Back AsyncComponent with a RemoteData tagged union

Introduce RemoteData<E,T> (Loading | Empty | Failure | Success) plus
fromResource and an exhaustive foldRemote. The data lives ON the state,
so "loaded without value" or "error with stale value" are unrepresentable.

AsyncComponent now derives a single rd() and pulls value/error out via the
fold instead of a loose State string. Public API (resource/isEmpty inputs,
the four slot directives, the ASYNC array) is unchanged, so the dashboard,
detail page, and async stories need no edits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/core/remote-data.ts

@@ -0,0 +1,47 @@
+import type { Resource } from '@angular/core';
+import { assertNever } from './fp';
+
+/**
+ * The four mutually-exclusive states of an async fetch, as a tagged union.
+ * Crucially the data lives ON the state: only `Failure` has an `error`, only
+ * `Success` has a `value`. "Loaded but no value" or "error with stale value"
+ * are unrepresentable — Richard Feldman's RemoteData.
+ */
+export type RemoteData<E, T> =
+  | { tag: 'Loading' }
+  | { tag: 'Empty' }
+  | { tag: 'Failure'; error: E }
+  | { tag: 'Success'; value: T };
+
+/** Project Angular's loosely-typed Resource into a RemoteData value. */
+export function fromResource<T>(
+  r: Resource<T>,
+  isEmpty: (v: T) => boolean = () => false,
+): RemoteData<Error | undefined, T> {
+  if (r.status() === 'error') return { tag: 'Failure', error: r.error() };
+  if (r.status() === 'loading') return { tag: 'Loading' };
+  if (r.hasValue()) {
+    const v = r.value();
+    return isEmpty(v) ? { tag: 'Empty' } : { tag: 'Success', value: v };
+  }
+  return { tag: 'Loading' };
+}
+
+/** Exhaustive fold: you must handle every case, checked at compile time. */
+export function foldRemote<E, T, R>(
+  rd: RemoteData<E, T>,
+  h: { loading: () => R; empty: () => R; failure: (e: E) => R; success: (v: T) => R },
+): R {
+  switch (rd.tag) {
+    case 'Loading':
+      return h.loading();
+    case 'Empty':
+      return h.empty();
+    case 'Failure':
+      return h.failure(rd.error);
+    case 'Success':
+      return h.success(rd.value);
+    default:
+      return assertNever(rd);
+  }
+}

src/app/molecules/async/async.component.ts

@@ -4,6 +4,7 @@ import type { Resource } from '@angular/core';
 import { SpinnerComponent } from '../../atoms/spinner/spinner.component';
 import { AlertComponent } from '../../atoms/alert/alert.component';
 import { ButtonComponent } from '../../atoms/button/button.component';
+import { fromResource, foldRemote } from '../../core/remote-data';
 
 /* Slot markers. Put on <ng-template> children of <app-async>. */
 @Directive({ selector: '[appAsyncLoaded]' })
@@ -23,27 +24,26 @@ export class AsyncErrorDirective {
   constructor(public tpl: TemplateRef<{ $implicit: Error | undefined; retry: () => void }>) {}
 }
 
-type State = 'loading' | 'error' | 'empty' | 'loaded';
-
 /**
  * Renders exactly ONE of loading / empty / error / loaded for a signal-based
- * resource (e.g. httpResource). The states are mutually exclusive by
- * construction, so the UI can never show two at once ("impossible states").
- * Unprovided slots fall back to sensible defaults.
+ * resource (e.g. httpResource). Built on a RemoteData tagged union (see
+ * core/remote-data.ts), so the states are mutually exclusive by construction —
+ * the UI can never show two at once ("impossible states"). Unprovided slots
+ * fall back to sensible defaults.
  */
 @Component({
   selector: 'app-async',
   imports: [NgTemplateOutlet, SpinnerComponent, AlertComponent, ButtonComponent],
   template: `
-    @switch (state()) {
-      @case ('loading') {
+    @switch (rd().tag) {
+      @case ('Loading') {
         @if (loadingTpl()) { <ng-container [ngTemplateOutlet]="loadingTpl()!.tpl" /> }
         @else { <app-spinner /> }
       }
-      @case ('error') {
+      @case ('Failure') {
         @if (errorTpl()) {
           <ng-container [ngTemplateOutlet]="errorTpl()!.tpl"
-            [ngTemplateOutletContext]="{ $implicit: resource().error(), retry: retry }" />
+            [ngTemplateOutletContext]="{ $implicit: error(), retry: retry }" />
         } @else {
           <app-alert type="error">Er ging iets mis bij het laden van de gegevens.</app-alert>
           <div style="margin-top:1rem">
@@ -51,11 +51,11 @@ type State = 'loading' | 'error' | 'empty' | 'loaded';
           </div>
         }
       }
-      @case ('empty') {
+      @case ('Empty') {
         @if (emptyTpl()) { <ng-container [ngTemplateOutlet]="emptyTpl()!.tpl" /> }
         @else { <p class="rhc-paragraph">Geen gegevens gevonden.</p> }
       }
-      @case ('loaded') {
+      @case ('Success') {
         <ng-container [ngTemplateOutlet]="loadedTpl().tpl"
           [ngTemplateOutletContext]="{ $implicit: value() }" />
       }
@@ -71,18 +71,17 @@ export class AsyncComponent<T> {
   emptyTpl = contentChild(AsyncEmptyDirective);
   errorTpl = contentChild(AsyncErrorDirective);
 
-  protected value = computed(() => {
-    const r = this.resource();
-    return r.hasValue() ? r.value() : undefined;
-  });
+  // Single source of truth: the resource projected into a RemoteData union.
+  protected rd = computed(() => fromResource(this.resource(), this.isEmpty()));
 
-  protected state = computed<State>(() => {
-    const r = this.resource();
-    if (r.status() === 'error') return 'error';
-    if (r.status() === 'loading') return 'loading';
-    if (r.hasValue()) return this.isEmpty()(r.value()) ? 'empty' : 'loaded';
-    return 'loading';
-  });
+  // value/error are pulled out via the exhaustive fold — only Success carries a
+  // value, only Failure carries an error, so these can't lie.
+  protected value = computed(() =>
+    foldRemote(this.rd(), { loading: () => undefined, empty: () => undefined, failure: () => undefined, success: (v) => v }),
+  );
+  protected error = computed(() =>
+    foldRemote(this.rd(), { loading: () => undefined, empty: () => undefined, failure: (e) => e, success: () => undefined }),
+  );
 
   retry = () => {
     const r = this.resource();