
=head1 Programming Parrot -- PMCs

=head2 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
L<http://www.parrot.org/>. Instructions for compiling
Parrot are available in the Parrot distribution itself. All
code examples in this article were tested with Parrot 0.8.1

=head2 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 and PMC. PIR registers are
referenced by a C<$> character, a capital letter signifying
the register type followed by the register number (C<$S15> is
String register number 15). Parrot programs consist of lines of
text where each line contains one opcode and its arguments.

Each subroutine will have as many registers available of each basic type
(int, num, string, and pmc) as necessary; a simple subroutine will only
need a few whereas complex subroutines with many calculations will need
a larger number of registers.  This is a fundamental difference from
hardware CPUs (and the original design of Parrot), in which there are a
fixed number of registers.

PIR also provides for a more "natural" syntax for opcodes than
the standard assembly language C<op arg, arg> format.
Rather than writing C<set $I1, 0> to assign a zero to the $I1
register, you may instead write C<$I1 = 0>.
PIR also provides easy syntax for creating named variables
and constants, subroutines, passing parameters to subroutines,
accessing parameters by name, etc.

Now, on to business ...

=head2 What's a PMC?

Integers, strings, and floating point numbers are common data types in
most programming languages, but what's a PMC?  PMC stands for
"I<P>olyI<M>orphic I<C>ontainer".  PMCs are how Parrot handles more
complicated structures and behaviors, such as arrays, hashes, and
objects. 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.

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
    String          Similar to a string register but in PMC form
    Integer         Similar to an int register but in PMC form
    Float           Similar to a num register but in PMC form
    Exception       The standard exception mechanism
    Timer           A timer of course :)

=head2 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 C<:main> pragma) 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:

=head3 Example 2: reading command line arguments, take 1

=begin PIR

    .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

=end PIR

The C<.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 C<args>. Since this is
the main subroutine of my program (as designated by the
C<:main> modifier to the subroutine), Parrot arranges for
the C<args> PMC to be an aggregate of some sort that
contains the command line arguments. We then repeatedly use
the C<shift> opcode to remove an element from the front of
C<args> and place it into a string register which I then
output. When the C<args> PMC is empty, it will evaluate as a
boolean false and the conditional on line 4 will cause the
program to end.

One problem with my program is that it's destructive to the
C<args> PMC. What if I wanted to use the C<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.

=head3 Example 3: reading command line arguments, take 2

=begin PIR

    .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

=end PIR

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, but as was explained
before, a local variable is in fact just a symbol indicating that
is mapped to a register), 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 C<$P0>, the statement C<$I0 = $P0> assigns 5 to
the register C<$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 many other
programming languages. Also note that I'm assigning to a
string register and then printing that register. Why didn't
I just do C<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 (there are cases in which this is not
the case, but those will be discussed later).
So, while there's an opcode to index into an aggregate and
an opcode to print a string, there is no opcode to do I<both>
of those things.

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

=head3 Example 4: Typing the C<args> PMC

=begin PIR

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

=end PIR

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

=head2 "You are standing in a field of PMCs"

Now, let's get back to that table above. The C<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 C<Env> do
something similar?

Nope. If you want to access environment variables I<you>
need to create a PMC of type C<Env>. This is accomplished by
the C<new> opcode like so: C<$P0 = new 'Env'> After that
statement, C<$P0> will contain a hash consisting of all of
the environment variables at that time.

But, both the keys and values the C<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--C<Iterator>

An C<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:

=head3 Example 5: output environment

=begin PIR

    .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

=end PIR

Lines 3 and 4 create my new PMCs. Line 3 creates a new
C<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 C<Iterator> PMC and
initializes it with the PMC that I wish to iterate over
(my newly created C<Env> PMC in this case). From that point
on, I treat the C<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 C<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 C<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
L<parrot/docs/pmc/iterator.pod>

So, to output the environment variables, I use the
C<Iterator> to walk the keys, and then index each key into
the C<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!

=head3 Example 6: reading command line arguments, take 3

=begin PIR

    .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

=end PIR

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

=head2 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 C<docs/vtables.pod>.
This file outlines the steps you need to take to create
a new PMC type.

=head2 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)

=head3 Example 7: Triggering an exception

=begin PIR

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

=end PIR

=head3 Example 8: Setting a timer

=begin PIR

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

    .sub expired
       say 'Timer has expired!'
       exit 1
    .end

    .sub main :main
       $P0 = new 'Timer'
       $P1 = get_global 'expired'

       $P0[.PARROT_TIMER_HANDLER] = $P1    # call sub in $P1 when timer goes off
       $P0[.PARROT_TIMER_SEC]     = 2      # trigger in 10 seconds
       $P0[.PARROT_TIMER_REPEAT]  = 1      # repeat indefinitely
       $P0[.PARROT_TIMER_RUNNING] = 1      # start timer immediately
       set_global 'timer', $P0             # keep the timer around

       $I0 = 0
    loop:
       print $I0
       say ': running...'
       inc $I0
       sleep 1                             # wait a second
       goto loop
    .end

=end PIR

=head2 Author

Jonathan Scott Duff <duff@pobox.com>

=head2 Thanks

=over 4

* Alberto Simões

=back

=cut
