urbit/pub/docs/dev/hoon/top-down/1-basic.mdy

---
logo: black
sort: 2
title: Basic Hoon
---

Assume they’ve navigated to the examples directory and have the code on their screens.

# Basic Hoon

The best way to teach is to get you programming interesting and
useful things as soon as possible. However, in order to get you
to where building interesting and useful things is practical, we
have to quickly cover some of the fundamentals of hoon. The best
way to do this is start by walking through two boring programs:
the first Project Euler problem and fizbuzz.

## Euler 1


```
If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

Find the sum of all the multiples of 3 or 5 below 1000.
```

Here is the hoon solution:


    ::    project euler 1
    ::    https://projecteuler.net/problem=1
    ::  run in dojo with +euler1
    ::
    ::::  /hoon/euler1/gen
      ::
    :-  %say  |=  *  
    :-  %noun
    =<  (sum 1.000)
    ::
    ::::  ~fintud-macrep
      ::
    |%
    ++  three 
      |=  a=@
      =|  b=@
      |-  ^-  @u
      ?:  (lth a b)
        0
      (add b $(b (add 3 b)))

    ++  five
      |=  a=@
      =|  b=@
      |-  ^-  @
      ?:  (lte a b)
        0
      ?:  =((mod b 3) 0)
        $(b (add b 5))
      (add b $(b (add b 5)))

    ++  sum
      |=  a=@u
      (add (five a) (three a))
    --


> Hoon is not generally whitespace sensitive, but we do have two
> different kinds of whitespace: a single space and a gap, which
> is two or more spaces or a linebreak. Tabs are taboo. Do not
> use them. Really. For a more detailed explanation of when to
> use spaces vs. gaps, see the syntax section before the first
> excercises.

To run this code, navigate to the `%examples` desk.  If you
haven't pulled the examples desk from `~wactex-ribmex`, do so
now.

    > |merge %examples ~wactex-ribmex %examples
    >=
    ; ~wactex-ribmex is your neighbor
    ; ~wactex-ribmex is your neighbor
    [time passes...]
    merged with strategy %init
    > %
    [~.~fintud-macrep ~.home ~.~2015.11.13..02.24.52..7bed ~]
    > =% /=examples
    =% /~fintud-macrep/examples/~2015.11.13..02.25.00..41e9/
    > +euler1
    233.168

> You can check your directory by just typing `%`.

### Lines 1-11:

Any line that begins with `::` is a comment.

    :-  %say  |=  *  
    :-  %noun
    =<  (sum 1.000)

All you need to know about the lines above is that they call the
`++sum` function with an argument of `1.000`. We'll cover them in
more detail later.

### How to form expressions

Hoon does not use reserved words to form expressions. Instead,
expressions are formed with runes, which are diagraphs of two
ascii symbols. Each rune takes a specific number of
children--either expressions formed by other runes or literals
that produce their own value.

For example, the rune `?:` from line 17 is the classic
'if-then-else' statement, and thus takes three children:

      ?:  (lth a b)           ::  if first child evals to true
        0                     ::  then produce result of second
      (add b $(b (add 3 b)))  ::  else, produce result of third

Since runes are such a fundamental structure in Hoon, we found
ourselves speaking them out loud frequently. It quickly grew
cumbersome to have to say "question mark, colon" to describe
`?:`. To alleviate this problem, we came up with our own naming
scheme: each ascii glyph has a single syllable pronunciation
phonetically designed to be both easily remembered and easily
pronounced in conjunction with the other glyphs (when forming a
rune).

See the entire naming schema below/or link to it:

```
    ace [1 space]   gal <               pel (
    bar |           gap [>1 space, nl]  per )
    bas \           gar >               sel [
    buc $           hax #               sem ;
    cab _           hep -               ser ]
    cen %           kel {               soq '
    col :           ker }               tar *
    com ,           ket ^               tec `
    doq "           lus +               tis =
    dot .           pam &               wut ?
    fas /           pat @               zap !
```

### Lines 12-34

Now let's quickly walk through this code line-by-line. Lines
12-34 are wrapped in a `|%` (pronounced 'bar cen'), which
produces a core, a fundamental datatype in hoon similar to a
struct, class, or object. Simply, a core is just a map of names
to any kind of code, whether it be functions or data. Each
element in this map begins with a `++` followed by the name and
the corresponding code. Since `|%` takes an arbitrary number of
children, it needs to be closed with a `--`.

> `++` is not technically a rune, since it is only used in core
> syntax as shown above

Let's step into each of the three arms within our core.

### ++  sum

    ++  sum
      |=  a=@
      (add (five a) (three a))
    --

`|=` produces a function, much like a lambda in lisp. It takes two children.

- The argument(s), in this case `a`, which here must be an atom,
  as denoted by `@`.

- The body of the function itself, which is run when the function
  is called (in this case, with `(sum 1.000)`). This particular
  function adds the results of evaluating the gates `++ five` and
  `++three` with each of their respective input parameters set
  to `a`. 

### ++  three

    ++  three 
      |=  a=@
      =|  b=@
      |-  ^-  @u
      ?:  (lth a b)
        0
      (add b $(b (add 3 b)))

As above, `++three` takes an integer argument, `a`, and then
executes the remainder of the code with `a` set to the actual
arguments.

Similarly, `=|` pushes its first child, `b` into our context (in
other words, it declares a variable `b`) and executes the
remainder of the code.  However, `b` is not an argument; `=|`
sets `b` to the default value of whatever type it is declared as.
Since the default value of an atom is `0`, b is set to `0`.

So now we have two variables: `a` is set to our input, and `b` is
initialized to `0`.

The easiest way to think about `|-` that it lays down a recursion
point. More on this later.

`^-` is just a cast that sets the result of the remainder of the
code to an atom.

In pseudocode, the last three lines read like this: if `a` is
less than `b`, produce zero. Else, add `b` to the result of
rerunning the segment of the function following the `|-` with the
value of `b` changed to `b` plus three.

The only thing that should look completely unfamiliar to you here
is the `$(b (add 3 b))`, which causes us to recurse back to our
last recursion point with the value of `b` set to `(add 3 b)`.
Note that we only specify what changes (`b` in this case).  If
you recurse by an actual function call, then you have to specify
every argument.

> If you're familiar with Clojure, `|-` is `loop` and `$()` is
> recur.


## Excercises

Please tweak your code to complete the following excercises.

There are a few runes and syntaxes that we have yet to cover but
you will need to complete the excercises below. For these, please
refer to our cheatsheat below the exercises.

1.  Read and understand `++five` line by line.

2.  Change `++sum` to accept two variables, `a` and `b`. Pass `a`
    to three and `b` to five. Then run the code with `a` set to
    `1.000` and b set to `2.000`.

3.  Check if this new result is under one thousand. If it is,
    return the string 'result is less than one thousand'. If not,
    return 'result is greater than or equal to one thousand'.

```
Review

|%  start core (collection of named ++ arms)
|=  define function
=|  define variable from type with default value
|-  drop a recursion point
^-  cast
?:  if-then-else
=(a b)  test equality
(function args ...)  call function

New material

- :-  make a cell of values. The irregular wide form of this is
  [a b] with two expressions separated by a single space.

- Cords are one datatype for text in hoon. They're just a big
  atom formed from adjacent unicode bytes -- a "c string". To
  produce a cord enclose text within single quotes. To set the type
  of an argument to a cord, use @t.

- There are two syntaxes for writing Hoon: tall form and wide
  form.

  In tall form, expressions are formed with either two spaces or
  a line break separating both a rune from its children and each
  of its children from one another. We use tall form when writing
  multiline expressions.

  For more concise expressions, we use wideform, which is always
  a single line. Wideform can be used inside tall form
  expressions, but not vice versa.
  
  Wideform expressions are formed with a rune followed by ()
  containing its children, all of which are separated by a
  single space. For example to make a cell of two elements:

  :-(a b)

  We've already seen wideform in action, for example with
  =((mod b 3) 0).  In this case = is actually an irregular form
  of .=, which tests its two children for equality.

  Another irregular form is [a b] for :-(a b)

  Surrounding a function with () is an irregular wide form
  syntax for calling a function with n arguments.
```
    

## The subject

Now we're going to cover the boiler plate that we skimmed over
earlier.

    :-  %say  |=  *  
    :-  %noun
    =<  (sum [1.000 2.000])
    
This program is a cell of two elements: the first, `%say`, tells
the interpreter what to produce--in this case a value.

The second element is `|=`, which we know produces a function.
`|=`'s first child is its argument(s), which in this case is any
noun (`*`).  Its second child is the remainder of the program.

Similarly, the rest of the program is a cell of the literal
`%noun`, which tells the shell that we're producing a value of
type `noun`, and the second child contains the code that we run
to actually produce our value of the type `noun`.

`=<` is a rune that takes two children. The second child is the
context against which we run the first child. So in this case, we
are running the expression `(sum 1.000)` against everything
contained within the `|%`. In Hoon, we call the code executed the
"formula" and its context the "subject".

    =<  (sum 1.000)             :: formula
    ::::::::::::::::::::::::::::::
    |%                          ::
    ++  three                   ::
      |=  a=@                   ::
      =|  b=@                   ::
      |-  ^-  @u                ::
      ?:  (lth a b)             ::
        0                       ::
      (add b $(b (add 3 b)))    ::
                                ::
    ++  five                    ::
      |=  a=@                   ::  subject
      =|  b=@                   ::
      |-  ^-  @                 ::
      ?:  (lte a b)             ::
        0                       ::
      ?:  =((mod b 3) 0)        ::
        $(b (add b 5))          ::
      (add b $(b (add b 5)))    ::
                                ::
    ++  sum                     ::
      |=  a=@u                  ::
      (add (five a) (three a))  ::
    --                          ::
    ::::::::::::::::::::::::::::::

In nearly every language there is a similar concept of a
"context" in which expressions are executed. For example, in C
this includes things like the call stack, stack variables, and so
on.

Hoon is unique in that this context is a first-class value.
Scheme allows a sort of reification of the context through
continutations, and some may see a parallel to Forth's stack, but
Hoon takes takes the concept one step further.

Our starting subject is the standard library, which is defined in
`/arvo/hoon.hoon` and `/arvo/zuse.hoon`.  This is where functions
like `add` are defined.  When we define a core with `|%`, we
don't throw away the subject (i.e. the standard library); rather,
we stack the new core on top of the old subject so that both are
accessible.

Show our own (add a b c) vs ^(add a b)

## Exercises:

1. Pass `++sum` its arguments (`2000` and `3000`) from the
   commandline.

2. Comment out all of the arms of the `|%`. Now add another arm
   and call it `++add`, have it accept two arguments and procduce
   42 (regardless of input).  Change the `=<` line to `[(add 5 7)
   (^add 5 7)]`.  Can you recognize what's happening?

3. Write fizbuzz:

  Write a program that prints the numbers from 1 to 100
  (entered from the command line). But for multiples of three
  print “Fizz” instead of the number and for the multiples of
  five print “Buzz”. For numbers which are multiples of both
  three and five print “FizzBuzz”.

    Different ways we can do this:

    i. with cubes
    ii. with cords
    iii. with tapes

Cheatsheet:

    - To pass arguments from the command line to a program, you
      replace the `*` in the first line of the boiler plate to 
      `[^ [[arg=TYPE ~] ~]]` where `TYPE` is replaced with the
      type of argument you're expecting.  Then `+euler1 a` from
      the dojo sets `arg` to `a`.
    - A list of strings is of type `(list ,@t)`, so the result of
      the fizzbuzz function is of this type (hint: you'll need to
      use `^-`)
    - The empty list is `~`
    - Lisp-style cons (construct a cell/prepend an element) is
      `[new-element list]`
    - For example, the first three positive integers are `[1 2 3
      ~]`
    - `gte` tests whether `a` is greater than or equal to `b`.
    - `mod` runs the modulo operation on two atoms.
    - See the [basic math section]() for more info.