Add API to get the type of object in a slot.

src/include/wren.h

@@ -162,12 +162,30 @@ typedef struct
   int heapGrowthPercent;
 } WrenConfiguration;
 
-typedef enum {
+typedef enum
+{
   WREN_RESULT_SUCCESS,
   WREN_RESULT_COMPILE_ERROR,
   WREN_RESULT_RUNTIME_ERROR
 } WrenInterpretResult;
 
+// The type of an object stored in a slot.
+//
+// This is not necessarily the object's *class*, but instead its low level
+// representation type.
+typedef enum
+{
+  WREN_TYPE_BOOL,
+  WREN_TYPE_NUM,
+  WREN_TYPE_FOREIGN,
+  WREN_TYPE_LIST,
+  WREN_TYPE_NULL,
+  WREN_TYPE_STRING,
+  
+  // The object is of a type that isn't accessible by the C API.
+  WREN_TYPE_UNKNOWN
+} WrenType;
+
 // Initializes [configuration] with all of its default values.
 //
 // Call this before setting the particular fields you care about.
@@ -263,6 +281,9 @@ int wrenGetSlotCount(WrenVM* vm);
 // It is an error to call this from a finalizer.
 void wrenEnsureSlots(WrenVM* vm, int numSlots);
 
+// Gets the type of the object in [slot].
+WrenType wrenGetSlotType(WrenVM* vm, int slot);
+
 // Reads a boolean value from [slot].
 //
 // It is an error to call this if the slot does not contain a boolean value.

src/vm/wren_vm.c

@@ -1512,6 +1512,20 @@ static void validateApiSlot(WrenVM* vm, int slot)
   ASSERT(slot < wrenGetSlotCount(vm), "Not that many slots.");
 }
 
+// Gets the type of the object in [slot].
+WrenType wrenGetSlotType(WrenVM* vm, int slot)
+{
+  validateApiSlot(vm, slot);
+  if (IS_BOOL(vm->apiStack[slot])) return WREN_TYPE_BOOL;
+  if (IS_NUM(vm->apiStack[slot])) return WREN_TYPE_NUM;
+  if (IS_FOREIGN(vm->apiStack[slot])) return WREN_TYPE_FOREIGN;
+  if (IS_LIST(vm->apiStack[slot])) return WREN_TYPE_LIST;
+  if (IS_NULL(vm->apiStack[slot])) return WREN_TYPE_NULL;
+  if (IS_STRING(vm->apiStack[slot])) return WREN_TYPE_STRING;
+  
+  return WREN_TYPE_UNKNOWN;
+}
+
 bool wrenGetSlotBool(WrenVM* vm, int slot)
 {
   validateApiSlot(vm, slot);