Udon (Markdown-esque)

Udon is a domain-specific language for composing documents. Udon is very similar to Markdown, but with some minor variation in syntax and some additional Urbit-related affordances.

Udon files are compiled to manxes (Urbit's XHTML/XML representation), so can easily be used to publish documents to the browser. Udon also allows you to embed arbitrary Sail syntax, which itself allows embedding arbitrary Hoon, so it can be quite powerful for dynamic content when compiled against an appropriate subject.

This document will walk through the basics of Udon and its syntax.

Basic example

Here's an example of an Udon file and its various allowed syntax.

# H1
## H2
### H3
#### H4
##### H5
###### H6
This is a paragraph with _italics_, *bold* and
`inline code`. Sentences can be hard wrapped.
- unordered
- list
+ ordered
+ list
fenced codeblock
(note language spec not supported)
horizontal rule:
> block quotes
may be hard-wrapped if indented
Backslash at end\
of line adds linebreak
Udon syntax may be prefixed with \*backslashes\* to escape.
Hoon atom literals like ~sampel-palnet and ~.foo will
be rendered as inline code.
;td: Arbitrary
;td: Sail
;td: is
;td: allowed

Syntax summary

  • The first line of a .udon document must be a single rune: ;>. This tells the compiler to interpret everything following as udon.
  • Paragraphs: Content on a single line will be made into a paragraph. Paragraphs may be hard-wrapped, so consecutive lines of text will become a single paragraph. The paragraph will be ended by an empty line or other block element.
  • Headers: lines beginning with 1-6 #s followed by a single space and then some content (e.g. ## foo) will be made into headers. The number of #s dictates the header level.
  • Italics: content wrapped in single _s (e.g. _foo_) will be made italic.
  • Bold: content wrapped in single *s (e.g. *foo*) will be made bold.
  • Unordered lists: lines beginning with - followed by a space will be made into items in a list. List lines can be hard-wrapped, with two spaces beginning each subsequent line to be included in the list. Lists can be nested by indenting the -s a further two spaces for each level of nesting.
  • Ordered lists: lines beginning with + followed by a space will be made into ordered lists, and numbered in the order they appear. These have the same wrapping and nesting logic as unordered lists.
  • Links: this is standard markdown syntax: square bracks containing the display content and then parentheses containing the URL, e.g. [foo](http://example.com). The URL may also be a relative link or an anchor link.
  • Images: this is also standard markdown; a link with an exclamation mark at the beginning, e.g. ![foo](http://example.com/image.png). The square brackets contain the alt-text and the the parentheses contain the image URL.
  • Inline code: text wrapped in single backticks will be rendered verbatim in a monospace font.
  • Fenced codeblocks: Triple-backticks on their own line begin and end a codeblock. All lines in between will be rendered verbatim in a monospace font. Note that udon does not support a language specification after the opening backticks like markdown does.
  • Horizontal rules: Three or more hyphens (---) will create a horizontal rule.
  • Block quotes: a line beginning with > creates a block quote. This may be hard-wrapped, as long as the next line is indented two spaces. Block quotes may contain anything, including other blockquotes.
  • Line breaks: A line ending in a single backslash will have a line break inserted at the end, so it will not flow together with the subsequent line as is usually the case.
  • Escape characters: You may prefix Udon syntax with a backslash to have it treated as the literal text.
  • Hoon literals and wings: Udon will automatically render any values with atom aura syntax as inline code. It'll also render arms like ++foo:bar, +$baz, and +*foo:bar:baz, as inline code.
  • Sail: this is hoon's native XML syntax. Udon will parse it, execute it, and include the +$manxes produced in the resulting document. This means you can embed arbitrary hoon in the document.

Note that Udon is quite strict on its syntax, and may fail to parse if it's incorrect.

Udon Mode

An Udon file has a .udon extension (an %udon mark).

The first thing in an Udon file must be the micgar rune: ;>

Micgar tells the Hoon compiler to interpret everything afterwards as Udon. Udon-mode ends at the end of the file; there's no way to terminate micgar before that. Udon is therefore useful for whole documents rather than embedding snippets in other Hoon files.

The Hoon compiler will produce a manx as a result.

To scry out a file, compile it against the standard library, and stringify the resulting XHTML, you can do:

%- crip
%- en-xml:html
!< manx
%+ slap !>(..zuse)
%- ream
.^(@t %cx /=the-desk=/the-file/udon)

Note you may want to provide more than just ..zuse in the subject (like a bowl), or if you're automatically building untrusted code, you may want to provide less. It depends on your use case.

You can alternatively import and build udon files at compile time with a /* (fastar) Ford rune specifying an %elem mark (which produces a manx), although note it compiles the Udon against an empty subject, so Hoon in embedded Sail won't have access to standard library functions. A mark conversion gate from %udon to %elem is another option.


The Docs App includes a a few files written in Udon which are useful as a reference.