Setup

Before we begin working on Hoon you should first (1) have Urbit installed and (2) boot an 'urbit' that you can use for trying out Hoon examples. Interactive learning is far superior to passive reading.

Installing Urbit

You can install Urbit on any Mac or Unix machine; follow the steps for creating a development ship. On Windows, make a virtual Linux machine using VirtualBox or a similar tool.

Once you're finished you can boot your very own urbit.

What is an urbit?

An urbit is an Urbit virtual computer with persistent state that can connect to the Urbit network. (Note the lowercase 'u' here. 'Urbit' is the entire software stack, whereas 'an urbit' is a local instance.) Each urbit is associated with a unique number that plays three distinct roles: (1) it's an address on the Urbit network, (2) it's a cryptographic identity, and (3) it's (in principle) a human memorable name. Normally an urbit's name is represented as a string starting with ~, as in ~zod or ~taglux-nidsep.

These may not look like numbers, but they are. Each urbit name is written in a base-256 format, where each 'digit' is a syllable. Imagine your phone number as a pronounceable string which sounds like a name in a foreign language. An ordinary user-level urbit is a 'planet', and it's named by a 32-bit number. The latter is represented as a four-syllable string; e.g., the planet name ~taglux-nidsep is the number 6,095,360.

For this tutorial you'll boot and run what we call a 'comet'. Comets are urbits whose names are 128-bits or 16 syllables, such as:

~hillyn-pitwet-hasdur-paswer--miszod-rabpex-divrup-fogdur

Comet names aren't quite as memorable as others, but they're disposable identities that anyone can make for free to join the live network. Thus, comets make the ideal urbit for playing around with Hoon, asking for help from other Urbit devs in Talk if you need it, and showing off Urbit to your friends.

Booting a Comet

To boot your comet, go into the command line and run the following command from the directory that was created during Urbit installation:

$ urbit -c mycomet

This will take a few minutes and will spin out a bunch of boot messages. Toward the end, you'll see something like:

ames: on localhost, UDP 31337.
http: live (insecure, public) on 8080
http: live ("secure", public) on 8443
http: live (insecure, loopback) on 12321
~palnul_nocser:dojo>

(There will likely be other messages as well.)

Look at the prompt, ~palnul_nocser:dojo>. To the left of the : you'll see an abbreviation of the comet name you just booted, ~palnul_nocser in this case. To the right is the name of a local application, dojo. Dojo is the Urbit command line app; it's also a Hoon REPL we'll use to run simple Hoon examples.

To make sure everything is working, type (add 2 2) at the prompt and hit return. Your screen now shows:

ames: on localhost, UDP 31337.
http: live (insecure, public) on 8080
http: live ("secure", public) on 8443
http: live (insecure, loopback) on 12321
> (add 2 2)
4
~palnul_nocser:dojo>

You just used a function from the Hoon standard library, add. Next, quit Urbit with ctrl-d:

> (add 2 2)
4
~palnul_nocser:dojo>
$

Your comet isn't running anymore and you're back at your computer's normal terminal prompt. You now have a mycomet directory for your Urbit comet; this directory is your comet's pier and it contains the entirety of your comet's state.

Right now the only thing in your pier is your comet's system files:

$ ls -a /path/to/mycomet
./  ../ .urb/

Restarting your already-created comet from the terminal is like creating a new one, but without the -c flag. (The -c is for "create".)

$ urbit mycomet

Because the comet has already been booted it won't take very long to get it running again. There are also fewer startup messages:

$ urbit mycomet
[...]
ames: on localhost, UDP 31337.
http: live (insecure, public) on 8080
http: live ("secure", public) on 8443
http: live (insecure, loopback) on 12321
~palnul_nocser:dojo>

Another Noun

You've already used a standard library function to produce one value, in the dojo. Now that your comet is running again, let's try another. Enter the number 17.

We won't show the ~palnul_nocser:dojo> prompt from here on out. We'll just show the echoed command along with its result.

You'll see:

> 17
17

You asked dojo to evaluate 17 and it echoed the number back at you. This value is a 'noun'. But what is a noun? Move on to lesson 1.2 for the answer.

Next Lesson: Nouns