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Wisp

Reactivity

State in the browser with no build step: a counter, a value derived from it, and a list that filters as you type.

src/routes/+page.wisp

---
let crates = vec!["serde", "tokio", "rand", "regex", "clap", "anyhow", "rayon", "chrono", "image", "wisp"];
---

<title>Reactivity</title>
<h1>Reactivity</h1>

<h2>A Counter</h2>
<p>
  <button on:click="count--" aria-label="One less">-</button>
  <strong class="big">{:count}</strong>
  <button on:click="count++" aria-label="One more">+</button>
</p>
<p class="muted">Double is {:double}, and it is {:even ? 'even' : 'odd'}.</p>

<h2>A Live Filter</h2>
<label>Filter crates <input bind:value="q" placeholder="Type a letter or two"></label>
<ul class="list">
  {:#each crates.filter((c) => c.includes(q.toLowerCase())) as c}
    <li>{:c}</li>
  {:else}
    <li class="muted">No crate has {:q} in its name.</li>
  {:/each}
</ul>

<script>
  let count = $state(0)
  let double = $derived(count * 2)
  let even = $derived(count % 2 === 0)
  $effect(() => {
    document.title = `Reactivity (${count})`
  })
</script>

<style>
  .big {
    display: inline-block;
    min-width: 3ch;
    font-size: 1.75rem;
    text-align: center;
  }
</style>

src/app.css

/* One small stylesheet for every example: light or dark with the system,
   violet accent, neutrals tinted toward it. */
:root {
  color-scheme: light dark;
  --bg: light-dark(oklch(0.99 0.004 295), oklch(0.2 0.014 290));
  --panel: light-dark(oklch(1 0 0), oklch(0.24 0.016 290));
  --sunk: light-dark(oklch(0.965 0.008 295), oklch(0.17 0.012 290));
  --line: light-dark(oklch(0.9 0.012 295), oklch(0.34 0.02 290));
  --ink: light-dark(oklch(0.22 0.02 290), oklch(0.96 0.006 295));
  --muted: light-dark(oklch(0.48 0.02 290), oklch(0.76 0.016 290));
  --accent: light-dark(oklch(0.5 0.21 290), oklch(0.72 0.15 292));
  --accent-strong: light-dark(oklch(0.43 0.2 290), oklch(0.8 0.12 292));
  --accent-soft: light-dark(oklch(0.95 0.03 295), oklch(0.3 0.06 290));
  --on-accent: light-dark(oklch(1 0 0), oklch(0.18 0.02 290));
  --warn: light-dark(oklch(0.52 0.17 40), oklch(0.78 0.12 50));
  --radius: 0.5rem;
  --space: 1rem;
  --mono: ui-monospace, "Cascadia Code", Menlo, monospace;
}

:root[data-theme="light"] {
  color-scheme: light;
}

:root[data-theme="dark"] {
  color-scheme: dark;
}

* {
  box-sizing: border-box;
}

body {
  max-width: 40rem;
  margin: 0 auto;
  padding: 1.5rem 1rem 3rem;
  background: var(--bg);
  color: var(--ink);
  font: 400 1rem/1.6 system-ui, -apple-system, "Segoe UI", sans-serif;
  -webkit-font-smoothing: antialiased;
}

/* Type */

h1 {
  margin: 0 0 1rem;
  font-size: clamp(1.5rem, 1.2rem + 1.5vw, 1.875rem);
  line-height: 1.15;
  letter-spacing: -0.02em;
  text-wrap: balance;
}

h2 {
  margin: 2rem 0 0.75rem;
  font-size: 1.125rem;
  letter-spacing: -0.01em;
}

h3 {
  margin: 0 0 0.5rem;
  font-size: 1rem;
}

p {
  margin: 0 0 var(--space);
}

a {
  color: var(--accent);
  text-underline-offset: 0.2em;
}

a:hover {
  color: var(--accent-strong);
}

:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

code {
  padding: 0.1em 0.35em;
  border-radius: 0.3em;
  background: var(--sunk);
  font: 0.875em var(--mono);
}

pre {
  margin: 0 0 var(--space);
  padding: 0.875rem 1rem;
  overflow-x: auto;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--sunk);
  font: 0.875rem/1.55 var(--mono);
}

pre code {
  padding: 0;
  background: none;
}

.muted {
  color: var(--muted);
}

/* Nav between routes: a header of links, the current one marked. */

header,
nav {
  display: flex;
  flex-wrap: wrap;
  gap: 0.25rem 1.25rem;
  margin: 0 0 1.5rem;
  padding: 0 0 0.75rem;
  border-bottom: 1px solid var(--line);
  font-weight: 600;
}

header a,
nav a {
  color: var(--muted);
  text-decoration: none;
}

header a:hover,
nav a:hover,
header a[aria-current],
nav a[aria-current] {
  color: var(--ink);
}

/* Forms: fields stack their label above, the button sits at the end of the row. */

form {
  display: flex;
  flex-wrap: wrap;
  align-items: end;
  gap: 1rem 0.75rem;
  margin: 0 0 1.75rem;
}

/* A lone button with an action is a form of its own: it sits in the line. */
form:has(> button:only-child) {
  display: inline-flex;
  margin: 0;
}

label {
  position: relative;
  display: grid;
  flex: 1 1 12rem;
  gap: 0.375rem;
  color: var(--muted);
  font-size: 0.875rem;
  font-weight: 600;
}

label:has(> input[type="checkbox"]) {
  display: flex;
  flex: 0 0 auto;
  align-items: center;
  align-self: center;
}

input,
select,
textarea {
  width: 100%;
  min-height: 2.75rem;
  padding: 0.5rem 0.75rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  color: var(--ink);
  font: inherit;
  font-size: 1rem;
  font-weight: 400;
  transition: border-color 0.15s, box-shadow 0.15s;
}

input:hover,
select:hover,
textarea:hover {
  border-color: var(--muted);
}

input:focus,
select:focus,
textarea:focus {
  border-color: var(--accent);
  outline: none;
  box-shadow: 0 0 0 3px var(--accent-soft);
}

input[type="checkbox"] {
  width: 1.125rem;
  min-height: 0;
  accent-color: var(--accent);
}

input[type="file"] {
  padding: 0.4rem;
}

[aria-invalid="true"] {
  border-color: var(--warn);
}

/* A field's problem sits right under it, so the button beside the field stays put. */
label .problem {
  position: absolute;
  top: calc(100% + 0.25rem);
}

form:has(.problem) {
  row-gap: 2.25rem;
  margin-bottom: 2.75rem;
}

.problem {
  display: block;
  color: var(--warn);
  font-size: 0.8125rem;
  font-weight: 500;
}

/* Buttons */

button,
.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 2.75rem;
  padding: 0 1.125rem;
  border: 1px solid var(--accent);
  border-radius: var(--radius);
  background: var(--accent);
  color: var(--on-accent);
  font: 600 0.9375rem/1 system-ui, sans-serif;
  text-decoration: none;
  white-space: nowrap;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s, transform 0.1s;
}

button:hover,
.button:hover {
  border-color: var(--accent-strong);
  background: var(--accent-strong);
  color: var(--on-accent);
}

button:active {
  transform: translateY(1px);
}

button:disabled,
button[aria-busy="true"] {
  opacity: 0.6;
  cursor: progress;
  transform: none;
}

button.quiet {
  min-height: 2.25rem;
  padding: 0 0.75rem;
  border-color: var(--line);
  background: var(--panel);
  color: var(--ink);
  font-weight: 500;
}

button.quiet:hover {
  border-color: var(--accent);
  background: var(--accent-soft);
  color: var(--ink);
}

.bar {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
  margin-top: 1rem;
}

/* Lists: plain ones keep their bullets; a .list is rows in a panel. */

ul,
ol {
  margin: 0 0 var(--space);
  padding-left: 1.25rem;
}

li + li {
  margin-top: 0.25rem;
}

ul.list {
  margin: 0 0 var(--space);
  padding: 0;
  overflow: hidden;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  list-style: none;
}

label + ul.list {
  margin-top: 0.75rem;
}

ul.list li {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 0.5rem 0.75rem;
  margin: 0;
  padding: 0.625rem 0.75rem 0.625rem 1rem;
}

ul.list li + li {
  border-top: 1px solid var(--line);
}

/* An empty list says so, quietly, in the middle. */
ul.list li.muted:only-child {
  justify-content: center;
  padding: 1.5rem 1rem;
  background: var(--sunk);
}

/* Tables */

table {
  width: 100%;
  margin: 0 0 var(--space);
  border-collapse: collapse;
  font-size: 0.9375rem;
}

th,
td {
  padding: 0.5rem 0.75rem;
  border-bottom: 1px solid var(--line);
  text-align: left;
}

th {
  color: var(--muted);
  font-size: 0.8125rem;
  font-weight: 600;
}

hr {
  margin: 1.5rem 0;
  border: 0;
  border-top: 1px solid var(--line);
}

@media (prefers-reduced-motion: reduce) {
  * {
    transition: none !important;
    animation: none !important;
  }
}

src/app.html

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  %wisp.head%
</head>
<body>
  %wisp.body%
</body>
</html>

How It Works

Everything is in src/routes/+page.wisp. The server renders the page, then the <script> makes the parts marked with {:...} live in the browser.

let count = $state(0) is the state. $derived makes double and even that follow it, and $effect sets document.title whenever count changes. The buttons use on:click="count++", and {:count} updates in place.

The filter uses bind:value="q" on the input. {:#each crates.filter(...) as c} rebuilds the list on each key, with {:else} for no match. The list itself, crates, is Rust in the --- block.

A single $ means the server fills it once. A {: means the browser keeps it up to date.

Try it in the preview: press the buttons and type in the filter.

Type to search every docs page and heading.