WebAssembly

lucid build --target web compiles a program to WebAssembly. The output is one file, NAME.js, which holds a small loader and two WebAssembly modules: the program and its runtime.

lucid build --target web hello.lcd   # writes hello.js
node hello.js                        # runs it under Node

A page loads it with one tag:

<!doctype html>
<html lang="en">
  <head><meta charset="utf-8"><title>Counter</title></head>
  <body>
    <script src="counter.js"></script>
  </body>
</html>

Every built feature works on the web, except those a page cannot have:

Feature	On the web
`spawn`, `std.net`, `std.http`	Error E0907
`extern` tasks and `.pointer()`	Error E0905
`std.fs`	Gives `Failed`
`env.get`, `args.all()`	`none`, `[]`
`time.sleep`	Traps
Channels and locks	Work, with one thread

Pages with std.dom

std.dom builds pages in the Elm architecture:

a model: any value that holds the page’s state,
view(model): gives a tree of dom.Node[Msg], where each handler gives a message,
update(model, message): gives the next model,
dom.app(init, view, update): shows view(init), and after each event runs update and then view, and changes only the nodes that differ.

use std.dom

choice Message
    Increment
    Decrement

task update(count: int, message: Message) -> int
    return match message
        Increment -> count + 1
        Decrement -> count - 1

task button(label: text, message: Message) -> dom.Node[Message]
    return dom.Element(tag: "button", attrs: [dom.On(event: "click", message: _ => message)], children: [dom.Text(content: label)])

task view(count: int) -> dom.Node[Message]
    return dom.Element(tag: "p", children: [button("-", Decrement), dom.Text(content: " {count} "), button("+", Increment)])

dom.app(0, view, update)

Constructor	Makes
`dom.Element(tag:, attrs: [], children: [])`	An element
`dom.Text(content:)`	Text, never read as HTML
`dom.Attribute(name:, value:)`	An attribute. `value` and `checked` also set an input’s state
`dom.On(event:, message:)`	A handler. `message` gets the event’s key, or the element’s value

std.dom works only with --target web. Elsewhere it is error E0906.

Safe by construction

std.dom never reads text as HTML. It traps on a script element, an attribute whose name starts with on, formaction, srcdoc, and a javascript: or data:text/html value. So a page runs only code that the program wrote, even when values come from a user.

Hash keys on the web come from crypto.getRandomValues, as they come from the system on native, so map order and collisions cannot be planned by whoever supplies the keys.