Day 24 – Make It Snow

Hello again, fellow sixers! Today I’d like to take the opportunity to highlight a little module of mine that has grown up in some significant ways this year. It’s called Terminal::Print and I’m suspecting you might already have a hint of what it can do just from the name. I’ve learned a lot from writing this module and I hope to share a few of my takeaways.

Concurrency is hard

Earlier in the year I decided to finally try to tackle multi-threading in Terminal::Print and… succeeded more or less, but rather miserably. I wrapped the access to the underlying grid (a two-dimensional array of Cell objects) in a react block and had change-cell and print-cell emit their respective actions on a Supply. The react block then handled these actions. Rather slowly, unfortunately.

Yet, there was hope. After jnthn++ fixed a constructor bug in OO::Monitors I was able to remove all the crufty hand-crafted handling code and instead ensure that method calls to the Terminal::Print::Grid object would only run in a single thread at any given time. (This is the class which holds the two-dimensional array mentioned before and was likewise the site of my react block experiment).

Here below are the necessary changes:

- unit class Terminal::Print::Grid;
+ use OO::Monitors;
+ unit monitor Terminal::Print::Grid;

This shift not only improved the readability and robustness of the code, it was significantly faster! Win! To me this is really an amazing dynamic of Perl 6. jnthn’s brilliant, twisted mind can write a meta-programming module that makes it dead simple for me to add concurrency guarantees at a specific layer of my library. My library in turn makes it dead simple to print from multiple threads at once on the screen! It’s whipuptitude enhancers all the the way down!

That said, our example today will not be writing from multiple threads. For some example code that utilizes async, I point you to examples/async.p6 and examples/matrix-ish.p6.

Widget Hero

Terminal::Print is really my first open source library in the sense that it is the first time that I have started my own module from scratch with the specific goal of filling a gap in a given language’s ecosystem. It is also the first time that I am not the sole contributor! I would be remiss not to mention japhb++ in this post, who has contributed a great deal in a relatively short amount of time.

In addition to all the performance related work and the introduction of a span-oriented printing mechanism, japhb’s work on widgets especially deserves its own post! For now let’s just say that it has been a real pleasure to see the codebase grow and extend even as I have been too distracted to do much good. My takeaway here is a new personal milestone in my participation in libre/open software (my first core contributor!) that reinforces all the positive dynamics it can have on a code base.

Oh, and I’ll just leave this here as a teaser of what the widget work has in store for us:


You can check it out in real-time and read the code at examples/rpg-ui.p6.

Snow on the Golf Course

Now you are probably wondering, where is the darn, snow! Well, here we go! The full code with syntax highlighting is available in examples/snowfall.p6. I will be going through it step by step below.

use Terminal::Print;

class Coord {
    has Int $.x is rw where * <= T.columns = 0;
    has Int $.y is rw where * <= T.rows = 0 ;

Here we import Terminal::Print. The library takes the position that when you import it somewhere, you are planning to print to the screen. To this end we export an instantiated Terminal::Print object into the importer’s lexical scope as T. This allows me to immediately start clarifying the x and y boundaries of our coordinate system based on run-time values derived from the current terminal window.

class Snowflake {
    has $.flake = ('❆','❅','❄').roll;
    has $.pos =;

sub create-flake {
    state @cols = ^T.columns .pick(*); # shuffled
    if +@cols > 0 {
        my $rand-x = @cols.pop;
        my $start-pos = x => $rand-x;
        return pos => $start-pos;
    } else {
        @cols = ^T.columns .pick(*);
        return create-flake;

Here we create an extremely simple Snowflake class. What is nice here is that we can leverage the default value of the $.flake attribute to always be random at construction time.

Then in create-flake we are composing a way to make sure we have hit every x coordinate as a starting point for the snowfall. Whenever create-flake gets called, we pop a random x coordinate out of the @cols state variable. The state variable enables this cool approach because we can manually fill @cols with a new randomized set of our available x coordinates once it is depleted.

draw( -> $promise {

start {
    my @flakes = create-flake() xx T.columns;
    my @falling; + 33).then: { $promise.keep };
    loop {
        # how fast is the snowfall?
        sleep 0.1; 
        if (+@flakes) {
            # how heavy is the snowfall?
            my $limit = @flakes > 2 ?? 2            
                                    !! +@flakes;
            # can include 0, but then *cannot* exclude $limit!
            @falling.push: |(@flakes.pop xx (0..$limit).roll);  
        } else {
            @flakes = create-flake() xx T.columns;
        for @falling.kv -> $idx, $flake {
            with $flake.pos.y -> $y {
                if $y > 0 {
                    T.print-cell: $flake.pos.x, ($flake.pos.y - 1), ' ';

                if $y < T.rows {
                    T.print-cell: $flake.pos.x, $flake.pos.y, $flake.flake;            

                try {
                    $flake.pos.y += 1;
                    CATCH {
                        # the flake has fallen all the way
                        # remove it and carry on!


Let’s unpack that a bit, shall we?

So the first thing to explain is draw. This is a handy helper routine that is also imported into the current lexical scope. It takes as its single argument a block which accepts a Promise. The block should include a start block so that keeping the argument promise works as expected. The implementation of draw is shockingly simple.

So draw is really just short-hand for making sure the screen is set up and torn down properly. It leverages promises as (I’m told) a “conv-var” which according to the Promises spec might be an abuse of promises. I’m not very futzed about it, to be honest, since it suits my needs quite well.

This approach also makes it quite easy to create a “time limit” for the snowfall by scheduling a promise to be kept at now + 33 — thirty three seconds from when the loop starts. then we keep the promise and draw shuts down the screen for us. This makes “escape” logic for your screensavers quite easy to implement (note that SIGINT also restores your screen properly. The most basic exit strategy works as expected, too :) ).

The rest is pretty straightforward, though I’d point to the try block as a slightly clever (but not dangerously so) combination of where constraints on Coord‘s attributes and Perl 6’s resumable exceptions.

Make it snow!

And so, sixers, I bid you farewell for today with a little unconditional love from ab5tract’s curious little corner of the universe. Cheers!


2 thoughts on “Day 24 – Make It Snow

  1. This is really cool (as in awesome)! I couldn’t get it to work at first, but after following these directions it worked fine (from

    # Only needed if you already have OO::Monitors installed
    $ zef uninstall “OO::Monitors:ver”

    # Installs OO::Monitors:ver and Terminal::Print
    $ zef install Terminal::Print

  2. Glad you were able to get it to work! Hopefully the dependency issues will be resolved with the next release of OO::Monitors, which should also speed up usage of this library quite a bit once precompilation becomes viable.

    Thanks for sharing your solution here!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s