Parrot Babysteps 01 - Getting Started

Introduction

I want to explore basic installation and usage of Parrot. The usage will include output, simple variables, and input. PIR, the Parrot Intermediate Representation language, will be my language of choice. This is the same pattern as my other Babysteps tutorials, but the fact that PIR is a lower level language means that these are going to be some pretty big steps. I encourage you to walk along with me if you’re curious about PIR.

As always, I welcome suggestions from both novice and experienced Parrot hackers on how to improve this material for newcomers to Parrot.

$ sudo apt-get install build-essential

Installation

You need to get Parrot before you can use it. The download page shows where you can get versions of Parrot appropriate to your system, including Windows packages.

$ cd ~/src
$ wget http://www.parrot.org/release/supported

$ sudo rm -rf /usr/local/lib/*parrot*

$ tar xfvz parrot-3.0.0.tar.gz
$ cd parrot-3.0.0
$ perl Configure.pl
$ make
$ make test

$ sudo cpan TAP::Harness::Archive
$ make smoke
...
All tests successful.
Files=381, Tests=13848, 155 wallclock secs ( 2.36 usr  1.43 sys + 102.37 cusr 28.89 csys = 135.05 CPU)
Result: PASS

TAP Archive created at /Users/brian/src/parrot/parrot-3.0.0/parrot_test_run.tar.gz
Test report successfully sent to Smolder at
http://smolder.parrot.org/app/projects/report_details/14424
You can see other recent reports at
http://smolder.parrot.org/app/projects/smoke_reports/1 .

$ sudo make install

$ cd editor
$ make vim-install

Output

It is time to start writing code. Move to your projects directory and create a new directory for the Parrot Babysteps project.

$ cd ~/projects
$ mkdir parrot-babysteps
$ cd parrot-babysteps
$ gvim example-01-01.pir

When I’m connected to the iMac via ssh I settle for:

$ vim example-01-01.pir

The steps are the more or less the same on Windows. First, open a command prompt by whatever means you’re comfortable with. I prefer typing cmd into the "Search programs and files" field of the Start menu. Then, move to your projects directory and create a new directory for the Parrot Babysteps project.

C:\Users\brian> cd \projects
C:\projects> mkdir parrot-babysteps
C:\projects> cd parrot-babysteps
C:\projects\parrot-babysteps> gvim example-01-01.pir

From here on, the differences between Windows and other platforms become small enough that I feel comfortable ignoring them until something really significant catches my attention.

We immediately see some benefit from installing editor support.

# Copyright (C) 2011 Parrot Foundation.

.sub 'main' :main
    # For Parrot developers.
.end

# Local Variables:
#   mode: pir
#   fill-column: 100
# End:
# vim: expandtab shiftwidth=4 ft=pir:

That looks like a heck of a lot for an otherwise empty file, but most of those lines are actually comments.

# This is a comment

.sub main :main
.end

# example-01-01.pir
.sub main :main
    say "Hello, world!"
.end

$ parrot example-01-01.pir
Hello, World!
$

Variables

What if I want to change the greeting, so that it displays my name as part of the greeting? While simply rewriting the phrase does technically work …

# example-01-02.pir
.sub main :main
    say "Hello, Brian!"
.end

… I want something a little more flexible. I want to be able to define a name, and then print a greeting with the name. This will require adding a variable. We use variables whenever we want the program to remember something.

There are four types of things that Parrot can keep track of:

Parrot has special rules for dealing with each of these types, so it is important to tell Parrot how you want your variable treated. One way to do that is with register variables. Registers can be thought of as variable buckets: one bucket for each type. Okay, maybe not a bucket. More like four rows of nicely organized variables, with each variable type in its own row.

Register variables are prefixed with the dollar sign $ and an uppercase letter indicating their type. The rest of the variable name is a whole number from zero on up. Here, let me show you:

# example-01-03.pir
.sub main :main
    $S0 = "Brian"
    $S1 = "Hello, " . $S0
    $S1 = $S1 . "!"
    say $S1
.end

Assignment to register variables is pretty straightforward, as you can see. You tell Parrot which position in which row is being assigned to on one side of the = symbol, and the value to assign on the other.

Oh, did you see what I did when creating $S1? I did mention that Parrot has special rules for handling each type of variable. One thing you can do with string variables is paste two of them together, making a whole new string. You use the . operator for that. But only for pasting two strings together. If you want more, you’ll need to do it on two lines like I did.

Back to register variables. No, it doesn’t really matter which numbers you use. I could have used $S9999 to hold the name and Parrot wouldn’t care. We just need to remember what purpose each variable serves.

Actually, that’s the main problem with register variables. Their names don’t tell you much about what purpose the variable serves. Sure, it’s easy enough to see that $S1 is a string, but will I still remember that it’s being used to hold the name once I’ve added a few hundred lines of code? Probably not. I’m getting flashbacks to programming BASIC in grade school, when I kept a cheat sheet of variable names handy.

Parrot is thankfully there for us with the .local directive. local allows us to create a readable name for our variable, asking Parrot to deal with the register side of things.

Creation of a local variable requires the .local directive, followed by type information and lastly a variable name.

.local <type> <name>

The type names match up to the Parrot types, of course:

Variables created with the .local directive can be treated the same as a register variable.

# example-01-04.pir
.sub main :main
    .local string name
    .local string greeting

    name = "Brian"
    greeting = "Hello, " . name
    greeting = greeting . "!"
    say greeting
.end

You know, that line where we append an exclamation to our greeting is longer than it needs to be. The PIR provides special operators for when you are doing something to a variable and then assigning the result back to the same variable. That means we could describe "append an exclamation point to `greeting`" this way, too:

# example-01-05.pir
# ...
greeting = "Hello, " . name
greeting .= "!"
# ...

Input

We’ve covered a lot of conceptual ground already. What has it earned us?

$ parrot example-01-05.pir
Hello, Brian!

This is not impressive. We’ve actually increased the amount of code significantly compared to just saying "Hello, Brian!". Splitting the name off into its own variable does lay the groundwork for a slightly more impressive task, though. Let’s make the program ask the user for his name when run.

# example-01-06.pir
.sub main :main
    .local string name
    .local string greeting
    .local pmc stdin

    stdin = getstdin
    name = stdin.'readline_interactive'("Please enter your name: ")
    greeting = "Hello, " . name
    greeting .= "!"
    say greeting
.end

We’ve only added a few lines, but these few lines have added a lot of functionality. The first thing you probably noticed was the pmc variable named stdin. PMCs are used in Parrot for complex types, like classes in other languages. They often have special subroutines called methods tied to them. Those methods can greatly simplify the creation of complex programs.

We use the opcode getstdin to figure out standard input and assign it to our stdin PMC. Standard input - widely referred to as stdin - is a FileHandle that describes where we expect user input to come from. That can be any number of places, but usually refers to the keyboard. The FileHandle method we care about most today is readline_interactive. readline_interactive prompts the user for input, and hands that input back to the program.

$ parrot example-01-06.pir
Please enter your name: Brian
Hello, Brian

Conclusion

We’ve done quite a bit today. We installed Parrot, created a simple script that prints a greeting to the console, then expanded our script to get the user’s name and incorporate the name into the greeting. It took less effort than I thought it would despite the fact that PIR is low-level compared to languages like Python and Perl 5.

The ability to handle user input makes it easier to create meaningful programs even with a bare minimum of knowledge, and can be very handy as we explore Parrot, the PIR syntax, and the many PMCs. From this starting point, we can go deeper into the Parrot and PIR documentation, and search the Web for Parrot tutorials.

Remember also that Parrot is released monthly. I do try to keep these pages up to date, but sometimes work and life demand more of my time, or I get distracted by bright shiny things. So don’t assume my version numbers are correct. Double-check them for yourself.

Most of all, have fun!