3 files changed, 64 insertions, 41 deletions
diff --git a/chapter-7/index.html b/chapter-7/index.html
index 17e5f54..e5c4372 100644
--- a/chapter-7/index.html
+++ b/chapter-7/index.html
@@ -7,5 +7,20 @@
     <script type="text/javascript" src="propagators.js"></script>
   </head>
   <body>
+    <article id="wasm-error" hidden="true">
+      <h1>Uh oh!</h1>
+      <p>
+        A browser with Wasm GC and tail call support is required for
+        this demo.
+      </p>
+      <p>
+        We recommend using either Firefox or Chrome.
+      </p>
+      <p>
+        Safari is currently unsupported.  Likewise, <emph>all browsers</emph> on
+        iOS are unsupported, as they are all secretly Safari under the
+        hood.
+      </p>
+    </article>
   </body>
 </html>
diff --git a/chapter-7/propagators.js b/chapter-7/propagators.js
index c392ad1..fe0034a 100644
--- a/chapter-7/propagators.js
+++ b/chapter-7/propagators.js
@@ -1,23 +1,30 @@
 window.addEventListener("load", async () => {
-  await Scheme.load_main("propagators.wasm", {
-    reflect_wasm_dir: "hoot",
-    user_imports: {
-      window: {
-        setTimeout: setTimeout
-      },
-      document: {
-        makeTextNode: Document.prototype.createTextNode.bind(document),
-        makeElement: Document.prototype.createElement.bind(document),
-        body: () => document.body,
-      },
-      element: {
-        appendChild: (parent, child) => parent.appendChild(child),
-        setAttribute: (elem, attr, value) => elem.setAttribute(attr, value),
-        getValue: (elem) => elem.value,
-        setValue: (elem, val) => elem.value = val,
-        replaceWith: (oldElem, newElem) => oldElem.replaceWith(newElem),
-        addEventListener: (elem, name, f) => elem.addEventListener(name, f)
-      },
+  try {
+    await Scheme.load_main("propagators.wasm", {
+      reflect_wasm_dir: "hoot",
+      user_imports: {
+        window: {
+          setTimeout: setTimeout
+        },
+        document: {
+          makeTextNode: Document.prototype.createTextNode.bind(document),
+          makeElement: Document.prototype.createElement.bind(document),
+          body: () => document.body,
+        },
+        element: {
+          appendChild: (parent, child) => parent.appendChild(child),
+          setAttribute: (elem, attr, value) => elem.setAttribute(attr, value),
+          getValue: (elem) => elem.value,
+          setValue: (elem, val) => elem.value = val,
+          replaceWith: (oldElem, newElem) => oldElem.replaceWith(newElem),
+          addEventListener: (elem, name, f) => elem.addEventListener(name, f)
+        },
+      }
+    });
+  } catch(e) {
+    if(e instanceof WebAssembly.CompileError) {
+      document.getElementById("wasm-error").hidden = false;
     }
-  });
+    throw e;
+  }
 });
diff --git a/chapter-7/propagators.scm b/chapter-7/propagators.scm
index e0096b6..3b54e47 100644
--- a/chapter-7/propagators.scm
+++ b/chapter-7/propagators.scm
@@ -197,25 +197,26 @@
 (define (add-cell-neighbor! cell neighbor)
   (set-cell-neighbors! cell (lset-adjoin eq? (cell-neighbors cell) neighbor)))
 
-(define (test-cell-content! cell)
+(define (add-cell-content! cell new)
   (match cell
-    (($ <cell> _ neighbors content strongest equivalent? _
+    (($ <cell> _ neighbors content strongest equivalent? merge
                find-strongest handle-contradiction)
-     (let ((strongest* (find-strongest content)))
-       (cond
-        ((equivalent? strongest strongest*)
-         (set-cell-strongest! cell strongest*))
-        ((contradiction? strongest*)
-         (set-cell-strongest! cell strongest*)
-         (handle-contradiction cell))
-        (else
-         (set-cell-strongest! cell strongest*)
-         (for-each alert-propagator! neighbors)))))))
-
-(define (add-cell-content! cell new)
-  (let ((merge (cell-merge cell)))
-    (set-cell-content! cell (merge (cell-content cell) new))
-    (test-cell-content! cell)))
+     (let ((content* (merge content new)))
+       (set-cell-content! cell content*)
+       (let ((strongest* (find-strongest content*)))
+         (cond
+          ;; New strongest value is equivalent to the old one.  No need
+          ;; to alert propagators.
+          ((equivalent? strongest strongest*)
+           (set-cell-strongest! cell strongest*))
+          ;; Uh oh, a contradiction!  Call handler.
+          ((contradiction? strongest*)
+           (set-cell-strongest! cell strongest*)
+           (handle-contradiction cell))
+          ;; Strongest value has changed.  Alert the propagators!
+          (else
+           (set-cell-strongest! cell strongest*)
+           (for-each alert-propagator! neighbors))))))))
 
 (define (alert-propagator! propagator)
   (queue-task! (propagator-activate propagator)))
@@ -242,20 +243,20 @@
            (add-cell-content! output (apply f args)))))
      (make-propagator name inputs (list output) activate))))
 
-(define (compound-propagator name inputs outputs to-build)
+(define (compound-propagator name inputs outputs build)
   (let ((built? #f))
     (define (maybe-build)
       (unless (or built?
                   (and (not (null? inputs))
                        (every unusable-value? (map cell-strongest inputs))))
         (parameterize ((current-parent (propagator-relations propagator)))
-          (to-build)
+          (build)
           (set! built? #t))))
     (define propagator (make-propagator name inputs outputs maybe-build))
     propagator))
 
-(define (constraint-propagator name cells to-build)
-  (compound-propagator name cells cells to-build))
+(define (constraint-propagator name cells build)
+  (compound-propagator name cells cells build))
 
 (define-record-type <reactive-id>
   (%make-reactive-id name clock)