If you are working on a Gall agent with any kind of web interface, it's likely you will encounter the problem of converting Hoon data structures to JSON and vice versa. This is what we'll look at in this document.

Urbit represents JSON data with the $json structure (defined in lull.hoon). You can refer to the json type section below for details.

JSON data on the web is encoded in text, so Urbit has two functions in zuse.hoon for dealing with this:

• +en-json:html - For printing $json to a text-encoded form.
• +de-json:html - For parsing text-encoded JSON to a $json structure.

You typically want $json data converted to some other noun structure or vice versa, so Urbit has three collections of functions for this purpose, also in zuse.hoon:

• +enjs:format - Functions for converting various atoms and structures to $json.
• +dejs:format - Many "reparsers" for converting $json data to atoms and other structures.
• +dejs-soft:format - Largely the same as +dejs:format except its reparsers produce units which are null upon failure rather than simply crashing.

The relationship between these types and functions look like this:

Note this diagram is a simplification - the +dejs:format and +enjs:format collections in particular are tools to be used in writing conversion functions rather than simply being used by themselves, but it demonstrates the basic relationships. Additionally, it is less common to do printing/parsing manually - this would typically be handled automatically by Eyre, though it may be necessary if you're retrieving JSON data via the web client vane Iris.

In practice

A typical Gall agent will have a number of structures defined in a file in the /sur directory. These will define the type of data it expects to be %pokeed with, the type of data it will %give to subscribers, and the type of data its scry endpoints produce.

If the agent only interacts with other agents inside Urbit, it may just use a %noun mark. If, however, it needs to talk to a web interface of some kind, it usually must handle $json data with a %json mark.

Sometimes an agent's interactions with a web interface are totally distinct from its interactions with other agents. If so, the agent could just have separate scry endpoints, poke handlers, etc, that directly deal with $json data with a %json mark. In such a case, you can include $json encoding/decoding functions directly in the agent or associated libraries, using the general techniques demonstrated in the $json encoding and decoding example section below.

If, on the other hand, you want a unified interface (whether interacting with a web client or within Urbit), a different approach is necessary. Rather than taking or producing either %noun or %json marked data, custom mark files can be created which specify conversion methods for both %noun and %json marked data.

With this approach, an agent would take and/or produce data with some mark like %my-custom-mark. Then, when the agent must interact with a web client, the webserver vane Eyre can automatically convert %my-custom-mark to %json or vice versa. This way the agent only ever has to handle the %my-custom-mark data. This approach is used by %graph-store with its %graph-update-2 mark, for example, and a number of other agents.

For details of creating a mark file for this purpose, the mark file example section below walks through a practical example.

The $json type

Urbit represents JSON data with the $json structure (defined in /sys/lull.hoon):

+$  json                    ::  normal json value
  $@  ~                     ::  null
  $%  [%a p=(list json)]    ::  array
      [%b p=?]              ::  boolean
      [%o p=(map @t json)]  ::  object
      [%n p=@ta]            ::  number
      [%s p=@t]             ::  string
  ==                        ::

The correspondence of $json to JSON types is fairly self-evident, but here's a table comparing the two for additional clarity:

Since the $json %o object and %a array types may themselves contain any $json, you can see how JSON structures of arbitrary complexity can be represented. Note the %n number type is a @ta rather than something like a @ud that you might expect. This is because JSON's number type may be either an integer or floating point, so it's left as a knot which can then be parsed to a @ud or @rd with the appropriate +dejs:format function.

$json encoding and decoding example

Let's have a look at a practical example. Here's a core with three arms. It has the structure arm $user, and then two more: +to-js converts a $user structure to $json, and +from-js does the opposite. Usually we'd define the structure in a separate /sur file, but for simplicity it's all in the one core.

json-test.hoon

|%
+$  user
  $:  username=@t
      name=[first=@t mid=@t last=@t]
      joined=@da
      email=@t
  ==
++  to-js
  |=  usr=user
  |^  ^-  json
  %-  pairs:enjs:format
  :~
    ['username' s+username.usr]
    ['name' name]
    ['joined' (sect:enjs:format joined.usr)]
    ['email' s+email.usr]
  ==
  ++  name
    :-  %a
    :~
      [%s first.name.usr]
      [%s mid.name.usr]
      [%s last.name.usr]
    ==
  --
++  from-js
  =,  dejs:format
  ^-  $-(json user)
  %-  ot
  :~
    [%username so]
    [%name (at ~[so so so])]
    [%joined du]
    [%email so]
  ==
--

Note: This example (and a couple of others in this guide) sometimes use a syntax of foo+bar. This is just syntactic sugar to tag the head of bar with the term constant %foo, and is equivalent to [%foo bar]. Since json data is a union with head tags of %b, %n, %s, %a, or %o, it's sometimes convenient to do s+'some string', b+&, etc.

Try it out

First we'll try using our $json encoding/decoding library, and afterwards we'll take a closer look at its construction. To begin, save the code above in /lib/json-test.hoon of the %base desk on a fake ship and |commit it:

> |commit %base
>=
+ /~zod/base/5/lib/json-test/hoon

Then we need to build it so we can use it. We'll give it a face of user-lib:

> =user-lib -build-file %/lib/json-test/hoon

Let's now create an example of a $user structure:

> =usr `user:user-lib`['john456' ['John' 'William' 'Smith'] now 'john.smith@example.com']
> usr
[ username='john456'
  name=[first='John' mid='William' last='Smith']
  joined=~2021.9.12..09.47.58..1b65
  email='john.smith@example.com'
]

Now we can try calling the +to-js function with our data to convert it to $json:

> =usr-json (to-js:user-lib usr)
> usr-json
[ %o
    p
  { [p='email' q=[%s p='john.smith@example.com']]
    [p='name' q=[%a p=~[[%s p='John'] [%s p='William'] [%s p='Smith']]]]
    [p='username' q=[%s p='john456']]
    [p='joined' q=[%n p=~.1631440078]]
  }
]

Let's also see how that $json would look as real JSON encoded in text. We can do that with +en-json:html:

> (crip (en-json:html (to-js:user-lib usr)))
'{"joined":1631440078,"username":"john456","name":["John","William","Smith"],"email":"john.smith@example.com"}'

Finally, let's try converting the $json back to a $user again with our +from-js arm:

> (from-js:user-lib usr-json)
[ username='john456'
  name=[first='John' mid='William' last='Smith']
  joined=~2021.9.12..09.47.58
  email='john.smith@example.com'
]

Analysis

Converting to $json

Here's our arm that converts a $user structure to $json:

++  to-js
  |=  usr=user
  |^  ^-  json
  %-  pairs:enjs:format
  :~
    ['username' s+username.usr]
    ['name' name]
    ['joined' (sect:enjs:format joined.usr)]
    ['email' s+email.usr]
  ==
  ++  name
    :-  %a
    :~
      [%s first.name.usr]
      [%s mid.name.usr]
      [%s last.name.usr]
    ==
  --

There are different ways we could represent our $user structure as JSON, but in this case we've opted to encapsulate it in an object and have the name as an array (since JSON arrays preserve order).

+enjs:formatincludes the convenient +pairs function, which converts a list of [@t json] to an object containing those key-value pairs. We've used this to assemble the final object. Note that if you happen to have only a single key-value pair rather than a list, you can use +frond instead of +pairs.

For the joined field, we've used the +sect function from +enjs to convert the @da to a Unix seconds timestamp in a $json number. The +sect function, like others in +enjs, takes in a noun (in this case a @da) and produces $json (in this case a [%n @ta] number). +enjs contains a handful of useful functions like this, but for the rest we've just hand-made the $json structure. This is fairly typical when encoding $json, it's usually decoding that makes more extensive use of the $json utility functions in +format.

For the name field we've just formed a cell of %a and a list of $json strings, since a $json array is [%a p=(list json)]. Note we've separated this part into its own arm and wrapped the whole thing in a |^ - a core with a $ arm that's computed immediately. This is simply for readability - our structure here is quite simple but when dealing with deeply-nested $json structures or complex logic, having a single giant function can quickly become unwieldy.

Converting from $json

Here's our arm that converts $json to our $user structure:

++  from-js
  =,  dejs:format
  ^-  $-(json user)
  %-  ot
  :~
    [%username so]
    [%name (at ~[so so so])]
    [%joined du]
    [%email so]
  ==

This is the inverse of the encoding function described in the previous section.

We make extensive use of +dejs:format functions here, so we've used =, to expose the namespace and allow succinct +dejs function calls.

We use the +ot function from +dejs:format to decode the $json object to a n-tuple. It's a wet gate that takes a list of pairs of keys and other +dejs functions and produces a new gate that takes the $json to be decoded (which we've given it in jon).

The +so functions just decode $json strings to cords. The +at function converts a $json array to a tuple, decoding each element with the respective function given in its argument list. Like +ot, +at is also a wet gate that produces a gate that takes $json. In our case we've used +so for each element, since they're all strings.

For joined, we've used the +du function, which converts a Unix seconds timestamp in a $json number to a @da (it's basically the inverse of the +sect:enjs:format we used earlier).

Notice how +ot takes in other +dejs functions in its argument. One of its arguments includes the +at function which itself takes in other +dejs functions. There are several +dejs functions like this that allow complex nested JSON structures to be decoded. For other examples of common +dejs functions like this, see the More +dejs section below.

There are dozens of different functions in +dejs:format that will cover a great many use cases. If there isn't a +dejs function for a particular case, you can also just write a custom function - it just has to take $json. Note there's also the +dejs-soft:format functions - these are similar to +dejs functions except they produce units rather than simply crashing if decoding fails.

More +dejs

We looked at the commonly used +ot function in the first example, now let's look at a couple more common +dejs functions.

+of

The +of function takes an object containing a single key-value pair, decodes the value with the corresponding +dejs function in a key-function list, and produces a key-value tuple. This is useful when there are multiple possible objects you might receive, and tagged unions are a common data structure in hoon.

Let's look at an example. Here's a gate that takes in some $json, decodes it with an +of function that can handle three possible objects, casts the result to a tagged union, switches against its head with ?-, performs some transformation and finally returns the result. You can save it as gen/of-test.hoon in the %base desk of a fake ship and |commit %base.

of-test.hoon

|=  jon=json
|^  ^-  @t
=/  =fbb
  (to-fbb jon)
?-  -.fbb
  %foo  (cat 3 +.fbb '!!!')
  %bar  ?:(+.fbb 'Yes' 'No')
  %baz  :((cury cat 3) p.fbb q.fbb r.fbb)
==
+$  fbb
  $%  [%foo @t]
      [%bar ?]
      [%baz p=@t q=@t r=@t]
  ==
++  to-fbb
=,  dejs:format
%-  of
:~  foo+so
    bar+bo
    baz+(at ~[so so so])
==
--

Let's try it:

> +of-test (need (de-json:html '{"foo":"Hello"}'))
'Hello!!!'

> +of-test (need (de-json:html '{"bar":true}'))
'Yes'

> +of-test (need (de-json:html '{"baz":["a","b","c"]}'))
'abc'

+ou

The +ou function decodes a $json object to an n-tuple using the matching functions in a key-function list. Additionally, it lets you set some key-value pairs in an object as optional and others as mandatory. The mandatory ones crash if they're missing and the optional ones are replaced with a given noun.

+ou is different to other +dejs functions - the functions it takes are $-((unit json) grub) rather than the usual $-(json grub) of most +dejs functions. There are only two +dejs functions that fit this - +un and +uf. These are intended to be used with +ou - you would wrap each function in the key-function list of +ou with either +un or +uf.

+un crashes if its argument is ~. +ou gives functions a ~ if the matching key-value pair is missing in the $json object, so +un crashes if the key-value pair is missing. Therefore, +un lets you set key-value pairs as mandatory.

+uf takes two arguments - a noun and a +dejs function. If the (unit json) it's given by +ou is ~, it produces the noun it was given rather than the product of the +dejs function. This lets you specify key-value pairs as optional, replacing missing ones with whatever you want.

Let's look at a practical example. Here's a generator you can save in the %base desk of a fake ship in gen/ou-test.hoon and |commit %base. It takes in a $json object and produces a triple. The +ou in +decode has three key-function pairs - the first two are mandatory and the last is optional, producing the bunt of a set if the %baz key is missing.

ou-test.hoon

|=  jon=json
|^  ^-  [@t ? (set @ud)]
(decode jon)
++  decode
=,  dejs:format
%-  ou
:~  foo+(un so)
    bar+(un bo)
    baz+(uf *(set @ud) (as ni))
==
--

Let's try it:

> +ou-test (need (de-json:html '{"foo":"hello","bar":true,"baz":[1,2,3,4]}'))
['hello' %.y {1 2 3 4}]

> +ou-test (need (de-json:html '{"foo":"hello","bar":true}'))
['hello' %.y {}]

> +ou-test (need (de-json:html '{"foo":"hello"}'))
[%key 'bar']
dojo: hoon expression failed

+su

The +su function parses a string with the given parsing rule. Hoon's functional parsing library is very powerful and lets you create arbitrarily complex parsers. JSON will often have data types encoded in strings, so this function can be very useful. The writing of parsers is outside the scope of this guide, but you can see the Parsing Guide and sections 4e to 4j of the standard library documentation for details.

Here are some simple examples of using +su to parse strings:

> `@ux`((su:dejs:format hex) s+'deadbeef1337f00D')
0xdead.beef.1337.f00d

> `(list @)`((su:dejs:format (most lus dem)) s+'1+2+3+4')
~[1 2 3 4]

> `@ub`((su:dejs:format ven) s+'+>-<->+<+')
0b11.1000.1101

Here's a more complex parser that will parse a GUID like 824e7749-4eac-9c00-db16-4cb816cd6f19 to a @ux:

su-test.hoon

|=  jon=json
^-  @ux
%.  jon
%-  su:dejs:format
%+  cook
|=  parts=(list [step @])
^-  @ux
(can 3 (flop parts))
;~  plug
  (stag 4 ;~(sfix (bass 16 (stun 8^8 six:ab)) hep))
  (stag 2 ;~(sfix qix:ab hep))
  (stag 2 ;~(sfix qix:ab hep))
  (stag 2 ;~(sfix qix:ab hep))
  (stag 6 (bass 16 (stun 12^12 six:ab)))
  (easy ~)
==

Save it in the /gen directory of the %base desk and |commit it. We can then try it with:

> +su-test s+'5323a61d-0c26-d8fa-2b73-18cdca805fd8'
0x5323.a61d.0c26.d8fa.2b73.18cd.ca80.5fd8

If we delete the last character it'll no longer be a valid GUID and the parsing will fail:

> +su-test s+'5323a61d-0c26-d8fa-2b73-18cdca805fd'
/gen/su-test/hoon:<[2 1].[16 3]>
/gen/su-test/hoon:<[3 1].[16 3]>
{1 36}
syntax error
dojo: naked generator failure

mark file example

Here's a simple mark file for the $user structure we created in the first example. It imports the json-test.hoon library we created and saved in our %base desk's /lib directory.

user.hoon

/+  *json-test
|_  usr=user
++  grab
  |%
  ++  noun  user
  ++  json  from-js
  --
++  grow
  |%
  ++  noun  usr
  ++  json  (to-js usr)
  --
++  grad  %noun
--

The Marks section of the Clay documentation covers mark files comprehensively and is worth reading through if you want to write a mark file.

In brief, a mark file contains a door with three arms. The door's sample type is the type of the data in question - in our case the $user structure. The +grab arm contains methods for converting to our mark, and the +grow arm contains methods for converting from our mark. The +noun arms are mandatory, and then we've added +json arms which respectively call the +from-js and +to-js functions from our json-test.hoon library. The final +grad arm defines various revision control functions, in our case we've delegated these to the %noun mark.

From this mark file, Clay can build mark conversion gates between the %json mark and our %user mark, allowing the conversion of $json data to a $user structure and vice versa.

Try it out

First, we'll save the code above as user.hoon in the /mar directory our of %base desk:

> |commit %base
>=
+ /~zod/base/9/mar/user/hoon

Let's quickly create a $json object to work with:

> =jon (need (de-json:html '{"joined":1631440078,"username":"john456","name":["John","William","Smith"],"email":"john.smith@example.com"}'))
> jon
[ %o
    p
  { [p='email' q=[%s p='john.smith@example.com']]
    [p='name' q=[%a p=~[[%s p='John'] [%s p='William'] [%s p='Smith']]]]
    [p='username' q=[%s p='john456']]
    [p='joined' q=[%n p=~.1631440078]]
  }
]

We'll also build our library so we can use its types from the dojo:

> =user-lib -build-file %/lib/json-test/hoon

Now we can ask Clay to build a mark conversion gate from a %json mark to our %user mark. We'll use a scry with a %f care which produces a static mark conversion gate:

> =json-to-user .^($-(json user:user-lib) %cf /===/json/user)

Let's try converting our $json to a $user structure with our new mark conversion gate:

> =usr (json-to-user jon)
> usr
[ username='john456'
  name=[first='John' mid='William' last='Smith']
  joined=~2021.9.12..09.47.58
  email='john.smith@example.com'
]

Now let's try the other direction. We'll again scry Clay to build a static mark conversion gate, this time from %user to %json rather than the reverse:

> =user-to-json .^($-(user:user-lib json) %cf /===/user/json)

Let's test it out by giving it our $user data:

> (user-to-json usr)
[ %o
    p
  { [p='email' q=[%s p='john.smith@example.com']]
    [p='name' q=[%a p=~[[%s p='John'] [%s p='William'] [%s p='Smith']]]]
    [p='username' q=[%s p='john456']]
    [p='joined' q=[%n p=~.1631440078]]
  }
]

Finally, let's see how that looks as JSON encoded in text:

> (crip (en-json:html (user-to-json usr)))
'{"joined":1631440078,"username":"john456","name":["John","William","Smith"],"email":"john.smith@example.com"}'

Usually (though not in all cases) these mark conversions will be performed implicitly by Gall or Eyre and you'd not deal with the mark conversion gates directly, but it's still informative to see them work explicitly.

Further reading

The Zuse library reference - This includes documentation of the JSON parsing, printing, encoding and decoding functions.

The Marks section of the Clay documentation - Comprehensive documentation of marks.

The External API Reference section of the Eyre documentation - Details of the webserver vane Eyre's external API.

The Iris documentation - Details of the web client vane Iris, which may be used to fetch external JSON data among other things.

Strings Guide - Atom printing functions like +scot will often be useful for JSON encoding - see the Encoding in Text section for usage.

Parsing Guide - Learn how to write functional parsers in hoon which can be used with +su.