Fix #14: Reset playground engine between runs and display errors

The ChaiScript WASM engine accumulated state (variables, functions) across
script executions, causing confusing errors when editing code. Additionally,
if the engine aborted due to an unhandled exception, no meaningful error was
shown and subsequent runs silently failed.

Each run now reloads the WASM module via resetEngine() to guarantee a clean
engine state. An onAbort handler catches WASM crashes and displays the error
message. Exception formatting handles string, Error, and unknown throw types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: leftibot

playground.html

@@ -270,32 +270,85 @@ <h3>Interactive Playground <span id="chai-status" class="loading">Loading&hellip
     outputEl.scrollTop = outputEl.scrollHeight;
   }
 
-  var Module = {
-    print: function(text) {
-      appendOutput(text);
-    },
-    printErr: function(text) {
-      appendOutput(text, 'chai-output-error');
-    },
-    onRuntimeInitialized: function() {
-      statusEl.textContent = 'Ready';
-      statusEl.className = 'ready';
-      btnRun.disabled = false;
-      runtimeReady = true;
-      selectExample(0);
+  var engineAborted = false;
+  var engineScript = null;
+  var pendingCode = null;
+  var firstLoad = true;
+
+  function setEngineStatus(text, className) {
+    statusEl.textContent = text;
+    statusEl.className = className;
+  }
+
+  function formatError(e) {
+    if (typeof e === 'string') { return e; }
+    if (e && e.message) { return e.message; }
+    try { return String(e); } catch (_) { return 'Unknown error'; }
+  }
+
+  function executePendingCode() {
+    if (!pendingCode) { return; }
+    var code = pendingCode;
+    pendingCode = null;
+    try {
+      Module.eval(code);
+    } catch (e) {
+      appendOutput('Error: ' + formatError(e), 'chai-output-error');
+    }
+  }
+
+  function createModuleConfig() {
+    return {
+      print: function(text) {
+        appendOutput(text);
+      },
+      printErr: function(text) {
+        appendOutput(text, 'chai-output-error');
+      },
+      onAbort: function(what) {
+        engineAborted = true;
+        setEngineStatus('Error \u2014 click Run to retry', 'error');
+        appendOutput('Engine error: ' + formatError(what), 'chai-output-error');
+      },
+      onRuntimeInitialized: function() {
+        engineAborted = false;
+        setEngineStatus('Ready', 'ready');
+        btnRun.disabled = false;
+        runtimeReady = true;
+        if (firstLoad) {
+          firstLoad = false;
+          selectExample(0);
+        } else {
+          executePendingCode();
+        }
+      }
+    };
+  }
+
+  var Module = createModuleConfig();
+
+  function resetEngine() {
+    runtimeReady = false;
+    btnRun.disabled = true;
+    setEngineStatus('Resetting\u2026', 'loading');
+    if (engineScript) {
+      engineScript.parentNode.removeChild(engineScript);
+      engineScript = null;
     }
-  };
+    Module = createModuleConfig();
+    window.Module = Module;
+    engineScript = document.createElement('script');
+    engineScript.src = '/playground/chaiscript.js';
+    document.body.appendChild(engineScript);
+  }
 
   function runCode() {
     var code = inputEl.value;
     if (!code.trim()) { return; }
 
     outputEl.innerHTML = '';
-    try {
-      Module.eval(code);
-    } catch (e) {
-      appendOutput('Error: ' + e.message, 'chai-output-error');
-    }
+    pendingCode = code;
+    resetEngine();
   }
 
   function scheduleLiveRun() {
@@ -331,7 +384,10 @@ <h3>Interactive Playground <span id="chai-status" class="loading">Loading&hellip
   });
 
   buildSidebar();
+
+  engineScript = document.createElement('script');
+  engineScript.src = '/playground/chaiscript.js';
+  document.body.appendChild(engineScript);
 </script>
-<script src="/playground/chaiscript.js"></script>
 
 </body>

test_playground.sh

@@ -64,7 +64,15 @@ assert_file_contains "playground.html" "example-item"
 assert_file_contains "playground.html" "debounceTimer"
 assert_file_contains "playground.html" "addEventListener.*input"
 
-# 9. Playground examples cover major ChaiScript features
+# 9. Playground engine is reset between runs (issue #14)
+assert_file_contains "playground.html" "resetEngine"
+assert_file_contains "playground.html" "onAbort"
+
+# 10. Playground catches and displays exceptions with detail (issue #14)
+assert_file_contains "playground.html" "chai-output-error"
+assert_file_contains "playground.html" "engineAborted"
+
+# 11. Playground examples cover major ChaiScript features
 assert_file_contains "playground.html" "Variables & Types"
 assert_file_contains "playground.html" "Functions"
 assert_file_contains "playground.html" "Loops"