( (1,2,3), :INITIALIZE );

Now, if you realize that:

my @b;

is actually short for:

my @b is Array;

it’s only a small step to realize that you can create your own class with customized array logic, that can replace the standard Array logic with your own. Observe:

class Foo {
    has @!array;
    method STORE(@!array) {
        say "STORED @!array[]";
        self
    }
}

my @b is Foo = 1,2,3;  # STORED 1 2 3

However, when you actually start using such an array, you are confronted with some weird results:

say @b[0]; # Foo.new
say @b[1]; # Index out of range. Is: 1, should be in 0..0

Without getting into the reasons for these results, it should be clear that to completely mimic an Array, a lot more is needed. Fortunately, there are ecosystem modules available to help you with that: Array::Agnostic for arrays, and Hash::Agnostic for hashes.

BUILD

The BUILD method also subtly changed its semantics. In Raku, method BUILD will be called as an object method and receive all of the parameters given to .new, after which it is fully responsible for initializing object attributes. This becomes more visible when you use the internal helper module BUILDPLAN. This module shows the actions that will be performed on an object of a class when built with the default .new method:

class Bar {
    has $.score = 42;
}
use BUILDPLAN Bar;
# class Bar BUILDPLAN:
#  0: nqp::getattr(obj,Foo,'$!score') = :$score if possible
#  1: nqp::getattr(obj,Foo,'$!score') = 42 if not set

This is internals speak for: – assign the value of the optional named argument score to the $!score attribute – assign the value 42 to the $!score attribute if it was not set already

Now, if we add a BUILD method to the class, the buildplan changes:

class Bar {
    has $.score = 42;
    method BUILD() { }
}
use BUILDPLAN Bar;
# class Bar BUILDPLAN:
#  0: call obj.BUILD
#  1: nqp::getattr(obj,Foo,'$!score') = 42 if not set

Note that there is no automatic attempt to take the value of the named argument score anymore. Which means that you need to do a lot of work in your custom BUILD method if you have many named arguments, and only one of them needs special handling. That’s why the TWEAK method was added:

class Bar {
    has $.score = 42;
    method TWEAK() { }
}
use BUILDPLAN Bar;
# class Bar BUILDPLAN:
#  0: nqp::getattr(obj,Foo,'$!score') = :$score if possible
#  1: nqp::getattr(obj,Foo,'$!score') = 42 if not set
#  2: call obj.TWEAK

Note that the TWEAK method is called after all of the normal checks and initializations. This is in most cases much more useful.

Conclusion

Although the idea of true polymorphic objects has been implemented in Raku, it turned out quite different from originally envisioned. In hindsight, one can see why it was decided to be unpractical to try to support an ever increasing list of special methods for all objects. Instead, a choice was made to only implement a few key methods from the proposal, and for the others the approach of automatic coercions was taken.

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