=pod

=head1 PMCs

Z<CHP-10>

PMCs are one of the four fundamental data types in Parrot, and definitely one
of the most useful. A PMC can contain a single scalar value (integer, floating
point number, string), an array of values, a subroutine, a namespace, or an
entire list of other data types as well. PMCs are the basis for all higher
order data types in Parrot, and are flexible enough to be used for any
purpose that an HLL needs.

All the common PMC types are included in the Parrot repository and built directy
into libparrot and the parrot executable. However, the system is not rigid; new
PMC types can be defined externally and loaded into Parrot at a later time. In
this way, HLLs and libraries and applications can add new data types to Parrot
at the C code level, which helps to ensure speed and efficiency. PMCs loaded
this way are called X<dynamic PMCs;dynpmcs> Dynamic PMCs or I<dynpmcs>.

=head2 PIR Classes

It's worth a quick diversion here to talk about the difference between a pure
PIR class object, and a PMC. Even though classes written in PIR can inherit from
an existing PMC type, they aren't all their own type of PMC. In fact, classes
written in PIR are all of the Object PMC type. In order to add a new fundamental
PMC type to Parrot, it needs to be written in C N<well, a superset of C anyway>
and it needs to be compiled using the X<PMC compiler> PMC compiler.

=head2 Writing PMCs

In the strictest sense, PMCs are written in C and are compiled by your local
C compiler into machine code for linking with libparrot or the parrot
executable. However, Parrot's build process makes use of a special PMC compiler
program that converts PMCs defined in a special C-like script to ordinary
C code. The PMC compiler adds all the necessary boiler plate code and installs
the PMC type into Parrot for use. The PMC script is like a macro superset
N<although the functionality is a litte bit more involved then is available
in the normal C preprocessor> over the C language. All valid C is valid in
PMC scripts, but there are also a few additions that help to make common tasks
a little easier.

The PMC script was born of conflicting necessities. The internals of Parrot
are all written according to the ISO C89 standard for maximum portability.
However, PIR and languages that are built on top of Parrot are typically
object-oriented (or have some OO capabilities). PMCs are like classes,
they have data and methods, and they can inherit from parent PMCs.

C is low-level and portable, which is desirable. But Parrot needed some
support for OO features that C doesn't have, and the C preprocessor
can't support directly. To support the necessary features, and to make
the task of writing PMCs just a little bit easier, the Parrot developers
created a PMC compiler program that takes a special PMC script and converts
it into standard ISO C89.

=head3 PMC Files

PMC files have a C<.pmc> file extension. They're written in a C-like
language with a few additions to help with creating PMCs. PMC files do
not natively allow POD documentation, so all such documentation must be
enclosed in C</* */> comments. All PMC files that ship with Parrot
include significant file-level and function-level documentation to help
explain how the PMCs operate.

=head3 C<pmclass> Definitions

A PMC file can contain a single PMC class definition and any other helper
functions or data structure definitions that are needed to support the
PMC. To define a PMC class in the PMC file, you use the C<pmclass>
statement. Everything outside the C<pmclass> definition will be ignored by
the PMC compiler and passed through verbatim into the generated C<.c> file.
Inside the C<pmclass> definition are going to be all the VTABLE and METHOD
declarations for the PMC.

A standard definition can contain a number of parts. Here's a pseudo-grammar
for them:

  pmclass CLASSNAME [extends PARENT]? [provides INTERFACE] [FLAGS]* {
      /* Attributes defined here */

      /* VTABLE and METHODs defined here. */

  }

The C<extends> keyword is optional, but allows us to specify that this
PMC class is a subtype of the given type. If we have an C<extends>
in the definition, we can use the C<SUPER> keyword throughout the PMC
file to refer to the parent type.

The C<FLAGS> are a series of flags that can be specified to determine
how the PMC behaves and how it's constructed internally. The C<need_ext>
flag assigns a special C<PMC_EXT> data structure to the PMC structure
internally. C<PMC_EXT> is necessary to handle data sharing between threads
or interpreters, storing attributes in the PMC, and a few other uses as
well. The C<singleton> flag means that there can only be one instantiated
object of this class. The C<is_ro> and C<has_ro> flags indicate that the
PMC class is read-only or that it contains read-only data, respectively.
The C<is_shared> flag indicates that the PMC is intended to be shared
between multiple interpreters, and therefore special synchronization
logic should be applied. The C<abstract> flag indicates that the PMC
class cannot be instantiated directly, but can be inherited from by a
non-abstract PMC class.

The C<provides> keyword is used to show that the PMC provides certain
standard interfaces. For instance, you can specify C<provides array>
and then Parrot will enable us to write things like C<$P0[2]> in PIR
code to access the PMC using integer indices. C<provides hash> means
that we can use string and PMC keys to access values in the PMC. These
C<provides> each correspond to a series of VTABLE interfaces that the
PMC must provide, or must inherit. Without the necessary VTABLE
interfaces available, Parrot may try to perform illegal operations and
things will go badly. We'll talk about all the available C<provides>
interfaces and the VTABLE interfaces that they must define.

=head3 Attributes

PMCs can be given a custom set of data field attributes using the C<ATTR>
keyword. ATTR allows the PMC to be extended to contain custom data
structures that are automatically managed by Parrot's memory subsystem.
Here's an example:

  pmclass Foo {
    ATTR INTVAL bar;
    ATTR PMC baz;

    ...
  }

The attributes are stored in a custom data structure that can be accessed
using a macro with the same name as the PMC, but all upper-case:

  Parrot_Foo_Attributes * attrs = PARROT_FOO(SELF);
  attrs->bar = 7;                 /* it's an INTVAL */
  attrs->baz = pmc_new( ... )     /* It's a PMC */

Notice how the type name of the attributes structure is C<Parrot_>,
followed by the name of the PMC with the same capitalization as is used
in the C<pmclass> definition, followed by C<_Attributes>. The macro to
return this structure is C<PARROT_> followed by the name of the PMC in
all caps.

=head3 C<INTERP>, C<SUPER> and C<SELF>

The PMC compiler enables us to use a few pre-defined variable names
throughout the file to make things easier. The C<INTERP> keyword always
contains a reference to the current interpreter structure. This keyword is
include by default in all VTABLE interfaces and all PMC methods. It is not
automatically included in any extra helper functions that you define in
the PMC file.

Here's an example of a VTABLE interface function:

  VTABLE Foo(INVAR PMC, INVAR INTVAL)
  {
      ...
  }

The PMC compiler will convert this to the following C function definition:

  void Foo(PARROT_INTERP, PMC *self, PMC *arg_1, INTVAL arg_2)
  {
      ...
  }

The C<interp> and C<self> variables are provided in all VTABLE interfaces,
even though you don't have to define them explicitly in the PMC file.

If the C<pmclass> definition uses the C<extends> keyword, a reference to
a member of the parent class is also contained in the C<SUPER> variable.
The C<SUPER()> function calls the VTABLE interface from the parent class.

  VTABLE destroy()
  {
      SUPER(); /* Call the parent PMC's VTABLE */
  }

The PMC compiler also allows the use of "method syntax" for the C<SELF> and
C<SUPER> variables:

  SUPER.clone()   /* Call the clone VTABLE interface on SUPER */
  SELF.destroy()  /* Call the destroy VTABLE interface on SELF */

Or, you can call the VTABLE interfaces more directly:

  VTABLE_clone(INTERP, SUPER)
  VTABLE_destroy(INTERP, SELF)

=head3 PMC Compiler

The PMC compiler is a small program written in Perl 5 that's part of the
normal Parrot build process. It converts all C<.pmc> files to C<.c> files
for final compilation. The long-term goal for Parrot is to not be dependent
on Perl 5 for configuration and building, but for now Perl 5 is required
when building Parrot.

=head2 VTABLE Function Definitions

=head3 VTABLE Functions Parameters

VTABLE functions are defined just like ordinary C functions, almost. Here's
a normal definition for a VTABLE method:

  VTABLE VTABLENAME (PARAMETERS) {
    /* ordinary C here, almost */
  }

You can't just name your VTABLE functions anything you want. There is a
predefined list of VTABLE function names, and you must name it exactly
the same as the one you are trying to implement. The PARAMETERS list
is pretty particular as well: Each VTABLE function type has a specific
parameter list that must be implemented exactly or else the compiler
will throw a warning.

=head2 Methods

VTABLES are standard, but they're rigid. They need to have the exact name
that Parrot expects, and they need to have the exact function signature
that Parrot expects too. VTABLES are responsible for the low-level basic
access operations that all data types need to implement. However, to get
more out of your PMCs, we need a more flexible want to interact with them.

Enter methods, which are ways to extend the functionality of your PMC
in ways that the PMC needs. Methods allow the developer to add all sorts
of arbitrary functionality to a PMC that the VTABLE functions themselves
cannot define.

=head2 Dynpmcs

=head3 Loading dynpmcs




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