# One first version of this article was published on TPR 2.3
#
# Please be free to edit it to suit latest parrot developments,
# and to be a good starting point for beginners.

=head1 Writing PIR

PIR (Parrot Intermediate Representation) is a way to program the
parrot virtual machine that is easier to use than PASM (Parrot
Assembler). PASM notation is like any other assembler-like format and
can be used directly, but it is more verbose and gives too much power
to the user. PIR abstracts common operations and conventions into a
syntax that more closely resembles a high-level language. PIR allows
the programmer to write code that more naturally expresses their
intent without worrying about setting up the exact details that PASM
requires to function properly.

This article will show the basics on programming in PIR. More advanced
topics will appear in later articles.

=head2 Getting Parrot

In order to test the PIR and PASM code in this article, a parrot virtual
machine is needed (henceforth just "parrot"). Parrot is available from
L<http://parrotcode.org>. Just download the latest release, or checkout
the current development version from the SVN tree. The programs in this
article were tested with Parrot 0.4.3.

Parrot is very easy to compile on unix-like operating systems: just
run C<perl Configure.pl && make> in the root directory of the parrot
source and, if everything works correctly, a C<parrot> executable should
appear. Normally, I use it directly from the root directory of the
parrot source, instead of installing it into the system.

If you do not want to compile your own Parrot you can download a pre-compiled
binary from http://www.parrotcode.org/source.html

=head2 Parrot Virtual Machine overview

Before we get started with the examples, here's a quick overview of
parrot's architecture.

Parrot is a register-based virtual machine. It provides 4 types of
registers. The register types are:

=over 4

=item I - integer

=item N - floating point

=item S - string

=item P - PMC (Parrot Magic Cookie)

=back

Registers are designated by the type of register followed by a number.
For instance, the integer registers are C<$I0>, C<$I1> and so on. The
PMC registers hold arbitrary data objects and are parrot's mechanism
for implementing more complex behavior than the ones that can be
expressed using the other 3 register types alone. PMCs will be covered
in more detail in a future article. Examples in this article will
focus on the first 3 register types.

=head2 Simple Operators

Let me start with a simple and typical example:

  .sub main :main
       print "hello world\n"
  .end

To run it, save the code in a C<hello.pir> file and pass it to the
parrot virtual machine:

   ./parrot hello.pir

Note that I am using a relative path to parrot given that I didn't
install it into the system.

The keywords starting with a dot (C<.sub> and C<.end>) are PIR directives.
They are used together to define subroutines. After the C<.sub> keyword
I use the name of the subroutine. The keyword that starts with a colon
(C<:main>) is an pragma that tells parrot that
this is the main body of the program and that it should start by
executing this subroutine. By the way, I could use C<.sub foo :main> and
Parrot will use the C<foo> subroutine as the main body of the program.
The actual name of the subroutine does not matter as long as it has the
C<:main> pragma.  The full set of pragmas are defined in
L<docs/pdds/pdd19_pir.pod>.

Before going into more details about subroutines and calling
conventions, let's compare some PIR syntax to the equivalent PASM.

If I want to add two integer registers using PASM I would use the
Parrot C<set> opcode to put values into registers, and the C<add>
opcode to add them, like this:

   set $I1, 5
   set $I2, 3
   add $I0, $I1, $I2   # $I0 yields 5+3

PIR includes infix operators for these common opcodes. I could write
this same code as

   $I1 = 5
   $I2 = 3
   $I0 = $I1 + $I2

There are the four arithmetic operators as you should be expecting, as
well as the six different comparison operators, which return a boolean
value:

   $I1 = 5
   $I2 = 3
   $I0 = $I1 <= $I2   # $I0 yields 0 (false)

I can also use the short accumulation-like operators, like C<+=>.

Another PIR perk is that local variable names may be declared and used
instead of register names. For that I just need to declare the
variable using the C<.local> keyword with any of the four data
types available on PIR: C<int>, C<string>, C<num> and C<pmc>:

   .local int size
   size = 5

Note that all registers, both numbered and named, are consolidated by
the Parrot register allocator, assigning these "virtual registers" to
actual registers as needed.  The register allocator even coalesces two
more more virtual names onto the same physical register when it can
prove that they have non-overlapping lifetimes, so there is no need to
be stingy with register names.  To see the actual registers used, use
C<disassemble> on the C<*.pbc> output.

=head2 Branching

Another simplification of PASM are branches. Basically, when I want to
test a condition and jump to another place in the code, I would write
the following PASM code:

   le $I1, $I2, LESS_EQ

Meaning, if C<$I1> is less or equal than C<$I2>, jump to label
C<LESS_EQ>. In PIR I would write it in a more legible way:

   if $I1 <= $I2 goto LESS_EQ

PIR includes the C<unless> keyword as well.

=head2 Calling Functions

Subroutines can easily be created using the C<.sub> keyword shown
before. If you do not need parameters, it just as simple as I show in
the following code:

  .sub main :main
     hello()
  .end

  .sub hello
    print "Hello World\n"
  .end

Now, I want to make my C<hello> subroutine a little more useful, such
that I can greet other people. For that I will use the C<.param>
keyword to define the parameters C<hello> can handle:

  .sub main :main
     hello("leo")
     hello("chip")
  .end

  .sub hello
     .param string person
     print "Hello "
     print person
     print "\n"
  .end

If I need more parameters I just need to add more C<.param> lines.

To return values from PIR subroutines I use the C<.return> keyword,
followed by one or more arguments, just like this:

  .return (10, 20, 30)

To accept values from such a function, use a parenthesized results list:

  ($I1, $I2, $I3) = compute_it($I8, $I9)

=head2 Factorial Example

Now, for a little more complicated example, let me show how I would
code Factorial subroutine:

  .sub main :main
     $I1 = factorial(5)
     print $I1
     print "\n"
  .end

  .sub factorial
     .param int i
     if i > 1 goto recur
     .return (1)
  recur:
     $I1 = i - 1
     $I2 = factorial($I1)
     $I2 *= i
     .return ($I2)
  .end

This example also shows that PIR subroutines may be recursive just as in
a high-level language.

=head2 Named Arguments

As some other languages as Python and Perl support named arguments,
PIR supports them as well.

As before, I need to use C<.param> for each named argument, but with one
of the two following syntaxes:

  .sub func
    .param int a :named("foo")    # or
    .param int "bar" => b

Both of these lines say the exact same thing: the subroutine will
receive an integer named "foo", and inside of the subroutine that
integer will be known as "a".

When calling the function, I need to pass the names of the
arguments. For that there are two syntaxes as well:

  func( 10 :named("foo") )    # or
  func( "foo" => 10 )

Note that with named arguments, you may rearrange the order of your
parameters at will.

  .sub foo
    .param string "name"    => a
    .param int    "age"     => b
    .param string "gender"  => c
    # ...
  .end

This subroutine may be called in any of the following ways:

  foo( "Fred", 35, "m" )
  foo( "gender" => "m", "name" => "Fred", "age" => 35 )
  foo( "age" => 35, "gender" => "m", "name" => "Fred" )
  foo( "m" :named("gender"), 35 :named("age"), "name" => "Fred" )

and any other permutation you can think of as long as you use the named
argument syntax.

It's also possible to use named syntax when returning values from
subroutines. Into the C<.return> command I'll use:

  .return ( "bar" => 20, "foo" => 10)

and when calling the function, I will do:

  ("foo" => $I0, "bar" => $I1) = func()

And C<$I0> will yield 10, and C<$I1> will yield 20, as expected.

=head2 Concluding

To conclude this first article on PIR and to let you test what you
learned, let me show you how to do input on PASM (hence, also in
PIR). There is a C<read> opcode to read from standard input.  Just
pass it a string register or variable where you wish the characters
read to be placed and the number of characters you wish to read:

  read $S1, 100

This line will read 100 characters (or until the end of the line) and
put the read string into C<$S1>. In case you need a number, just
assign the string to the correct register type:

  read $S1, 100
  $I1 = $S1

With the PIR syntax shown in this article you should be able to start
writing simple programs. Next article we will look into available
PMCs, and how they can be used.

=head2 Author

Alberto Simões

=head2 Thanks

=over 4

=item * Jonathan Scott Duff

=back

=cut

