Address review: make ChaiScript engine an opaque handle, drop singleton

Replace the static ChaiScript singleton in the Emscripten wrapper with a
handle-based registry symmetric to the existing State registry. JS callers
now create an engine with chaiscript_create(), pass the resulting handle to
the eval/state helpers, and release it with chaiscript_destroy(). Multiple
independent engines are now possible, and a state snapshot can be restored
onto any engine. Updated the playground HTML and the three native regression
tests to exercise the new API.

Requested by @lefticus in PR #699 review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

emscripten/chaiscript.html

@@ -185,6 +185,8 @@ print("10! = " + to_string(factorial(10)))
       outputEl.scrollTop = outputEl.scrollHeight;
     }
 
+    var chaiHandle = 0;
+
     var Module = {
       print: function(text) {
         appendOutput(text);
@@ -193,6 +195,7 @@ print("10! = " + to_string(factorial(10)))
         appendOutput(text, 'output-error');
       },
       onRuntimeInitialized: function() {
+        chaiHandle = Module.create();
         statusEl.textContent = 'Ready';
         statusEl.className = 'ready';
         btnRun.disabled = false;
@@ -205,7 +208,7 @@ print("10! = " + to_string(factorial(10)))
 
       appendOutput('> Running...', 'output-line');
       try {
-        Module.eval(code);
+        Module.eval(chaiHandle, code);
       } catch (e) {
         appendOutput('Error: ' + e.message, 'output-error');
       }

emscripten/chaiscript_em.cpp

@@ -13,6 +13,8 @@
 #include <emscripten/bind.h>
 
 EMSCRIPTEN_BINDINGS(chaiscript) {
+  emscripten::function("create", &chaiscript_create);
+  emscripten::function("destroy", &chaiscript_destroy);
   emscripten::function("eval", &chaiscript_eval);
   emscripten::function("evalString", &chaiscript_eval_string);
   emscripten::function("evalBool", &chaiscript_eval_bool);

emscripten/chaiscript_eval.hpp

@@ -7,21 +7,26 @@
 // Shared eval helper functions for the ChaiScript Emscripten wrapper.
 // These functions provide typed evaluation of ChaiScript expressions,
 // used by both the Emscripten/WebAssembly build and native tests.
+//
+// The interface is opaque and handle-based: JS callers create one or more
+// engines via chaiscript_create(), pass the resulting handle to the eval and
+// state helpers, and release the engine with chaiscript_destroy() when done.
+// State snapshots are themselves opaque handles produced by
+// chaiscript_save_state() and consumed by chaiscript_restore_state() /
+// chaiscript_release_state(). Hiding ChaiScript and ChaiScript::State behind
+// integer handles keeps embind from having to manage their lifetimes and
+// avoids forcing a static singleton on the C++ side.
 
 #ifndef CHAISCRIPT_EMSCRIPTEN_EVAL_HPP_
 #define CHAISCRIPT_EMSCRIPTEN_EVAL_HPP_
 
 #include <chaiscript/chaiscript.hpp>
 #include <map>
+#include <memory>
 #include <string>
 #include <unordered_map>
 
 namespace detail {
-  inline chaiscript::ChaiScript &get_chai_instance() {
-    static chaiscript::ChaiScript chai;
-    return chai;
-  }
-
   // ChaiScript::State captures globals/functions/types but not the top-level
   // scripting locals (variables created by `var x = ...` at the script's
   // outermost scope). The playground's reset-between-runs use case needs both,
@@ -31,66 +36,89 @@ namespace detail {
     std::map<std::string, chaiscript::Boxed_Value> locals;
   };
 
+  inline std::unordered_map<int, std::unique_ptr<chaiscript::ChaiScript>> &chai_registry() {
+    static std::unordered_map<int, std::unique_ptr<chaiscript::ChaiScript>> registry;
+    return registry;
+  }
+
   inline std::unordered_map<int, Snapshot> &state_registry() {
     static std::unordered_map<int, Snapshot> registry;
     return registry;
   }
 
-  inline int next_state_handle() {
+  inline int next_handle() {
     static int handle = 0;
     return ++handle;
   }
+
+  inline chaiscript::ChaiScript &get_chai(const int handle) {
+    return *chai_registry().at(handle);
+  }
 } // namespace detail
 
-inline void chaiscript_eval(const std::string &input) {
-  detail::get_chai_instance().eval(input);
-}
-
-inline std::string chaiscript_eval_string(const std::string &input) {
-  return detail::get_chai_instance().eval<std::string>(input);
-}
-
-inline bool chaiscript_eval_bool(const std::string &input) {
-  return detail::get_chai_instance().eval<bool>(input);
-}
-
-inline int chaiscript_eval_int(const std::string &input) {
-  return detail::get_chai_instance().eval<int>(input);
-}
-
-inline float chaiscript_eval_float(const std::string &input) {
-  return detail::get_chai_instance().eval<float>(input);
-}
-
-inline double chaiscript_eval_double(const std::string &input) {
-  return detail::get_chai_instance().eval<double>(input);
-}
-
-// Snapshot the current engine state and return an opaque handle. The handle
-// is owned by the caller; release it with chaiscript_release_state when it is
-// no longer needed.
-inline int chaiscript_save_state() {
-  const int handle = detail::next_state_handle();
-  auto &chai = detail::get_chai_instance();
-  detail::state_registry().emplace(handle, detail::Snapshot{chai.get_state(), chai.get_locals()});
+// Construct a fresh ChaiScript engine and return an opaque handle. The handle
+// is owned by the caller and must be released with chaiscript_destroy().
+inline int chaiscript_create() {
+  const int handle = detail::next_handle();
+  detail::chai_registry().emplace(handle, std::make_unique<chaiscript::ChaiScript>());
   return handle;
 }
 
-// Restore a previously snapshotted state. Unknown handles are silently
-// ignored so JS callers do not need to track validity defensively.
-inline void chaiscript_restore_state(const int handle) {
-  const auto it = detail::state_registry().find(handle);
+// Destroy an engine handle. Unknown handles are silently ignored so JS callers
+// do not need to track validity defensively.
+inline void chaiscript_destroy(const int handle) {
+  detail::chai_registry().erase(handle);
+}
+
+inline void chaiscript_eval(const int handle, const std::string &input) {
+  detail::get_chai(handle).eval(input);
+}
+
+inline std::string chaiscript_eval_string(const int handle, const std::string &input) {
+  return detail::get_chai(handle).eval<std::string>(input);
+}
+
+inline bool chaiscript_eval_bool(const int handle, const std::string &input) {
+  return detail::get_chai(handle).eval<bool>(input);
+}
+
+inline int chaiscript_eval_int(const int handle, const std::string &input) {
+  return detail::get_chai(handle).eval<int>(input);
+}
+
+inline float chaiscript_eval_float(const int handle, const std::string &input) {
+  return detail::get_chai(handle).eval<float>(input);
+}
+
+inline double chaiscript_eval_double(const int handle, const std::string &input) {
+  return detail::get_chai(handle).eval<double>(input);
+}
+
+// Snapshot the engine identified by chai_handle and return a fresh opaque
+// state handle. Release it with chaiscript_release_state() when no longer
+// needed.
+inline int chaiscript_save_state(const int chai_handle) {
+  const int state_handle = detail::next_handle();
+  auto &chai = detail::get_chai(chai_handle);
+  detail::state_registry().emplace(state_handle, detail::Snapshot{chai.get_state(), chai.get_locals()});
+  return state_handle;
+}
+
+// Restore a previously snapshotted state onto the engine identified by
+// chai_handle. Unknown state handles are silently ignored.
+inline void chaiscript_restore_state(const int chai_handle, const int state_handle) {
+  const auto it = detail::state_registry().find(state_handle);
   if (it != detail::state_registry().end()) {
-    auto &chai = detail::get_chai_instance();
+    auto &chai = detail::get_chai(chai_handle);
     chai.set_state(it->second.engine_state);
     chai.set_locals(it->second.locals);
   }
 }
 
-// Release a handle returned by chaiscript_save_state. Releasing an unknown
-// handle is a no-op.
-inline void chaiscript_release_state(const int handle) {
-  detail::state_registry().erase(handle);
+// Release a state handle returned by chaiscript_save_state. Releasing an
+// unknown handle is a no-op.
+inline void chaiscript_release_state(const int state_handle) {
+  detail::state_registry().erase(state_handle);
 }
 
 #endif /* CHAISCRIPT_EMSCRIPTEN_EVAL_HPP_ */

unittests/emscripten_eval_test.cpp

@@ -16,36 +16,57 @@
 #include <string>
 
 int main() {
+  const int chai = chaiscript_create();
+  assert(chai > 0 && "create must return a positive handle");
+
   // Test eval (void return) - same as Emscripten eval()
-  chaiscript_eval("var x = 42");
+  chaiscript_eval(chai, "var x = 42");
 
   // Test evalString - same as Emscripten evalString()
-  [[maybe_unused]] std::string s = chaiscript_eval_string("to_string(x)");
+  [[maybe_unused]] const std::string s = chaiscript_eval_string(chai, "to_string(x)");
   assert(s == "42");
 
   // Test evalInt - same as Emscripten evalInt()
-  [[maybe_unused]] int i = chaiscript_eval_int("1 + 2");
+  [[maybe_unused]] const int i = chaiscript_eval_int(chai, "1 + 2");
   assert(i == 3);
 
   // Test evalBool - same as Emscripten evalBool()
-  [[maybe_unused]] bool b = chaiscript_eval_bool("true");
+  [[maybe_unused]] bool b = chaiscript_eval_bool(chai, "true");
   assert(b == true);
 
-  b = chaiscript_eval_bool("false");
+  b = chaiscript_eval_bool(chai, "false");
   assert(b == false);
 
   // Test evalFloat - same as Emscripten evalFloat()
-  [[maybe_unused]] float f = chaiscript_eval_float("1.5f");
+  [[maybe_unused]] const float f = chaiscript_eval_float(chai, "1.5f");
   assert(std::abs(f - 1.5f) < 0.001f);
 
   // Test evalDouble - same as Emscripten evalDouble()
-  [[maybe_unused]] double d = chaiscript_eval_double("3.14");
+  [[maybe_unused]] const double d = chaiscript_eval_double(chai, "3.14");
   assert(std::abs(d - 3.14) < 0.001);
 
   // Test a more complex expression
-  chaiscript_eval("def square(n) { return n * n; }");
-  [[maybe_unused]] int sq = chaiscript_eval_int("square(7)");
+  chaiscript_eval(chai, "def square(n) { return n * n; }");
+  [[maybe_unused]] const int sq = chaiscript_eval_int(chai, "square(7)");
   assert(sq == 49);
 
+  // A second engine is fully independent of the first.
+  const int chai2 = chaiscript_create();
+  assert(chai2 != chai && "create must mint a fresh handle each call");
+  [[maybe_unused]] bool caught = false;
+  try {
+    chaiscript_eval(chai2, "x");
+  } catch (const chaiscript::exception::eval_error &) {
+    caught = true;
+  }
+  assert(caught && "engines must not share globals");
+
+  chaiscript_destroy(chai2);
+  chaiscript_destroy(chai);
+
+  // Destroying an unknown handle is a no-op.
+  chaiscript_destroy(chai);
+  chaiscript_destroy(99999);
+
   return 0;
 }

unittests/emscripten_exception_test.cpp

@@ -22,12 +22,14 @@ int main() {
   // through the eval wrapper functions. In WASM builds without exception
   // support, these would abort instead of throwing.
 
+  const int chai = chaiscript_create();
+
   [[maybe_unused]] bool caught = false;
 
   // Test 1: eval with undefined variable should throw
   caught = false;
   try {
-    chaiscript_eval("this_variable_does_not_exist");
+    chaiscript_eval(chai, "this_variable_does_not_exist");
   } catch (const chaiscript::exception::eval_error &) {
     caught = true;
   }
@@ -36,7 +38,7 @@ int main() {
   // Test 2: evalString with a type mismatch should throw
   caught = false;
   try {
-    chaiscript_eval_string("1 + 2");
+    chaiscript_eval_string(chai, "1 + 2");
   } catch (const chaiscript::exception::bad_boxed_cast &) {
     caught = true;
   }
@@ -45,7 +47,7 @@ int main() {
   // Test 3: evalInt with invalid syntax should throw
   caught = false;
   try {
-    chaiscript_eval_int("def {}");
+    chaiscript_eval_int(chai, "def {}");
   } catch (const chaiscript::exception::eval_error &) {
     caught = true;
   }
@@ -54,7 +56,7 @@ int main() {
   // Test 4: eval with throw statement should propagate exception
   caught = false;
   try {
-    chaiscript_eval("throw(\"user exception\")");
+    chaiscript_eval(chai, "throw(\"user exception\")");
   } catch (const chaiscript::Boxed_Value &) {
     caught = true;
   } catch (...) {
@@ -63,11 +65,13 @@ int main() {
   assert(caught && "ChaiScript throw must propagate as an exception");
 
   // Test 5: Verify normal operation still works after caught exceptions
-  chaiscript_eval("var post_exception_test = 100");
+  chaiscript_eval(chai, "var post_exception_test = 100");
 
-  [[maybe_unused]] const int result = chaiscript_eval_int("post_exception_test");
+  [[maybe_unused]] const int result = chaiscript_eval_int(chai, "post_exception_test");
   assert(result == 100 && "normal eval must work after caught exceptions");
 
+  chaiscript_destroy(chai);
+
   std::cout << "All emscripten exception tests passed.\n";
   return 0;
 }

unittests/emscripten_state_test.cpp

@@ -17,34 +17,37 @@
 #include <string>
 
 int main() {
+  const int chai = chaiscript_create();
+  assert(chai > 0 && "create must return a positive handle");
+
   // Capture a baseline before any user code has been evaluated.
-  const int baseline = chaiscript_save_state();
+  const int baseline = chaiscript_save_state(chai);
   assert(baseline > 0 && "saveState must return a positive handle");
 
   // Distinct calls produce distinct handles.
-  const int second = chaiscript_save_state();
+  const int second = chaiscript_save_state(chai);
   assert(second != baseline && "saveState must mint a fresh handle each call");
   chaiscript_release_state(second);
 
   // Define a global that did not exist in the baseline.
-  chaiscript_eval("var post_baseline_marker = 123");
-  assert(chaiscript_eval_int("post_baseline_marker") == 123);
+  chaiscript_eval(chai, "var post_baseline_marker = 123");
+  assert(chaiscript_eval_int(chai, "post_baseline_marker") == 123);
 
   // Restoring the baseline must drop the post-baseline definition: evaluating
   // it should now fail with eval_error because the variable is undefined.
-  chaiscript_restore_state(baseline);
+  chaiscript_restore_state(chai, baseline);
 
   [[maybe_unused]] bool caught = false;
   try {
-    chaiscript_eval("post_baseline_marker");
+    chaiscript_eval(chai, "post_baseline_marker");
   } catch (const chaiscript::exception::eval_error &) {
     caught = true;
   }
   assert(caught && "restoreState must drop globals defined after the snapshot");
 
   // The engine remains usable after a restore.
-  chaiscript_eval("var after_restore = 7");
-  assert(chaiscript_eval_int("after_restore * 6") == 42);
+  chaiscript_eval(chai, "var after_restore = 7");
+  assert(chaiscript_eval_int(chai, "after_restore * 6") == 42);
 
   // Releasing the handle is required so the registry does not grow unbounded.
   chaiscript_release_state(baseline);
@@ -55,8 +58,19 @@ int main() {
   chaiscript_release_state(99999);
 
   // Restoring an unknown handle is a no-op: state must be unchanged.
-  chaiscript_restore_state(99999);
-  assert(chaiscript_eval_int("after_restore") == 7);
+  chaiscript_restore_state(chai, 99999);
+  assert(chaiscript_eval_int(chai, "after_restore") == 7);
+
+  // A snapshot taken on one engine can be restored onto a different engine,
+  // confirming that state is decoupled from any particular instance.
+  const int snapshot_with_after_restore = chaiscript_save_state(chai);
+  const int chai2 = chaiscript_create();
+  chaiscript_restore_state(chai2, snapshot_with_after_restore);
+  assert(chaiscript_eval_int(chai2, "after_restore") == 7);
+
+  chaiscript_release_state(snapshot_with_after_restore);
+  chaiscript_destroy(chai2);
+  chaiscript_destroy(chai);
 
   return 0;
 }