July 2015 Archives

I made a small scripting language that runs in the browser. It is very ninja. To see the ninja, first open Javascript console and write:

function recur() {recur()};recur();

Hopefully it blew the stack. Then type this into the Scape REPL:

def recur() recur(); recur();

When you are convinced it will infinitely loop without blowing the stack, hit ctrl-c to stop further processing.

Rincewinds rave, that is black magic! Also called tail call elimination. Scape code is not evaluated by snarfing functions from Javascript (JS functions do not have tail call elimination before ECMAScript 6), but instead is compiled to its own set of instructions, which are then run on a stack machine (running in the Javascript VM). During parsing Scape functions are checked for whether they can use tail call elimination. If they can, they get different instructions that reuse the existing stack frame.

More magic

Scape has forward mode automatic differentiation as a language feature. Automatic differentiation allows you to compute the derivative of a function, without having to define the derivative explicitly.

Without automatic differentiation, this would be the way to compute the partial derivative of the function $f(x,y) = x^{2}y^{2}$ for $x$ and $y$ :

def fun(x,y) * (* x x) (* y y);
def diff_fun(x,y) [* (* 2 x) (* y y),* (* 2 y) (* x x)];
diff_fun(4,5);
[200, 160]

With automatic differentiation in Scape, this is how it is done:

def fun(x,y) * (* x x) (* y y);
diff(fun(4,5));
[200, 160]

This is very useful for a number of numerical methods involving derivatives. The feature is currently experimental, it might interact with non-double types in funky ways.

Wai?

Mostly just for fun. I also started toying with the idea to make a safe scripting language for use in networked games. A language and runtime that would allow the player to define custom logic during gameplay without being able to ruin the experience for other players.

A dream would be a personalized Starcraft where it is you and your custom control scripts versus the other player and his scripts.

I hope to create a simple real time multiplayer game to show how I imagine it working. For now, playing with the Scape REPL is the only way to try the language.

Sayōnara

My 3 year old daughter’s current favourite book is Jakob Martin Strid’s Den utrolige historien om den kjempestore pæra. In the story a giant pear is made hollow, and eventually turned into a boat. I decided to try and make a similar upright floating pear-boat out of a normal pear. One that my my daughter could play with once finished. Off I went to buy a pear and tools, the pear I ended up with have these specs:

Weighs around 238 g.
Displaces approximately 225 ml of water. This means it is slightly more dense then water since the weight of that water would be approximately 225 g.
Judging from the pears available in the shop, the one I chose was slightly more symmetric then most pears.

In addition to the pear, the tools I used are shown in the image below:

pear-tools — Pear, knife, melon baller (the MVT in *pear pram* construction), coins, thread.

The first step in pear pram creation, is to cut the windows using a knife. Once that is done carve out the inside using the melon baller. I recommend making the windows close to the top of the pear. If you make them too low they will result in water intake when launched. It is better to create the windows high initially and expand them downwards once you have a feeling for how the pear floats.

Another great reason for making the windows high up is lowering the center of mass. Pears have not yet been cultivated to float upright, and the center of mass is way too high for that. Having a too high center of mass on a ship is catastrophic. In its unmodified form a hollow pear will most likely perform worse then the Vasa on its maiden voyage.

To lower the center of mass even more, it is important to carve out as much as possible of the “roof”. Since the stalk extends into the pear this is difficult. It is easy to ruin the pear if you use too much force; be careful.

Even with the windows high up and a very light roof, the center of mass is still too high. Initially I added coins on the inside as ballast, this worked if I put the pear very carefully into the water, but it was still prone to capsizing.

Pear in bowl — Coins on a thread prevents capsizing. The pear is finally approved for transport of Lego men

To remedy this I moved the center of mass even lower by adding ballast on a thread below the pear. This was done by creating a knot on the thread and using a needle to get it through the bottom of the pear. Then the coins were added as shown in the picture. To ensure that water would not leak along the thread into the pear, I greased the entry points.

In its finished form the pear is quite stable, it could hold a surprising amount of Lego men without capsizing. I never got around to test adding a sail, but I think it might be stable enough to support a small sail.

Sadly rot is an inevitability facing all pear prams. If you have a great idea for prolonging pear pram life, please leave a comment.

Month: July 2015

Scape – a very ninja scripting language

More magic

Wai?

Pear pram construction