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urbit.org/docs/programming/core/demo.md
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6# Demo
8Here's everyone's favorite whiteboard example, FizzBuzz. Each
9line is commented with its number:
11```
12:gate end/atom :: 1
13:var count 1 :: 2
14:loop :: 3
15:if =(end count) :: 4
16 ~ :: 5
17:cons :: 6
18 :if =(0 (mod count 15)) :: 7
19 "FizzBuzz" :: 8
20 :if =(0 (mod count 5)) :: 9
21 "Fizz" :: 10
22 :if =(0 (mod count 3)) :: 11
23 "Buzz" :: 12
24 (text :wrap(count)) :: 13
25:moar(count (add 1 count)) :: 14
26```
28Can you understand this code by just looking at it? Hopefully,
29but let's explain it anyway.
31Hoon is a functional language. FizzBuzz is usually defined as a
32side effect, but Hoon has no side effects. This code will
33*make* a list of strings, not *print* a list of strings.
35Line 1 defines a function of one argument, `end`, an unsigned
36integer.
38Line 2 declares a variable, `count`, with initial value `1`.
40Line 3 begins a loop.
42Line 4 checks if `count` equals `end`. If so, line 5 (and the
43whole loop) produces the value `~`, null. If not, line 6 begins
44an ordered pair.
46The head of the pair is "FizzBuzz", or "Fizz", or "Buzz", if the
47respective tests hit. If not, line 13 wraps a runtime type
48around `count`, then prints the type-value pair to a string (like
49C `sprintf()`). This string is the first item in the output list.
51The tail of the pair is line 14, which repeats the loop (like
52Clojure `recur`), with `count` incremented. This list of strings
53is the rest of the output list.
55You can test this code by mounting your home desk with `|mount %home`;
56copying it into `home/gen/fizzbuzz.hoon` in your Urbit pier; then
57typing `+fizzbuzz 100` in the Urbit dojo (shell).