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1---
2sort: 1
3next: true
4title: Hoon refresher / crash course
5---
7# Basic Hoon
9This recapitulates material you've already learned in Core Hoon.
10It will be a very abrupt tutorial if you don't know Hoon already,
11but it may be enough in and of itself. If you don't find it obvious at first
12glance, read it over again.
14Our goal is to get you programming interesting and useful things
15as soon as possible. To get there, we have to quickly cover some
16of the fundamentals of hoon. To do this, we'll walk through two
17simple programs: the first [Project
18Euler](https://projecteuler.net/) problem and
19[fizzbuzz](https://en.wikipedia.org/wiki/Fizz_buzz).
21To run this code, you'll need an urbit, and will have to copy the [examples repo](https://github.com/urbit/examples) into it.
23If you haven't installed
24urbit yet, check out the [installation
25instructions](/docs/user/install). Once urbit is
26installed, take a look at the [basic
27operation](/docs/user/basic) of your urbit.
29If you haven't copied in the examples repo, running the following commands from your urbit directory should do the trick:
31`cp -r {wherever-you've-pulled-to}/examples/gall/*/ arvo/`
32`cp -r {wherever-you've-pulled-to}/examples/dojo/*/ arvo/`
34Run an example to ensure it worked:
36 ~fintud-macrep:dojo> +examples-euler1
37 233.168
39## Euler 1
41Let's check out the code for Euler 1. First, the problem:
43```
44If we list all the natural numbers below 10 that are multiples of
453 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
47Find the sum of all the multiples of 3 or 5 below 1000.
48```
50Here is the hoon solution (which should be in your pier directory
51under `/gen/examples/euler1`):
53```
54:: project euler 1
55:: https://projecteuler.net/problem=1
56:: run in dojo with +examples-euler1
57::
58:::: /hoon/euler1/examples/gen
59 ::
60:- %say |= *
61:- %noun
62=< (sum 1.000)
63::
64:::: ~sivtyv-barnel
65 ::
66|%
67++ three
68 |= a/@
69 =| b/@
70 |- ^- @u
71 ?: (lth a b)
72 0
73 (add b $(b (add 3 b)))
75++ five
76 |= a/@
77 =| b/@
78 |- ^- @
79 ?: (lte a b)
80 0
81 ?: =((mod b 3) 0)
82 $(b (add b 5))
83 (add b $(b (add b 5)))
85++ sum
86 |= a/@u
87 (add (five a) (three a))
88--
89```
92<blockquote class="blockquote">
93Hoon is not generally whitespace sensitive, but we do have two
94different kinds of whitespace: a single space and a gap, which
95is two or more spaces or a linebreak. Tabs are taboo. Do not
96use them. Really. For a more detailed explanation of when to
97use spaces vs. gaps, see the syntax section before the first
98excercises.
99</blockquote>
102### Lines 1-11:
104Any line that begins with `::` is a comment.
106 :- %say |= *
107 :- %noun
108 =< (sum 1.000)
110All you need to know about the lines above is that they call the
111`++sum` function with an argument of `1.000`. We'll cover them in
112more detail later.
114### How to form expressions
116Hoon does not use reserved words to form expressions. (We did recently implement an alternative keyword syntax, which is not covered here. For more information, see the [syntax section](/docs/programming/core/twig/syntax/). Instead,
117expressions are formed with runes--diagraphs of two
118ascii symbols. Each rune takes a specific number of
119children--either expressions formed by other runes or literals
120that produce their own value (some runes take n children, and are usually closed with `==`).
122For example, the rune `?:` from line 17 is the classic
123'if-then-else' statement, and thus takes three children:
125 ?: (lth a b) :: if first child evals to true
126 0 :: then produce result of second
127 (add b $(b (add 3 b))) :: else, produce result of third
129Since runes are such a fundamental structure in Hoon, we found
130ourselves speaking them out loud frequently. It quickly grew
131cumbersome to have to say "question mark, colon" to describe
132`?:`. To alleviate this problem, we came up with our own naming
133scheme: each ascii glyph has a single syllable pronunciation
134phonetically designed to be both easily remembered and easily
135pronounced in conjunction with the other glyphs (when forming a
136rune).
138See the entire naming schema below/or link to it:
140```
141 ace [1 space] gal < pel (
142 bar | gap [>1 space, nl] per )
143 bas \ gar > sel [
144 buc $ hax # sem ;
145 cab _ hep - ser ]
146 cen % kel { soq '
147 col : ker } tar *
148 com , ket ^ tec `
149 doq " lus + tis =
150 dot . pam & wut ?
151 fas / pat @ zap !
152```
154Using our naming scheme `?:` is said 'wut col'.
156### Lines 12-34
158Now let's quickly walk through this code line-by-line. Lines
15912-34 are wrapped in a `|%` (pronounced 'bar cen'), which
160produces a core, a fundamental datatype in hoon similar to a
161struct, class, or object. A core is just a map of names
162to any kind of code, whether it be functions or data. Each
163element in this map begins with a `++` followed by the name and
164the corresponding code. Since `|%` takes an arbitrary number of
165children, it needs to be closed with a `--`.
167<blockquote class="blockquote">
168`++` is not technically a rune, since it is only used in core
169syntax as shown above
170</blockquote>
172Let's step into each of the three arms within our core.
174### `++ sum`
176 ++ sum
177 |= a/@
178 (add (five a) (three a))
179 --
181`|=` produces a function, much like a lambda in lisp. It takes two children:
183- A set of argument(s). In this case our argument set only
184 contains one: `a`, which is required to be an atom or natural
185 number, denoted by `@`.
187- The body of the function itself, which is executed when the
188 function is called (in this case, with `(sum 1.000)`). This
189 particular function adds the results of evaluating the gates `++
190 five` and `++three` with each of their respective input
191 parameters set to `a`.
193### ++ three
195 ++ three
196 |= a/@
197 =| b/@
198 |- ^- @u
199 ?: (lth a b)
200 0
201 (add b $(b (add 3 b)))
203As above, `++three` takes an integer argument, `a`, and then
204executes the remainder of the code with `a` set to the actual
205arguments.
207Similarly, `=|` pushes its first child, `b` into our context (in
208other words, it declares a variable `b`) and executes the
209remainder of the code. However, `b` is not an argument; `=|`
210sets `b` to the default value of whatever type it is declared as.
211Since the default value of an atom is `0`, b is set to `0`.
213So now we have two variables: `a` is set to our input, and `b` is
214initialized to `0`.
216The easiest way to think about `|-` that it lays down a recursion
217point. More on this later.
219`^-` is just a cast that sets the result of the remainder of the
220code to an unsigned integer, `@u`.
222In pseudocode, the last three lines read like this: if `a` is
223less than `b`, produce zero. Else, add `b` to the result of
224rerunning the segment of the function following the `|-` with the
225value of `b` changed to `b` plus three.
227The only thing that should look completely unfamiliar to you here
228is the `$(b (add 3 b))`, which causes us to recurse back to our
229last recursion point with the value of `b` set to `(add 3 b)`.
230Note that we only specify what changes (`b` in this case). If
231you recurse by an actual function call, then you have to specify
232every argument.
234<blockquote class="blockquote">
235If you're familiar with Clojure, `|-` is `loop` and `$()` is
236recur.
237</blockquote>
239**Exercises**:
241Please tweak your code to complete the following excercises.
243There are a few runes and some syntaxes that we have yet to cover that
244you will need to complete the excercises below. For these, please
245refer to our cheatsheat at the bottom.
247- Read and understand `++five` line by line.
249- Change `++sum` to accept two variables, `a` and `b`. Pass `a`
250 to three and `b` to five. Then run the code with `a` set to
251 `1.000` and ` q` set to `2.000`.
253- Check if this new result is under one thousand. If it is,
254 return the string 'result is less than one thousand'. If not,
255 return 'result is greater than or equal to one thousand'.
257```
258Review
260|% start core (collection of named ++ arms; an "object")
261|= define function
262=| define variable from type with default value
263|- drop a recursion point
264^- cast
265?: if-then-else
266=(a b) test equality
267(function args ...) call function
269New material
271- There are two syntaxes for writing Hoon: tall form and wide
272 form.
274 In tall form, expressions are formed with either two spaces or
275 a line break separating both a rune from its children and each
276 of its children from one another. We use tall form when writing
277 multiline expressions.
279 For more concise expressions, we use wideform, which is always
280 a single line. Wideform can be used inside tall form
281 expressions, but not vice versa.
283 Wideform expressions are formed with a rune followed by ()
284 containing its children, all of which are separated by a
285 single space. For example to make a cell of two elements:
287 :-(a b)
289 We've already seen wideform in action, for example with
290 =((mod b 3) 0). In this case = is actually an irregular form
291 of .=, which tests its two children for equality.
293 Surrounding a function with () is an irregular wide form
294 syntax for calling a function with n arguments. More on this later.
296- :- makes a cell of values. The irregular wide form of this is
297 [a b], with two expressions separated by a single space. (While the regular form of this rune takes a fixed number of children--2--its irregular wide form can accept n expressions:[a1..an])
299- Cords are one datatype for text in hoon. They're just a big
300 atom formed from adjacent unicode bytes -- a "c string". To
301 produce a cord enclose text within single quotes. To set the type
302 of an argument to a cord, use @t.
304```