Programming Parrot -- PMCs

Preliminaries

To run the example code in this article, you'll need to obtain a copy of Parrot and build it for your system. For information on obtaining Parrot, see http://www.parrotcode.org/. Instructions for compiling Parrot are available in the Parrot distribution itself. All code examples in this article were tested with Parrot 0.4.5

A quick review of Parrot

As mentioned by Alberto Manuel Simões in TPR 2.3, Parrot is a register-based virtual machine with 4 register types: Integer, String, Number, PMC. There are 32 registers for each register type (0 through 31) and registers are referenced by a capital letter signifying the register type followed by the register number (S15 is String register number 15). Parrot programs consist of lines of text where each line contains one opcode and its arguments.

Utilizing a slightly higher-level syntax called PIR (Parrot Intermediate Representation) you may obtain an arbitrary number of each register type by prefixing the register with a $ (For example, the virtual register $I51 is perfectly valid). PIR also provides for a more "natural" syntax for opcodes. Rather than saying set I1, 0 to assign a zero to the I1 register, you may say instead I1 = 0. PIR also provides syntax for easily creating named variables and constants, subroutines, passing parameters to subroutines, accessing parameters by name, etc.

Now, on to business ...

What's a PMC?

Integers, strings, and arbitrary floating point numbers are common data types in most programming languages, but what's a PMC? PMC stands for "Parrot Magic Cookie". PMCs are how Parrot handles more complicated structures and behaviors (hence the magic :) Some examples of PMC usage would be for arrays, hashes, data structures, objects, etc. Anything that can't be expressed using just integers, floating point numbers and strings can be expressed with a PMC.

Parrot comes with many types of PMC that encapsulate common, useful behavior. Here's a program that lists all of the PMC types that are builtin to Parrot:

Example 1: list all PMC types

    .sub _ :main
        $I0 = 1                         # The first PMC
      loop:
        $I1 = valid_type $I0            
        unless $I1 goto end_loop        # stop on invalid PMC number
        $S0 = typeof $I0                # get PMC type name
        print $I0                       # output PMC number
        print "\t"
        print $S0                       # output PMC type name
        print "\n"
        inc $I0
        goto loop
      end_loop:
    .end

In this program, the typeof opcode is used to determine the type of each PMC available to Parrot. When given an integer argument, typeof will return the name of the PMC type corresponding to that integer as a string.

Many of the PMC type names give clues as to how they are used. Here's a table that gives a short description of several interesting and useful PMC types:

    PMC type        Description of PMC
    --------        ------------------
    Env             access environment variables
    Iterator        iterate over aggregates such as arrays or hashes
    Array           A generic, resizable array
    Hash            A generic, resizable hash
    Random          Obtain a random number
    String          Similar to a string register but in PMC form
    Integer         Similar to an integer register but in PMC form
    Float           Similar to a number register but in PMC form
    Exception       The standard exception mechanism
    Timer           A timer of course :)

Your wish is my command line

Before I take a closer look at some of these PMC types, let's look at a common thing that people want to know how to do-- read command line arguments. The subroutine designated as the main program (by the :main modifier) has an implicit parameter passed to it that is the command line arguments. Since previous examples never had such a parameter to the main program, Parrot simply ignored whatever was passed on the command line. Now I want Parrot to capture the command line so that I can manipulate it. So, let's write a program that reads the command line arguments and outputs them one per line:

Example 2: reading command line arguments, take 1

    .sub _ :main
        .param pmc args
      loop:
        unless args goto end_loop           # line 4
        $S0 = shift args
        print $S0
        print "\n"
        goto loop
      end_loop:
    .end

The .param directive tells parrot that I want this subroutine to accept a single parameter and that parameter is some sort of PMC that I've named args. Since this is the main subroutine of my program (as designated by the :main modifier to the subroutine), Parrot arranges for the args PMC to be an aggregate of some sort that contains the command line arguments. We then repeatedly use the shift opcode to remove an element from the front of args and place it into a string register which I then output. When the args PMC is empty, it will evaluate as a boolean false and the conditional on line will cause the program to end.

One problem with my program is that it's destructive to the args PMC. What if I wanted to use the args PMC later in the program? One way to do that is to use an integer to keep an index into the aggregate and then just print out each indexed value.

Example 3: reading command line arguments, take 2

    .sub _ :main
        .param pmc args
        .local int argc
        argc = args                         # line 4
        $I0 = 0
      loop:
        unless $I0 < argc goto end_loop
        print $I0
        print "\t"
        $S0 = args[$I0]                     # line 10
        print $S0
        print "\n"
        inc $I0
        goto loop
      end_loop:
    .end

Line 4 shows something interesting about aggregates. Similar to perl, when you assign an aggregate to an integer thing (whether it be a register or local variable), Parrot puts the number of elements in the aggregate into the integer thing. (e.g., if you had a PMC that held 5 things in $P0, the statement $I0 = $P0 assigns 5 to the register $I0)

Since I know how many things are in the aggregate, I can make a loop that increments a value until it reaches that number. Line 10 shows that to index an aggregate, you use square brackets just like you would in Perl and other programming languages. Also note that I'm assigning to a string register and then printing that register. Why didn't I just do print args[$I0] instead? Because this isn't a high level language. PIR provides a nicer syntax but it's still really low level. Each line of PIR still essentially corresponds to one opcode. So, while there's an opcode to index into an aggregate and an opcode to print a string, there is no opcode to do both of those things.

BTW, what type of aggregate is the args PMC anyway? Another way to use the typeof opcode is to pass it an actual PMC:

Example 4: Typing the args PMC

    .sub _ :main
        .param pmc args
        $S0 = typeof args
        print $S0
        print "\n"
    .end

When you run this program it should output "ResizableStringArray". If you assign the result of the typeof opcode to a string thing, you get the name of the PMC type. If you assign the result to an integer thing, you get the PMC type number.

"You are standing in a field of PMCs"

Now, let's get back to that table above. The Env PMC can be thought of as a hash where the keys are environment variable names and the values are the corresponding environment variable values. But where does the actual PMC come from? For the command line, the PMC showed up as an implicit parameter to the main subroutine. Does Env do something similar?

Nope. If you want to access environment variables you need to create a PMC of type Env. This is accomplished by the new opcode like so: $P0 = new 'Env' After that statement, $P0 will contain a hash consisting of all of the environment variables at that time. All built-in types are prefixed with a dot, so if the PMC type is OrderedHash, to create a new one you'd do $P0 = new .OrderedHash.

But, both the keys and values the Env hash are strings, so how do I iterate over them as I did for the command line? We can't do the same as I did with the command line and use an integer index into the PMC because the keys are strings, not integers. So, how do I do it? The answer is another PMC type--Iterator

An Iterator PMC is used, as its name implies, to iterate over aggregates. It doesn't care if they are arrays or hashes or something else entirely, it just gives you a way to walk from one end of the aggregate to the other.

Here's a program that outputs the name and value of all environment variables:

Example 5: output environment

    .sub _ :main
        .local pmc env, iter
        .local string key, value

        env  = new 'Env'                    # line 3
        iter = new 'Iterator', env          # line 4
      iterloop:
        unless iter goto iterend
        key = shift iter                    # line 8
        value = env[key]
        print key
        print ":"
        print value
        print "\n"
        goto iterloop
      iterend:
    .end

Lines 3 and 4 create my new PMCs. Line 3 creates a new Env PMC which at the moment of its existence contains a hash of all of the environment variables currently in the environment. Line 4 creates a new Iterator PMC and initializes it with the PMC that I wish to iterate over (my newly created Env PMC in this case). From that point on, I treat the Iterator much the same way I first treated the PMC of command line arguments. Test if it's "empty" (the iterator has been exhausted) and shift elements from the Iterator in order to walk from one end of the aggregate to the other. A key difference is however, I'm not modifying the original aggregate, just the Iterator which can be thrown away or reset so that I can iterate the aggregate over and over again or even have two iterators iterating the same aggregate simultaneously. For more information on iterators, see "docs/pmc/iterator.pod" in parrot

So, to output the environment variables, I use the Iterator to walk the keys, and then index each key into the Env PMC to get the value associated with that key and then output it. Simple. Say ... couldn't I have iterated over the command line this same way? Sure!

Example 6: reading command line arguments, take 3

    .sub _ :main
        .param pmc args
        .local pmc cmdline
        cmdline = new 'Iterator', args
      loop:
        unless cmdline goto end_loop
        $S0 = shift cmdline
        print $S0
        print "\n"
        goto loop
      end_loop:
    .end

Notice how this code approaches the simplicity of the original that destructively iterated the args PMC. Using indexes can quickly become complicated by comparison.

How do I create my own PMC type?

That's really beyond the scope of this article, but if you're really interested in doing so, get a copy of the Parrot source and read the file docs/vtables.pod. This file outlines the steps you need to take to create a new PMC type.

A few more PMC examples

I'll conclude with a few examples without explanation. I encourage you to explore the Parrot source code and documentation to find out more about these (and other) PMCs. A good place to start is the docs directory in the Parrot distribution (parrot/docs)

Example 7: Output random numbers

    .sub _ :main
        $P0 = new 'Random'
        $N0 = $P0
        print $N0
        print "\n"
        $N0 = $P0
        print $N0
        print "\n"
    .end

Example 8: Triggering an exception

    .sub _ :main
        $P0 = new 'Exception'
        $P0['_message'] = "The sky is falling!"
        throw $P0
    .end

Example 9: Setting a timer

    .include "timer.pasm"                   # for the timer constants

    .sub expired
       print "Timer has expired!\n"
    .end

    .sub _ :main
       $P0 = new 'Timer'
       $P1 = global "expired"

       $P0[.PARROT_TIMER_HANDLER] = $P1    # call sub in $P1 when timer goes off
       $P0[.PARROT_TIMER_SEC] = 2          # trigger every 2 seconds
       $P0[.PARROT_TIMER_REPEAT] = -1      # repeat indefinitely
       $P0[.PARROT_TIMER_RUNNING] = 1      # start timer immediately
       global "timer" = $P0                # keep the timer around

       $I0 = 0
      loop:
       print $I0
       print ": running...\n"
       inc $I0
       sleep 1                             # wait a second
       goto loop
    .end

Author

Jonathan Scott Duff <duff@pobox.com>

Thanks

* Alberto Simões