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422 lines
13 KiB
Plaintext
---
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logo: black
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sort: 1
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next: true
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title: Basic Hoon
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---
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# Basic Hoon
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Our goal is to get you programming interesting and useful things
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as soon as possible. To get there we have to quickly cover some
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of the fundamentals of hoon. To do this we'll walk through two
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simple programs: the first [Project
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Euler](https://projecteuler.net/) problem and
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[fizzbuzz](https://en.wikipedia.org/wiki/Fizz_buzz).
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To run this code, you'll need an urbit, and you'll need the
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`%examples` desk from `~wactex-ribmex`. If you haven't installed
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urbit yet, check out the [installation
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instructions](http://urbit.org/docs/user/install). Once urbit is
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intalled, take a look at the [basic
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operation](http://urbit.org/docs/user/basic) of your urbit.
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If you haven't pulled the examples desk from `~wactex-ribmex`, do
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so now:
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~fintud-macrep:dojo> |merge %examples ~wactex-ribmex %examples
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>=
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; ~wactex-ribmex is your neighbor
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; ~wactex-ribmex is your neighbor
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[time passes...]
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merged with strategy %init
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The merge could take several minutes; you'll know it's done when
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"merged with strategy %init" is printed. Mount the new files to
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your Unix pier directory:
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~fintud-macrep:dojo> |mount /=examples=
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Switch desks to run commands from the `%examples` desk:
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~fintud-macrep:dojo> =dir /=examples=
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=% /~fintud-macrep/examples/~2015.11.13..02.25.00..41e9/
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Run an example:
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~fintud-macrep:dojo> +euler1
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233.168
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## Euler 1
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Let's check out the code for Euler 1. First, the problem:
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```
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If we list all the natural numbers below 10 that are multiples of
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3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
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Find the sum of all the multiples of 3 or 5 below 1000.
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```
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Here is the hoon solution (which should be in your pier directory
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under `/examples/gen/euler1.hoon`):
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```
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:: project euler 1
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:: https://projecteuler.net/problem=1
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:: run in dojo with +euler1
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::
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:::: /hoon/euler1/gen
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::
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:- %say |= *
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:- %noun
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=< (sum 1.000)
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::
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:::: ~fintud-macrep
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::
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|%
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++ three
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|= a=@
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=| b=@
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|- ^- @u
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?: (lth a b)
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0
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(add b $(b (add 3 b)))
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++ five
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|= a=@
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=| b=@
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|- ^- @
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?: (lte a b)
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0
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?: =((mod b 3) 0)
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$(b (add b 5))
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(add b $(b (add b 5)))
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++ sum
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|= a=@u
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(add (five a) (three a))
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--
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```
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> Hoon is not generally whitespace sensitive, but we do have two
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> different kinds of whitespace: a single space and a gap, which
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> is two or more spaces or a linebreak. Tabs are taboo. Do not
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> use them. Really. For a more detailed explanation of when to
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> use spaces vs. gaps, see the syntax section before the first
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> excercises.
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### Lines 1-11:
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Any line that begins with `::` is a comment.
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:- %say |= *
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:- %noun
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=< (sum 1.000)
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All you need to know about the lines above is that they call the
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`++sum` function with an argument of `1.000`. We'll cover them in
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more detail later.
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### How to form expressions
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Hoon does not use reserved words to form expressions. Instead,
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expressions are formed with runes, which are diagraphs of two
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ascii symbols. Each rune takes a specific number of
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children--either expressions formed by other runes or literals
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that produce their own value.
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For example, the rune `?:` from line 17 is the classic
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'if-then-else' statement, and thus takes three children:
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?: (lth a b) :: if first child evals to true
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0 :: then produce result of second
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(add b $(b (add 3 b))) :: else, produce result of third
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Since runes are such a fundamental structure in Hoon, we found
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ourselves speaking them out loud frequently. It quickly grew
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cumbersome to have to say "question mark, colon" to describe
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`?:`. To alleviate this problem, we came up with our own naming
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scheme: each ascii glyph has a single syllable pronunciation
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phonetically designed to be both easily remembered and easily
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pronounced in conjunction with the other glyphs (when forming a
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rune).
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See the entire naming schema below/or link to it:
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```
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ace [1 space] gal < pel (
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bar | gap [>1 space, nl] per )
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bas \ gar > sel [
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buc $ hax # sem ;
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cab _ hep - ser ]
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cen % kel { soq '
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col : ker } tar *
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com , ket ^ tec `
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doq " lus + tis =
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dot . pam & wut ?
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fas / pat @ zap !
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```
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Using our naming scheme `?:` is said 'wut col'.
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### Lines 12-34
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Now let's quickly walk through this code line-by-line. Lines
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12-34 are wrapped in a `|%` (pronounced 'bar cen'), which
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produces a core, a fundamental datatype in hoon similar to a
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struct, class, or object. A core is just a map of names
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to any kind of code, whether it be functions or data. Each
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element in this map begins with a `++` followed by the name and
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the corresponding code. Since `|%` takes an arbitrary number of
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children, it needs to be closed with a `--`.
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> `++` is not technically a rune, since it is only used in core
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> syntax as shown above
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Let's step into each of the three arms within our core.
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### `++ sum`
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++ sum
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|= a=@
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(add (five a) (three a))
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--
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`|=` produces a function, much like a lambda in lisp. It takes two children:
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- A set of argument(s). In this case our argument set only
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contains one: `a` which is required to be an atom or natural
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number, denoted by `@`.
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- The body of the function itself, which is executed when the
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function is called (in this case, with `(sum 1.000)`). This
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particular function adds the results of evaluating the gates `++
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five` and `++three` with each of their respective input
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parameters set to `a`.
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### ++ three
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++ three
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|= a=@
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=| b=@
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|- ^- @u
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?: (lth a b)
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0
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(add b $(b (add 3 b)))
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As above, `++three` takes an integer argument, `a`, and then
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executes the remainder of the code with `a` set to the actual
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arguments.
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Similarly, `=|` pushes its first child, `b` into our context (in
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other words, it declares a variable `b`) and executes the
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remainder of the code. However, `b` is not an argument; `=|`
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sets `b` to the default value of whatever type it is declared as.
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Since the default value of an atom is `0`, b is set to `0`.
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So now we have two variables: `a` is set to our input, and `b` is
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initialized to `0`.
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The easiest way to think about `|-` that it lays down a recursion
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point. More on this later.
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`^-` is just a cast that sets the result of the remainder of the
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code to an unsigned integer, `@u`.
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In pseudocode, the last three lines read like this: if `a` is
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less than `b`, produce zero. Else, add `b` to the result of
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rerunning the segment of the function following the `|-` with the
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value of `b` changed to `b` plus three.
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The only thing that should look completely unfamiliar to you here
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is the `$(b (add 3 b))`, which causes us to recurse back to our
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last recursion point with the value of `b` set to `(add 3 b)`.
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Note that we only specify what changes (`b` in this case). If
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you recurse by an actual function call, then you have to specify
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every argument.
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> If you're familiar with Clojure, `|-` is `loop` and `$()` is
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> recur.
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## Excercises
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Please tweak your code to complete the following excercises.
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There are a few runes and some syntax that we have yet to cover that
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you will need to complete the excercises below. For these, please
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refer to our cheatsheat at the bottom.
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1. Read and understand `++five` line by line.
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2. Change `++sum` to accept two variables, `a` and `b`. Pass `a`
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to three and `b` to five. Then run the code with `a` set to
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`1.000` and b set to `2.000`.
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3. Check if this new result is under one thousand. If it is,
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return the string 'result is less than one thousand'. If not,
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return 'result is greater than or equal to one thousand'.
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```
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Review
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|% start core (collection of named ++ arms)
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|= define function
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=| define variable from type with default value
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|- drop a recursion point
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^- cast
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?: if-then-else
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=(a b) test equality
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(function args ...) call function
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New material
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- :- make a cell of values. The irregular wide form of this is
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[a b] with two expressions separated by a single space.
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- Cords are one datatype for text in hoon. They're just a big
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atom formed from adjacent unicode bytes -- a "c string". To
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produce a cord enclose text within single quotes. To set the type
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of an argument to a cord, use @t.
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- There are two syntaxes for writing Hoon: tall form and wide
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form.
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In tall form, expressions are formed with either two spaces or
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a line break separating both a rune from its children and each
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of its children from one another. We use tall form when writing
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multiline expressions.
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For more concise expressions, we use wideform, which is always
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a single line. Wideform can be used inside tall form
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expressions, but not vice versa.
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Wideform expressions are formed with a rune followed by ()
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containing its children, all of which are separated by a
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single space. For example to make a cell of two elements:
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:-(a b)
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We've already seen wideform in action, for example with
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=((mod b 3) 0). In this case = is actually an irregular form
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of .=, which tests its two children for equality.
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Another irregular form is [a b] for :-(a b)
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Surrounding a function with () is an irregular wide form
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syntax for calling a function with n arguments.
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```
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## The subject
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Now we're going to cover the boiler plate that we skimmed over
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earlier.
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:- %say |= *
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:- %noun
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=< (sum [1.000 2.000])
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This program is a cell of two elements: the first, `%say`, tells
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the interpreter what to produce--in this case a value.
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The second element is `|=`, which we know produces a function.
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`|=`'s first child is its argument(s), which in this case is any
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noun (`*`). Its second child is the remainder of the program.
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Similarly, the rest of the program is a cell of the literal
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`%noun`, which tells the shell that we're producing a value of
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type `noun`, and the second child contains the code that we run
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to actually produce our value of the type `noun`.
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`=<` is a rune that takes two children. The second child is the
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context against which we run the first child. So in this case, we
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are running the expression `(sum 1.000)` against everything
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contained within the `|%`. In Hoon, we call the code executed the
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"formula" and its context the "subject".
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```
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::::::::::::::::::::::::::::::
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=< (sum 1.000) :: formula
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::::::::::::::::::::::::::::::
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|% ::
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++ three ::
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|= a=@ ::
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=| b=@ ::
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|- ^- @u ::
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?: (lth a b) ::
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0 ::
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(add b $(b (add 3 b))) ::
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::
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++ five ::
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|= a=@ :: subject
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=| b=@ ::
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|- ^- @ ::
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?: (lte a b) ::
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0 ::
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?: =((mod b 3) 0) ::
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$(b (add b 5)) ::
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(add b $(b (add b 5))) ::
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::
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++ sum ::
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|= a=@u ::
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(add (five a) (three a)) ::
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-- ::
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::::::::::::::::::::::::::::::
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```
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In nearly every language there is a similar concept of a
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"context" in which expressions are executed. For example, in C
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this includes things like the call stack, stack variables, and so
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on.
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Hoon is unique in that this context is a first-class value.
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Scheme allows a sort of reification of the context through
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continutations, and some may see a parallel to Forth's stack, but
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Hoon takes takes the concept one step further.
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Our starting subject is the standard library, which is defined in
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`/arvo/hoon.hoon` and `/arvo/zuse.hoon`. This is where functions
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like `add` are defined. When we define a core with `|%`, we
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don't throw away the subject (i.e. the standard library); rather,
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we stack the new core on top of the old subject so that both are
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accessible.
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## Exercises:
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4. Pass `++sum` its arguments (`2000` and `3000`) from the
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commandline.
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5. Comment out all of the arms of the `|%`. Now add another arm
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and call it `++add`, have it accept two arguments and procduce
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42 (regardless of input). Change the `=<` line to `[(add 5 7)
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(^add 5 7)]`. Can you recognize what's happening?
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6. Write fizbuzz:
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Write a program that prints the numbers from 1 to 100
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(entered from the command line). But for multiples of three
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print 'Fizz' instead of the number and for the multiples of
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five print 'Buzz'. For numbers which are multiples of both
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three and five print 'FizzBuzz'.
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Cheatsheet:
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- To pass arguments from the command line to a program, you
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replace the `*` in the first line of the boiler plate to
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`[^ [[arg=TYPE ~] ~]]` where `TYPE` is replaced with the
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type of argument you're expecting. Then `+euler1 a` from
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the dojo sets `arg` to `a`.
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- A list of strings is of type `(list ,@t)`, so the result of
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the fizzbuzz function is of this type (hint: you'll need to
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use `^-`)
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- The empty list is `~`
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- Lisp-style cons (construct a cell/prepend an element) is
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`[new-element list]`
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- For example, the first three positive integers are `[1 2 3
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~]`
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- `gte` tests whether `a` is greater than or equal to `b`.
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- `mod` runs the modulo operation on two atoms.
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- See the [basic math section]() for more info.
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