TITLE

Виртуальные таблицы Паррота

Реализация типов переменных при помощи Виртуальных таблиц

Это руководство по созданию ващих собственных PMC (Parrot Magic Cookie) классов. Оно рассказывает, что необходимо нописать надлежащим образом, что бы добавить новые переменный типы переменных в Паррот.

Обзор

Внутренности Паррот интерпритатора, в упошенной схеме (или, если ты хочешь меннее принебрежительно, скептически), - это лабиринт поведения типов переменных. Стандартный пример - различия между скалярами Перла и скалярами Питона. В Перле, если ты имеешь

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

ты получишь, что $a равно b0. Это благодаря магии оперетора инкрементирования в Перле. В Питоне, с другой стороны, ты получишь ошибку выполнения.

a

PythonString

b

PythonNumber

PythonString

PerlScalar

Такое поведение является функцией PMC ``типа'', естественно рассматиривать различные типы PMC как классы в объектно-ориентированных системах. Паррот интерпритатор вызывает методы индивидуальных PMC объектов чтобы манипулировать ими. Так что привер, приведенный выше, выгледял бы подобно этому:

"a9"

И если ты заменишь PerlScalar на PythonString, ты получишь различное поведение, но для основных внутренностей интерпритатора, инструкции будет такими же самыми. PMC являются абстрактными виртуальными классами; Интрепритатор вызывает методы, PMC объект делает требуемый вещи, и интепритатор не заботится особенно, что это требуемая вешь случается чтобы быть.

Hence, добавление новых типов данный в Паррот -это вопрос об обеспечении методов, ктороые выполняют ожидаемые от типов данных действия. Давайте взлянем, как сделать это.

Поехали

Если ты добавляешь тип данных в ядро Паррота, (и ты сверяешься с Dan и/или Simon, что вы, как предполагается, делаете) ты должен создать файл в classes/ подкаталоге; это там встроенные PMC классы живут. (И хороший источник примеров, чтобы разграбить, даже если Вы не пишете основной тип данных.)

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

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

This will produce a skeleton C file (to be preprocessed by the pmc2c.pl program) with stubs for all the methods you need to fill in. The function 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 enum_class_FoobyNumber in the init function. For built-in classes, this is automatically defined in 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 init function to return struct _vtable, and then return temp_base_vtable instead of assigning to the Parrot_base_vtables array.

To finish up adding a built-in class:

What You Can and Cannot Do

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

Flags are accessed by 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 Pobj_private0_FLAG .. 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 foobynumber.c.

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

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 set_integer method has multiple forms; the default set_integer means that you are being called with a PMC, and you should probably use the get_integer method of the PMC to find its integer value; set_integer_native means you're being passed an INTVAL. The final form is slightly special; if the interpreter calls 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, (native and same) and floating point numbers.

Methods you need to implement

The master list of vtable methods can be found in vtable.tbl in the root directory of the Parrot source; since that's not exactly verbose, here's a better description of the methods that you need to implement:

type: Return the enumeration value of your class.
name: Return a string containing your class name.
init: Do any data set-up you need to do.
is_equal: True if the passed-in PMC has the same 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 is_same.

Methods, you might need, depending on your class

clone: Copy your data and state into the passed-in destination PMC.
morph: Turn yourself into the specified type.
destroy: Do any data shut-down and finalization you need to do. To have this method called, you must set the Pobj_active_destroy_FLAG.
get_integer: Return an integer representation of yourself.
get_number: Return a floating-point representation of yourself.
get_string: Return a string representation of yourself (a STRING* object), this should be a 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.
get_bool: Return a boolean representation of yourself.
get_value: Return your private data as a raw pointer.
is_same: True if the passed-in PMC refers to exactly the same data as you. (Contrast is_equal)
set_integer: Set yourself to the passed-in integer value. This is an integer multimethod.
set_number: Set yourself to the passed-in float value. This is an floating-point multimethod.
set_string: Set yourself to the passed-in string. This is a string multimethod.
set_value: Set your private data to the raw pointer passed in. This will only be used in exceptional circumstances.
add: Fetch the number part of value and add your numeric value to it, storing the result in dest. (Probably by calling its set_integer or set_number method) This is a numeric multimethod.
subtract: Fetch the number part of value and subtract your numeric value from it, storing the result in dest. (Probably by calling its set_integer or set_number method) This is a numeric multimethod.
multiply
divide
modulus: You get the picture.
concatenate: Fetch the string part of value and catenate it to yourself, storing the result in dest. (Probably by calling its set_string method) This is a string multimethod.
logical_or
logical_and: Perform the given short-circuiting logical operations between your boolean value and the value passed in, storing the result in dest.
logical_not: Set yourself to be a logical negation of the value passed in.
match: Execute the given regular expression on value and store the result.
repeat: Repeat your string representation value times and store the result in dest.

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 through 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 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 perldoc -F pmc2c.pl to view the POD.)