Urbit / Docs

Introduction to Sail

Sail is Hoon markup that’s used to render a web page with XML. But what makes it special is that you can run arbitrary Hoon code within such a web page without using a separate markup language. In this tutorial, we will be dealing with Sail as it renders to HTML, a subset of XML.

Getting Started

Before starting, make sure that your urbit is mounted to Unix

To host that output, your ship also has a web-server, called Eyre, that can be found at http://localhost:8080/ if it’s your first ship that’s running on the a machine, http://localhost:8081/ if it’s the second ship on that same machine, and so on. In the startup messages, a ship will tell you which HTTP port it's using.

The /web directory of the %home desk is served by default. You can change which desk you're serving by using the command |serve in the Dojo. If you have already created and mounted a desk named %sandbox, for example, you can switch to serving it with this command:

|serve %sandbox

The first time you serve a desk, /web will be included as default. After that, you will need to explicitly append /web to the desk you're switching to -- otherwise, you'll need to write http://localhost:8080/web every time you navigate to your web-server. Newly served desks not automatically having /web as their root directory is a bug.

To switch back to your %home desk and set /web as your root directory, run:

|serve %/web

We'll be serving the %home desk for the examples in this tutorial.

Rendering

That server has access to source files located in /<your-urbit>/home/web and any sub-folders thereof. For the purposes of this tutorial, we will only be using files that are located in the /<your-urbit>/home/web/pages path.

There is an important distinction between these two paths. According to /ren/urb.hoon, any file inside of /web or any of its sub-directories is put through a tree of renderers specified at /ren/urb/tree.hoon -- except for files inside of /web/pages. All of the files in /web and its non-/pages children directories have various operations on files, such as wrapping your Sail the html, head, and body nodes. This rendering tool, called simply Tree, also adds certain formatting polish, such as a sidebar that lists sibling files in the same directory.

The /web/pages sub-directory is a unique case with its own rendering rules. The Tree is not used when rendering files in this path. The existence of no tag is inferred, and no special formatting is applied. Such simplicity is ideal for introducing the basics of Sail. Any source file located in /web/pages must produce a complete HTML node with “head” and “body” nodes as children.

Understanding the details of the rendering pipeline is not necessary to complete this tutorial. But, if you are curious, you might find it illuminating to explore the contents of the /ren directory.

So, if a Sail source file named sailtest.hoon is saved in your /web/pages folder on your only running urbit process, you can view that source rendered into a web-page at localhost:8080/pages/sailtest/.

The Basics

It’s easy to see how Sail can directly translate to HTML:

Sail code

;html
  ;head
    ;title: My first webpage
  ==
  ;body
    ;h1: Welcome!
    ; Hello, world!
    ; We're on the web.
    ;div;
    ;[email protected]"http://unsafely.tracking.you/cookiemonster.js";
  ==
==

HTML code

<html>
  <head>
    <title>My first webpage</title>
  </head>
  <body>
    <h1>Welcome!</h1>
    Hello, world!
    We’re on the web.
    <div></div>
    <script src="http://unsafely.tracking.you/cookiemonster.js"></script>
  </body>
</html>

Save the above Sail code in /home/web/pages/firstsail.hoon, and the above HTML code in /home/web/pages/firsth.html. Access the resulting pages by navigating to http://localhost:8080/pages/firstsail and http://localhost:8080/pages/firsth.html, respectively, in your browser. The resulting pages should be identical!

It shouldn't be hard to see the similarities here. So let's go into more detail about what the differences are.

Tags and Closing

In Sail, tag heads are written with the tag name prepended by ;. Unlike in HTML, there are different ways on closing tags, depending on the needs of the tag. One of the nice things about Hoon is that you don’t have to be constantly closing expressions; Sail inherits this convenience.

Empty

Empty tags are closed with a ; following the tag. Example:

;div;

Equals:

<div></div>

Filled

Filled tags are closed via line-break. To fill text inside, add : after the tag name, then insert your plain text following a space. Example:

;h1: The title

Equals:

<h1>The title</h1>

Nested

To nest tags, simply create a new line. Nested tags need to be closed with ==, because they expect a list of sub-tags. If we nest a line of plain text with no tag, a <p> is prepended.

Sail, like Hoon, is white-space sensitive. Unlike in Hoon, however, two spaces (called a “gap”) is not necessarily equivalent to a line-break in Sail.

;body
  ;h1: Blog title
  This is some good content.
==

Equals:

<body>
    <h1>Blog title</h1>
    <p>This is some good content.</p>
</body>

Conversely, if we want to write a string with no tag at all, then we can prepend those untagged lines with ;.

;body
  ;h1: Welcome!
  ; Hello, world!
  ; We’re on the web.
==

Equals:

<body>
    <h1>Welcome!</h1>
    Hello, world!
    We’re on the web.
</body>

Attributes

Attributes are key-value pairs that go into an HTML node.

Adding attributes is simple: just add the desired attribute between parentheses, right after the tag name without a space. We separate different attributes of the same node by using ,.

Attributes can be used in two forms: flat, which uses one line; and tall, which uses multiple lines, when a single line would not be practical. Flat forms and flat forms are two syntaxes of semantically equivalent expressions.

Generic

The code below produces a “Submit” button on the page.

Wide-form Sail:

;input(type "submit", value "Submit");

Tall-form Sail:

;input
  =type  "submit"
  =value  "Submit";
==

Equals:

<input type="submit" value="Submit">`

IDs

;nav#header: Menu

Equals:

<nav id="header">Menu</nav>

Add # after tag name to add an ID.

Classes

;h1.text-blue: Title equals <h1 class="text-blue">Title</h1>

Add . after tag name to add a class. However, if you want a class name that contains a space, you will need to use use the syntax of a generic attribute:

;div(class "logo inverse"); equals <div class="logo inverse"></div>

Image

;[email protected]"example.png";

Equals

<img src="example.png"/>

Add @ after the tag name to link your source.

To add attributes to the image, like size specifications, add the desired attribute after the " of the image name and before the final ; of the img tag.

For example:

;[email protected]"https://urbit.org/example.png"(width "100%");

Linking

;a/"urbit.org": A link to Urbit.org

Equals

<a href="urbit.org">A link to Urbit.org</a>

Add / after tag name to start an href.

Hoon in Sail

Our first example is useful to understand how Sail syntax translates to HTML syntax, but Sail finds its usefulness in the fact that it can be used seamlessly with Hoon code.

One place where Sail diverges from HTML is that it supports string interpolation. In Sail, the contents of all tags are a string type called tape unless otherwise designated. Tapes are delineated by the double quotes "". That’s important, because tapes can be interpolated with Hoon expressions, provided that those expressions produce something a tape as well. Interpolation is done by wrapping the code to be inserted in {}. In your Dojo prompt, try the command below (without the >):

> "I am a l{(mul 3 11)}t Urbit user."
! exit

Oh no! That command produced a nest-fail error, which indicates that there was a problem related to Hoon type-system. We used the proper {} syntax for interpolation, and we used the proper function-call syntax inside of it. So what happened?

Trying the mul function on its own will help answer this question. Try the following command in your Dojo:

> (mul 3 11)
33

This should work, producing 33, an atom. That’s our problem. The interpolation expression expects a tape, but our first command tried to interpolate with something that results in an atom. Try this now:

> <(mul 3 11)>
"33"

We get a similar result, but we know that it’s a tape because it’s wrapped in "". Bingo! With this knowledge, let’s perform a successful interpolation:

> "I am a l{<(mul 3 11)>}t Urbit user."
"I am a l33t Urbit user."

Great!

The Subject

The default subject of a Sail file is different from the subject that expressions in the Dojo or a Hoon generator are evaluated against.

To see this difference, first save the code below to a file at home/web/pages/sub.hoon and view the rendering at http://localhost:8080/pages/sub.

;html
  ;head;
  ;body
    {<.>}
  ==
==

Then enter the following command in the Dojo:

> .

You'll notice that the Sail subject is like a stripped-down version of the Dojo subject. Its final line should match the Dojo subject.

Now swap the code below into your Sail file.

/=  gas  /$  fuel:html
;html
  ;head;
  ;body
    {<.>}
  ==
==

You'll find that the resulting Sail subject is much larger. If you look closely, you'll see that there's some new similarities with the Dojo subject, such as containing the name of your ship. There's also some new differences, since fuel:html contains things that are specifically useful for performing web-related operations.

A More Interesting Example

Now let’s apply knowledge of string interpolation and the Sail subject in a Sail source file. The following code produces more interesting things than the example we saw before.

/=  gas  /$  fuel:html
=/  show-list  &
=+  wid=(met 3 p.bem.gas)
=/  what-kind
?:  (lte wid 1)   "galaxy"
?:  =(2 wid)      "star"
?:  (lte wid 4)   "planet"
?:  (lte wid 8)   "moon"
  "comet"
;html
  ;head
    ;meta(charset "utf-8");
    ;title: A more advanced page
  ==
  ;body
  ;h1: This is the Title
    ; This is a
      ;a/"http://urbit.org": link to Urbit.org.
    ;span:  Check it out!
    ;+  ?:  show-list
        ;ol
          ;li(style "color: green"): We're doing interesting stuff now.
          ;li: We're pretty-printing this sum with Hoon: {<(add 50 50)>}.
          ;li: The code above is shorthand for {(scow %ud (add 50 50))}.
          ;li: I am {(trip '~lodleb-ritrul')}.
          ;li: Actually, my name is {<p.bem.gas>}. I'm a {what-kind}.
        ==
        ;div: My name is {<p.bem.gas>}. I'm a {what-kind}.
  ==
==

Save the above Sail code in to home/web/pages/secondsail.hoon and access the resulting page by navigating to http://localhost:8080/pages/secondsail in your browser.

There are some interesting things here. Let’s go through this code piece by piece.

/=  gas  /$  fuel:html

Later in this code we want produce the name of the ship is hosting it, something that looks like ~lodleb-ritrul. In a generator, we could produce our ship name by writing our -- try it in the Dojo. But remember that Sail is rendered against its own subject, our is not part of the default subject. To augment our subject with the information that we need, we use /= gas /$ fuel:html.

The above expression uses two Ford runes, /= and /$, to add the relevant information to the current subject. We can pull the ship’s name out of this augmented subject later. It's important to note that these runes are not part of Hoon. They are part of Ford, our build system.

/$ is used to get data from the environment, and /= adds that data to subject in the face gas. The Ford user manual has details on these and other Ford runes, but understanding these runes isn’t necessary for the purposes of this tutorial.

fuel:html gets extra data from your ship’s Eyre module, which handles all things HTTP, and sticks it in the subject when Eyre renders a page. This includes the name of the ship, which is what we look for in the next line, but it includes much more.

p.bem.gas is an expression that looks for p within bem within gas, the face that we stored the above fuel:html expression in. That wing happens to resolve to the name of the ship that is hosting the web-server. If we wanted to access the current desk, we would use the wing expression q.bem.gas.

You can take a look at what you can access by typing fuel:html in the Dojo, and then explore it by modifying your wing expression in your Sail file accordingly.

=/  show-list  &

The above line of code stores a variable used later for a conditional. & means true, meaning that any evaluation of show-list will be true until changed in the source code.

=+  wid=(met 3 p.bem.gas)
=/  what-kind
  ?:  (lte wid 1)   "galaxy"
  ?:  =(2 wid)      "star"
  ?:  (lte wid 4)   "planet"
  ?:  (lte wid 8)   "moon"
  "comet"

This code chunk is a series of conditionals that checks the host ship’s name to see what its value is.

The first line, =+ wid=(met 3 p.bem.gas), combines a new noun with the subject and gives it the face wid. This noun is produced by the the standard-library function met, which measures the number of blocks of size a within b. Blocks are units that have a bitwidth of 2^a. So, in this case, it measures how many bytes (blocks of size 3 are bitwidths of 8; 2^3 = 8) are fully contained in your ship's address.

Galaxies contain zero to one 3-blocks; stars contain two 3-blocks; planets contain three to four 3-blocks; moons contain five to eight 3-blocks; comets contain nine to sixteen 3-blocks. Each test uses lte, the less-than-or-equal-to function, to determine if wid is of a given size. The result of this series of tests is added to the subject and given the face what-kind.

Ship names are simply another representation of atoms, and ship categories are different ranges of possible atomic values. Because of this, we perform less-than tests to see what kind of ship is running the web-server.

;html
  ;head
    ;meta(charset "utf-8");
    ;title: A more advanced page
  ==

Sail proper begins here. These four lines are four nodes. The ;html node indicates that the contained code should be rendered as HTML; this tag is closed on the final line. The ;head node contains metadata, and is closed three lines later. The ;meta node sets the character set to UTF-8, and the ;title node sets the title of the page that shows up in the browser tab.

  ;body
  ;h1: This is the Title
    ; This is a
      ;a/"http://urbit.org": link to Urbit.org.
    ;span:  Check it out!

The first line here opens, of course, the body node. The second line creates header text.

The third, fourth, and fifth lines of code here construct a single line with an inline link. ; This is a begins the rendered line with plain text.

;a/"http://urbit.org": link to Urbit.org. continues that same rendered line, and creates hyperlinked text out of link to Urbit.org.

;span: Check it out! continues that same rendered line further. ;span: has the same use as <span> in HTML, and by following it with just a : and no attributes, it continues the line without formatting. Following ;span: with two spaces causes there to be single space following the produced <span> element, because one of those spaces is syntactically necessary.

In the following lines, we beginning to use some Hoon expressions again.

  ;+  ?:  show-list
          ;ol

In the top line in the chunk above, we use the ;+ Sail rune, which resolves an expression to a single node. That expression, in this case, is ?:, which tests the truth value of show-list, which we assigned the value & (meaning “true”) at the beginning of the program. It’s closed by the == on the last line.

Thus, this program resolves to the first child of show-list, the node ;ol. That node creates an ordered list, and the items contained within that node.

            ;li(style "color: green"): We're doing more interesting stuff now.

The line above is the only element of this list that does not use Hoon proper. The line renders as green, as interpreted by the browser, because of the Sail attribute.

The next items in the list use {}. Recall that these braces, pronounced “lob” and “rob” in Hoon-speak, allow for the interpolation of code within a string.

            ;li: We're pretty-printing this sum with Hoon: {<(add 50 50)>}.

The line above is interpolated with Hoon code that produces the sum of 50 plus 50, which is wrapped in <> to put the product in double quotes to make it a tape, like the rest of the line.

            ;li: The code above is shorthand for {(scow %ud (add 50 50))}.

The {(scow %ud (add 50 50)} on this line is just a different way writing the {<(add 50 50)>} expression. scow is a Hoon function that turns a noun into a tape, Hoon’s string type. %ud tells scow that its argument should be displayed in the form of an unsigned decimal.

            ;li: I am {(trip '~lodleb-ritrul')}.

trip is a function that takes a cord and produces a tape. A piece of data of the cord type is one big atom formed from adjacent unicode bytes and delineated by ''. Remember that all text rendered in Sail is treated as a tape. So, by converting ~lodleb-ritrul to a tape, it can be included in the tape that we are trying to insert it into without getting a type error.

            ;li: Actually, my name is {<p.bem.gas>}. I'm a {what-kind}.
          ==

The line above has interpolated code that uses a wing expression to access the name of the host ship, and then accesses the face what-kind contains a string describing what type of ship it is. We declared gas on the very first line so that we could add that sort of information to the subject that our Sail is rendered against. The wing expression p.bem.gas looks for p within bem within gas, which happens to be where the name of the ship is located.

Note that {what-kind} does not have a <> wrapper. Why do you think this is?

        ;div; My name is {<p.bem.gas>}. I'm a {ship}.
  ==
==

The ;div; node above is only rendered, as an alternative to the above list, if show-list is evaluated as true. This means it won’t be shown with your default code. To show this line, change the & on line 2 to | (“false”).

Sail Runes

In the previous example, we used a few special runes that aren’t used in typical Hoon code. But there are others that are worth learning about at this point.

;+ is an expression that resolves to a single node. We saw this on line 20 in our Sail example above: ;+ ?: show-list resolves to the node ;ol and all of that node’s children.

;* is an expression that resolves to a list of nodes. If we want to render multiple nodes side-by-side, and not just the children of a single node, this is the expression that we use.

;= turns a tuple of nodes into a list of nodes.

To see how these runes work, let’s take a chunk of our previous code starting at line 20 and modify it a bit:

  ;*  ?:  show-list
        ;=
        ;li: I am {(trip '~lodleb-ritrul')}.
        ;li: Actually, my name is {<p.bem.gas>}.
        ==
      ;=
    ;div: My name is {<p.bem.gas>}. I'm a {what-kind}.
    ;div: No list here!
      ==
  ==

We changed the conditional branches so that each contains two Sail nodes, side-by-side -- notice that the ;ol is absent from the first branch.

We begin with the ;* rune, which resolves to a list of nodes, instead of the ;+ rune, which only resolves to a single node.

But that’s not enough on its own. ;* needs to resolve to a list, and a few Sail nodes aren’t lists until they are explicitly made into lists. Until we wrap those nodes in ;=, the nodes treated as mere tuples. All branches must be valid lists for the ;* ?: show-list expression to resolve.

Conclusion

You should now have foundational knowledge for making web pages with Sail. There is, of course, more to learn. If you’re curious, there’s more advanced resources out there. The Ford manual can show you how to access various ship resources for user in your pages. To learn more about how the renderer works, take a look at the /home/ren/urb.hoon file inside your urbit.