Add reactive always, when & bind features

docs/reactivity-design.md

@@ -0,0 +1,92 @@
+# Reactivity Design Notes
+
+Internal reference for the reactive features: `always`, `when`, and `bind`.
+
+## Why this works without explicit signals
+
+SolidJS (and most JS reactive frameworks) use explicit signal primitives:
+
+```js
+const [count, setCount] = createSignal(0);
+count()      // read — registers a subscription
+setCount(5)  // write — notifies subscribers
+```
+
+This is necessary because JavaScript has no way to intercept plain variable
+reads and writes. `myVar = 5` is invisible to framework code. So Solid must
+wrap values in function calls to run custom logic on access.
+
+_hyperscript doesn't have this constraint. It is its own language with its own
+execution engine. Every variable read goes through `resolveSymbol()` in the
+runtime, and every write goes through `setSymbol()`. The runtime is already
+the middleman for all variable access, so we hook tracking and notification
+directly into these existing paths:
+
+- **Read** (`resolveSymbol`): if an effect is currently evaluating, record the
+  variable as a dependency.
+- **Write** (`setSymbol`): notify all effects that depend on this variable.
+
+The user gets reactive behavior without any API ceremony. `$count` is just a
+variable. Writing `when $count changes ...` is all it takes to make it watched.
+
+## The JS interop boundary
+
+This approach is airtight for pure _hyperscript code, but leaky at the JS
+boundary:
+
+- **Writes from JS** (`window.$count = 99`) bypass `setSymbol`, so no
+  notification fires.
+- **Reads inside `js()` blocks** (`js(x) return window.$price * window.$qty end`)
+  bypass `resolveSymbol`, so no dependencies are tracked.
+- **`Object.assign` in the js feature** writes directly to `globalScope`,
+  also bypassing `setSymbol`.
+
+This is an accepted trade-off. For typical _hyperscript usage the reactivity
+is invisible and correct. Users mixing JS interop with reactive expressions
+should be aware that the tracking boundary is the _hyperscript runtime.
+
+## What creates reactivity
+
+Variables are not signals. `set $count to 0` just stores a value on
+`globalScope` (i.e. `window`). Nothing reactive happens.
+
+Reactivity is created by `always`, `when`, and `bind`. All three use
+`createEffect()` under the hood, which evaluates code with tracking
+enabled. During that evaluation, `resolveSymbol` sees that an effect
+is active and records dependencies. Future writes via `setSymbol` notify
+all subscribed effects.
+
+The variable itself has no idea it's being watched. The reactivity lives
+entirely in the effect and the subscription maps.
+
+## The three reactive features
+
+Each serves a distinct purpose:
+
+- **`always`** declares relationships. The block runs as one unit with
+  automatic dependency tracking and re-runs when any dependency changes.
+  Used for derived values, DOM updates, and conditional state. For
+  independent tracking, use separate `always` statements.
+
+- **`when`** reacts to changes. Watches a specific expression and runs a
+  command body when the value changes. `it` refers to the new value. Used
+  for side effects, async work, and events.
+
+- **`bind`** syncs two values bidirectionally. Includes same-value dedup to
+  prevent infinite loops. Used for form inputs and shared state.
+
+`always` runs its entire command block inside the tracking context (the
+block IS the effect). `when` separates the tracked expression from the
+handler commands. `bind` creates two `when`-style effects pointing at
+each other.
+
+## Why styles and computed styles are not tracked
+
+`*opacity`, `*computed-width`, and other style references are not reactive.
+There is no efficient DOM API for "notify me when a computed style changes."
+`MutationObserver` only catches inline style attribute edits, not changes
+from classes, media queries, CSS animations, or the cascade. No reactive
+framework (SolidJS, Vue, Svelte) tracks computed styles either.
+
+To react to style-affecting changes, track the cause instead: the variable,
+attribute, or class that drives the style.

src/_hyperscript.js

@@ -9,6 +9,7 @@ import {Runtime} from './core/runtime/runtime.js';
 import {HyperscriptModule} from './core/runtime/collections.js';
 import {config} from './core/config.js';
 import {conversions} from './core/runtime/conversions.js';
+import {reactivity} from './core/runtime/reactivity.js';
 
 // Import parse element modules
 import * as Expressions from './parsetree/expressions/expressions.js';
@@ -36,6 +37,9 @@ import * as WorkerFeatureModule from './parsetree/features/worker.js';
 import * as BehaviorFeatureModule from './parsetree/features/behavior.js';
 import * as InstallFeatureModule from './parsetree/features/install.js';
 import * as JsFeatureModule from './parsetree/features/js.js';
+import * as WhenFeatureModule from './parsetree/features/when.js';
+import * as BindFeatureModule from './parsetree/features/bind.js';
+import * as AlwaysFeatureModule from './parsetree/features/always.js';
 
 const globalScope = typeof self !== 'undefined' ? self : (typeof global !== 'undefined' ? global : this);
 
@@ -76,6 +80,9 @@ kernel.registerModule(WorkerFeatureModule);
 kernel.registerModule(BehaviorFeatureModule);
 kernel.registerModule(InstallFeatureModule);
 kernel.registerModule(JsFeatureModule);
+kernel.registerModule(WhenFeatureModule);
+kernel.registerModule(BindFeatureModule);
+kernel.registerModule(AlwaysFeatureModule);
 
 // ===== Public API =====
 
@@ -111,7 +118,7 @@ const _hyperscript = Object.assign(
         },
 
         internals: {
-            tokenizer, runtime,
+            tokenizer, runtime, reactivity,
             createParser: (tokens) => new Parser(kernel, tokens),
         },