=head1 TITLE

Parrot Vtables

=head1 Implementing Variable Types with Vtables

This is a guide to creating your own PMC (Parrot Magic Cookie) classes.
It tells you what you need to write in order to add new variable types
to Parrot.

=head2 Overview

The guts of the Parrot interpreter are by design ignorant (or, if you
want to be less disparaging, agnostic) of the intricacies of variable
type behaviour. The standard example is the difference between Perl
scalars and Python scalars. In Perl, if you have

    $a = "a9";
    $a++;

you end up with C<$a> being C<b0>. This is because of the magic of the Perl
increment operator. In Python, on the other hand, you'd get a
runtime error.

=over 3

=item *

To be perfectly honest, this is a slightly flawed example, since it's
unlikely that there will be a distinct "Python scalar" PMC class.
The Python compiler could well type-inference variables such that
C<a> would be a C<PythonString> and C<b> would be a C<PythonNumber>.
But the point remains - incrementing a C<PythonString> is very different
from incrementing a C<PerlScalar>.

=back

Since the behaviour is a function of the "type" of the PMC, it's
natural to consider the various different types of PMC as classes
in an object-oriented system. The Parrot interpreter calls methods on
the individual PMC objects to manipulate them. So the example above
would translate to something like:

=over 3

=item 1.

Construct a new PMC in the PerlScalar class.

=item 2.

Call a method setting its string value to C<"a9">.

=item 3.

Call a method to tell it to increment itself.

=back

And if you replace PerlScalar with PythonString, you get different
behaviour but to the fundamental guts of the interpreter, the
instructions are the same. PMCs are an abstract virtual class; the
interpreter calls a method, the PMC object does the right thing, and the
interpreter shouldn't have to care particularly what that right thing
happens to be.

Hence, adding a new data type to Parrot is a question of providing
methods which implement that data type's expected behaviour. Let's
now look at how one is supposed to do this.

=head2 Starting out

If you're adding data types to the core of Parrot, (and you've
checked with Dan and/or Steve that you're supposed to be doing so)
you should be creating a file in the F<classes/> subdirectory; this is
where all the built-in PMC classes live. (And a good source of examples
to plunder even if you're not writing a core data type.)

You should almost always start by running F<genclass.pl> found in the
F<classes/> subdirectory to generate a skeleton for the class. Let's
generate a number type for the beautifully non-existent Fooby language:

    perl -I../lib genclass.pl FoobyNumber > foobynumber.pmc

This will produce a skeleton C file (to be preprocessed by the
F<pmc2c.pl> program) with stubs for all the methods you need to fill in.
The function C<init> allows you to set up anything you need to set up.

Now you'll have to do something a little different depending on whether
you're writing a built-in class or an extension class. If you're writing
a built-in class, then you'll see a reference to
C<enum_class_FoobyNumber> in the C<init> function. For built-in classes,
this is automatically defined in F<pmc.h> when you run Configure.pl.
If you're not writing a built-in class, you need to indicate this by
using the 'extension' keyword after the 'pmclass YOURCLASS' declaration
in classes/YOURCLASS.pmc. Then, change the
type of the C<init> function to return C<struct _vtable>, and then
return C<temp_base_vtable> instead of assigning to the
C<Parrot_base_vtables> array.

To finish up adding a built-in class:

=over 4

=item 1.

Add classes/YOURCLASS.pmc to the MANIFEST

=item 2.

Rerun Configure.pl to add your new PMC to the set of built-in PMCs.

=back

=head2 What You Can and Cannot Do

The usual way to continue from the F<genclass.pl>-generated skeleton
is to define a structure that will hook onto the C<data>, if your data
type needs to use that, and then also define some user-defined flags.

Flags are accessed by C<< pmc->flags >>. Most of the bits in the flag word
are reserved for use by parrot itself, but a number of them have been
assigned for general use by individual classes. These are referred to as
C<Pobj_private0_FLAG> .. C<Pobj_private7_FLAG>. (The '7' may change during the
early development of parrot, but will become pretty fixed at some point.)

Normally, you will want to alias these generic bit names to something
more meaningful within your class:

    enum {
        Foobynumber_is_bignum = Pobj_private0_FLAG,
        Foobynumber_is_bigint = Pobj_private1_FLAG,
        ....
    };

You're quite at liberty to declare these in a separate header file, but I
find it more convenient to keep everything together in F<foobynumber.c>.
To manipulate the flags, use the macros listed in F<pobj.h>.

You may also use the C<cache> union in the PMC structure to remove
some extraneous dereferences in your code if that would help.

=head2 Multimethods

One slightly (potentially) tricky element of implementing vtables
is that several of the vtable functions have variant forms depending on
the type of data that they're being called with.

For instance, the C<set_integer> method has multiple forms; the
default C<set_integer> means that you are being called with a PMC,
and you should probably use the C<get_integer> method of the PMC to
find its integer value; C<set_integer_native> means you're being passed
an C<INTVAL>. The final form is slightly special; if the
interpreter calls C<set_integer_same>, you know that the PMC that you
are being passed is of the same type as you. Hence, you can break the
class abstraction to save a couple of dereferences - if you want to.

Similar shortcuts exist for strings, (C<native> and C<same>) and
floating point numbers.

=head2 Implementing methods

The master list of vtable methods can be found in F<vtable.tbl> in
the root directory of the Parrot source, with documentation in
F<docs/pdds/pdd02_vtables.pod>. A few of these are very important, 
for instance:

=over 3

=item C<type>

Return the enumeration value of your class.

=item C<name>

Return a string containing your class name.

=item C<init>

Do any data set-up you need to do.

=item C<is_equal>

True if the passed-in PMC has the same B<value> as you. For instance,
a Perl integer and a Python integer could have the same value, but
could not be the same thing as defined by C<is_same>.

=item C<clone>

Copy your data and state into the passed-in destination PMC.

=back

Others are methods you may or may not need, depending on your type:

=over 3

=item C<morph>

Turn yourself into the specified type.

=item C<destroy>

Do any data shut-down and finalization you need to do. To have this
method called, you must set the C<Pobj_active_destroy_FLAG>.

=item C<get_integer>

Return an integer representation of yourself.

=item C<get_number>

Return a floating-point representation of yourself.

=item C<get_string>

Return a string representation of yourself (a STRING* object), this
should be a B<copy> of whatever string you are holding, not just a
pointer to your own string so that anything that calls this method can
happily modify this value without making a mess of your guts.

=item C<get_bool>

Return a boolean representation of yourself.

=item C<get_value>

Return your private data as a raw pointer.

=item C<is_same>

True if the passed-in PMC refers to exactly the same B<data> as
you. (Contrast C<is_equal>)

=item C<set_integer>

Set yourself to the passed-in integer value. This is an integer
multimethod.

=item C<set_number>

Set yourself to the passed-in float value. This is an floating-point
multimethod.

=item C<set_string>

Set yourself to the passed-in string. This is a string multimethod.

=item C<set_value>

Set your private data to the raw pointer passed in. This will only
be used in exceptional circumstances.

=item C<add>

Fetch the number part of C<value> and add your numeric value to it,
storing the result in C<dest>. (Probably by calling its C<set_integer>
or C<set_number> method) This is a numeric multimethod.

=item C<subtract>

Fetch the number part of C<value> and subtract your numeric value from it,
storing the result in C<dest>. (Probably by calling its C<set_integer>
or C<set_number> method) This is a numeric multimethod.

=item C<multiply>

=item C<divide>

=item C<modulus>

You get the picture.

=item C<concatenate>

Fetch the string part of C<value> and catenate it to yourself,
storing the result in C<dest>. (Probably by calling its C<set_string>
method) This is a string multimethod.

=item C<logical_or>

=item C<logical_and>

Perform the given short-circuiting logical operations between your boolean
value and the value passed in, storing the result in C<dest>.

=item C<logical_not>

Set yourself to be a logical negation of the value passed in.

=item C<repeat>

Repeat your string representation C<value> times and store the result
in C<dest>.

=back

If any method doesn't fit into your class, just don't implement it
and don't provide an empty function body. The default class, which all
classes inherit from, will throw an exception, if this method would
be called.

If your class is a modification of an existing class, you may wish to
use inheritance. At the beginning of your vtable specification in
classes/YOURCLASS.pmc, add the C<extends SUPERCLASS> phrase. For
example:

  pmclass PackedArray extends Array { ...

See the POD documentation in classes/pmc2c.pl for a list of useful
keywords that you may use in the .pmc file. (Run
C<perldoc -F pmc2c.pl> to view the POD.)
