Generators are the most straightforward way to write programs for Urbit. They are used for doing computations that do not require persistence: they take an input and produce an output, then disappear. Generators might make sense for listing directory contents, or running unit tests, or fetching the contents of a URL.
There are four kinds of generators: naked,
A naked generator is simply a
gate; that is, it is an anonymous function that takes a
sample (argument) and produces a noun. All you need to do is write a
gate and put it into a file in the
/gen directory. Let's take a look at a very simple one:
|= a=* a
This generator takes one argument of any noun and produces it without any changes. If we put this into a file named
echo.hoon in the
/gen directory, we can run it from the Dojo:
> +echo 42 42
This command just passes in 42 and gets 42 back. But what about when we pass in
> +echo "asdf" [97 115 100 102 0]
We just get a pile of numbers back in the form of a raw
noun. The numbers that compose that noun are called
atoms, and how they are interpreted is a matter of what
aura is being applied to them. For the purposes of this lesson, an
aura tells the pretty-printer how to display an
atom, but keep in mind that an
aura is type metadata and does other things, too.
In the example above, we didn't specify an
aura, leaving the printer to fend for itself.
"asdf" is a
tape, a type that is simply a
cord is itself an atom represented as a string of UTF-8 characters. When used as part of a
cord is only a single character. So each atom in the
[97 115 100 102 0] output corresponds to a component of the
tape: 97 is
a, 115 is
s, 100 is
d, 102 is
0 is the “null” that every list is terminated with.
We can tell the Dojo to cast -- apply a specific type to -- the output of our generator to see something more familiar:
> _tape +echo "asdf" "asdf"
Now let's create a generator with two arguments instead of one. Save the code below as
add.hoon in the
|= [[email protected] [email protected]] (add a b)
Now, run the generator:
> +add [3 4] 7
You may notice our generator takes a cell containing two
@ud. This is actually one of the limitations of naked generators. We can only pass one argument. We can get around this, of course, by passing a cell, but it's less than ideal. Also, a naked generator cannot be called without an argument. This brings us to
%say generators when we want to provide something else in Arvo, the Urbit operating system, with metadata about the generator's output. This is useful when a generator is needed to pipe data to another program, a frequent occurrence.
To that end,
%say generators use
marks to make it clear, to other Arvo computations, exactly what kind of data their output is. A
mark is akin to a MIME type on the Arvo level. A
mark describes the data in some way, indicating that it's an
%atom, or that it's a standard such as
%json, or even that it's an application-specific data structure like
marks are not specific to
%say generators; whenever data moves between programs in Arvo, that data is marked.
So, more formally, a
%say generator is a
cell. The head of that cell is the
%say tag, and the tail is a
gate that produces a
cask -- a pair of the output data and the
mark describing that data.
Below is an example of a
%say generator. Save it to
add.hoon in the
:- %say |= * :- %noun (add 40 2)
Now run the generator as below:
> +add 42
Notice that we used no argument, something that is possible with
%say generators but impossible with naked generators. We'll explain that in a moment. For now, let's focus on the code that is necessary to make something a
Recall that the rune
:- produces a cell, with the first following expression as its head and the second following expression as its tail.
The expression above creates a cell with
%say as the head. The tail is the
|= * expression on the line that follows.
|= * :- %noun (add 40 2)
|= * constructs a gate that takes a noun. This gate will itself produce a
cask, which is cell formed by the prepending
:-. The head of that
%noun and the tail is the rest of the program,
(add 40 2). The tail of the
cask will be our actual data produced by the body of the program: in this case, just adding 40 and 2 together.
%saygenerators with arguments
We can modify the boilerplate code to allow arguments to be passed into a
%say generator, but in a way that gives us more power than we would have if we just used a naked generator.
Naked generators are limited because they have no way of accessing data that exists in Arvo, such as the date and time or pieces of fresh entropy. In
%say generators, however, we can access that kind of subject by identifying them in the gate's sample, which we only specified as
* in the previous few examples. But we can do more with
%say generators if we do more with that sample. Any valid sample will follow this 3-tuple scheme:
[[now, eny, beak] [list of unnamed arguments] [list of named arguments]]
This entire structure is a noun, which is why
* is a valid sample if we wish to not use any of the information here in a generator. But let's look at each of these three elements, piece by piece.
The first part of the above 3-tuple is a noun that is composed of three atoms:
now is the current time.
eny is 512 bits of entropy for seeding random number generators.
beak contains the current ship, desk, and case.
You can access each of those pieces of data by typing their names into the Dojo. But in a generator, we need to put faces (variable names) onto that data so we can easily use it in the program. We can do so like this:
|= [[[email protected] [email protected] bec=beak] ~ ~]
Any of those pieces of data could be omitted by replacing part of the noun with
* rather than giving them faces. For example,
[[email protected] * bec=beak] if we didn't want
[* * bec=beak] if we only wanted
The second part of the sample is a
list of arguments that must be passed to the generator as it is run. Because it's a
list, this element needs to be terminated with a
~. In the example above, we used
~, the empty list, to represent this second part. But that would mean we don't want to use any new faces for our generator. So a sample where we want to declare a single required argument would look like this:
|= [* [[email protected] ~] ~]
In the above code, we use a
* to ignore any of the information that would go in the first part of the sample, somewhat similar to how we use a
~ to say “nothing here” for the second or third parts of the sample.
But to use both parts together, our gate and sample would look like this:
|= [[[email protected] [email protected] bec=beak] [[email protected] ~] ~]
The third part of the sample is a list of optional arguments. These arguments may be passed to the program as it's being run, but the program doesn't require them. The syntax for the third part is just like the syntax for the second part, besides its position:
|= [* ~ [[email protected] ~]]
Let's look at an example that uses all three parts. Save the code below in a file called
dice.hoon in your
:- %say |= [[[email protected] [email protected] bec=beak] [[email protected] ~] [[email protected] ~]] :- %noun [(~(rad og eny) n) bet]
This is a very simple dice program with an optional betting functionality. In the code, our sample specifies faces on all of the Arvo data, meaning that we can easily access them. We also require the argument
[[email protected] ~], and allow the optional argument
[[email protected] ~].
But there's something new:
(~(rad og eny) n). This code pulls the
rad arm out of the
og core with the subject of
eny. Recall that
eny is our entropy value, so this is used to seed the generator. The
rad arm will give us a pseudorandom number between 0 and
n. Then we form a cell with the result and
bet, the optional named argument specified previously.
We can run this generator like so:
> +dice 6, =bet 2 [4 2] > +dice 6 [5 0] > +dice 6 [2 0] > +dice 6, =bet 200 [0 200] > +dice nest-fail
We get a different value from the same generator between runs, something that isn't possible with a naked generator. Another novelty is the ability to choose to not use the second argument.
Also unlike a naked generator, we don't need to put our arguments together into a cell. Swap the code below into your
:- %say |= [* [[email protected] [email protected] ~] ~] :- %noun (add a b)
In the above code we're again creating a
%say generator, but the sample of our gate is a little different than before. We are using
* for the first part of the sample, because we don't use any Arvo data in this program. We are using a
~ for the third part, because we aren't allowing optional arguments -- the list of optional arguments is empty.
Run a command like the one below in the Dojo. Notice that you can use two arguments that aren't in the cells.
> +add 40 2 42
We use the
%ask generator when we want to create an interactive program that prompts for inputs as it runs, rather than expecting arguments to be passed in at the time of initiation.
Code-wise, two things are needed to create a useful
The first is
sole-result, which lives in
/sur/sole.hoon. It's a mold-builder that's required by any part of Arvo, such as the Dojo, that wants to understand the output of the
%ask generator. Because it's the only type of
%ask data that is intelligible to the system, the ultimate output of an
%ask generator must be
sole-result. You can think of it as a function that takes molds and digests them into a type for the operating system. The simplest way to use
sole-result is by calling the
produce gate from
The second is
prompt, a gate that lives in
prompt allows you to get information back from the user, which is essential to the typical desired operation of an
Since this isn't easy to digest in the abstract, we'll use an example generator to explain how this kind of generator works.
The code below is an
%ask generator that checks if the user inputs “blue” when prompted. Save it as
/- sole /+ generators =, [sole generators] :- %ask |= * ^- (sole-result (cask tang)) %+ print leaf+"What is your favorite color?" %+ prompt [%& %prompt "color: "] |= t=tape %+ produce %tang ?: =(t "blue") :~ leaf+"Oh. Thank you very much." leaf+"Right. Off you go then." == :~ leaf+"Aaaaagh!" leaf+"Into the Gorge of Eternal Peril with you!" ==
Run the generator from the Dojo:
> +axe What is your favorite color? : color:
Something new happened. Instead of simply returning something, your Dojo's prompt changed from
~your-urbit:dojo: color:, and now expects additional input. Let's give it an input:
: color: red Into the Gorge of Eternal Peril with you! Aaaaagh!
Let's go over what exactly is happening in this code.
/- sole /+ generators =, [sole generators]
Here we bring in some of the types we are going to need from
/sur/sole and gates we will use from
/lib/generators. We use some special runes for this.
/- is a Ford rune used to import types from
/+ is a Ford rune used to import libraries from
=, is a rune that allows us to expose a namespace. We do this to avoid having to write
sole-result:sole instead of
print:generators instead of
:- %ask |= *
This code might be familiar. Just as with their
%ask generators need to produce a
cell, the head of which specifies what kind of generator we are running.
|= *, we create a gate and ignore the standard arguments we are given, because we're not using them.
^- (sole-result (cask tang))
%ask generators need to have the second half of the cell be a gate that produces a
sole-result, one that in this case contains a
tang. We use the
^- rune to constrain the generator's output to such a
cask is a pair of a
mark name and a noun. Recall that a
mark can be thought of as an Arvo-level MIME type for data.
tang is a
tank, and a
tank is a structure for printing data. There are three types of
leaf is for printing a single noun, a
rose is for printing rows of data, and a
palm is for printing backstep-indented lists.
%+ print leaf+"What is your favorite color?" %+ prompt [%& %prompt "color: "] |= t=tape %+ produce %tang
Because we imported
generators, we can access its contained gates, three of which we use in
tank to the console.
In our example,
%+ is the rune to call a gate, and our gate
tank to print. The
+ here is syntactic sugar for
[leaf "What is your favorite color?"] that just makes it easier to write.
prompt is used to construct a prompt for the user to provide input. It takes a single argument that is a tuple. Most
%ask generators will want to use the
The first element of the
prompt sample is a flag that indicates whether what the user typed should be echoed out to them or hidden.
%& will produce echoed output and
%| will hide the output (for use in passwords or other secret text).
The second element of the
prompt sample is intended to be information for use in creating autocomplete options for the prompt. This functionality is not yet implemented.
The third element of the
prompt sample is the
tape that we would like to use to prompt the user. In the case of our example, we use
produce is used to construct the output of the generator. In our example, we produce a
Our gate here takes a
tape that was produced by
prompt. If we needed another type of data we could use
parse to obtain it.
The rest of this generator should be intelligible to those with Hoon knowledge at this point.
One quirk that you should be aware of, though, is that
tang prints in reverse order from how it is created. The reason for this is that
tang was originally created to display stack trace information, which should be produced in reverse order. This leads to an annoyance: we either have to specify our messages backwards or construct them in the order we want and then
list. This is a known issue to be resolved.
%get generators are used for making HTTP requests through
eyre. For this section we'll assume knowledge of generators covered in previous sections, so we can jump in with an example:
/- sole /+ generators =, generators :- %get |= [* [url=tape ~] ~] ^- (sole-request:sole (cask json)) %+ curl (scan url auri:de-purl:html) |= hit/httr:eyre ?~ r.hit !! =/ my-json (de-json:html q.u.r.hit) ?~ my-json !! =, dejs:format %+ produce %json %. %title %~ got by %- (om same) u.my-json
The above generator was written with the Studio Ghibli API. It uses this API to print the title of a Studio Ghibli film. If you save it as
ghib.hoon you could run this generator by providing an api endpoint like so.
/- sole /+ generators =, generators
Here we import types from
/sur/sole.hoon and gates from
/lib/generators.hoon. We also use
=, to expose the namespace of
generators that we just imported.
:- %get |= [* [url=tape ~] ~]
%ask generators, we must include the “boilerplate”: code that produces a generator-specific
cell. The head of that cell is
%get, and the tail is a gate that takes a
tape of the URL that we are trying to access.
^- (sole-request:sole (cask json))
Here we make sure our gate is producing a
sole-request which contains a
cask of the type
%+ curl (scan url auri:de-purl:html) |= hit/httr:eyre
Here we can see the call to
curl, the sample of which will parse our url and make sure it's in the proper format.
The second part of the
curl call is the
gate that will get called with the result provided by the actual HTTP request.
?~ r.hit !!
Here we verify that the
r face of our response is not null before continuing. If it is null, the generator will crash, as there is nothing left to do.
=/ my-json (de-json:html q.u.r.hit)
Here we verify that data was returned to us in the
json Hoon type.
q.u.r.hit is the actual data we care about as a
hit is actually an entire response object, including headers, which we don't care about for this example.
de-json out of
html will parse the
cord and produce a
json type for us that we can then use to inspect the data.
?~ my-json !! =, dejs:format
After parsing the JSON structure we want to make sure we got back a valid
json type. To do that, we test for null and crash if that test passes. If it doesn't pass, we use
=, on the
dejs:format core so that we can more easily pull arms out of it.
%+ produce %json %. %title %~ got by
Now we come to our
produce call. Recall that
produce is used in all the non-naked generators to output the final result. Here we want to produce a
json, so we use the mark
%json to indicate that.
Next we have what may be an unfamiliar rune:
%.. This rune is simply
%- with the argument order reversed.
%title is the argument passed to the next gate that is called.
by is the core that is the
map engine. A Hoon
map is a key-value pair structure, sometimes referred to as a dictionary in other languages.
got is the arm in
by that will produce a gate that we can call to access members of a given
The idiomatic way to write Hoon is to have the “heaviest” code paths lowest in the program, which is why
%.is preferred in this case over
%-. Use of this style is recommended, as it results in code that is much easier to read.
At the moment, however, we do not have a
map, but instead have a
%- (om same) u.my-json
Here we use
om, which is an arm in
dejs:format that will take a
json and return a
same is a wet gate from the standard library that takes an argument of any type and returns it as that type. We could use any other gate to modify the data if that were useful for the particular data we are going to be processing, but here we want the data exactly as it is.
my-json is a
json, the actual data we are looking for is in
u.my-json. We take this and give it to the gate produced by the call to
om. This gate is going to transform our data by taking in the
json structure and producing a
map. Then we can look up keys in this map using
got from the
%. performs the same function as
%-, the rune used to call a function, except with the arguments reversed so that the function is last and the arguments of that function come first. It's used here to keep with the principle that heaviest, or most complex, expressions should go to the bottom.
The key we are looking up in our new
map we've created from
%title, which is an element in the original source, to produce the final result of the generator.