Add Fiber.try().

doc/site/classes.markdown

@@ -17,7 +17,7 @@ This creates a class named `Unicorn` with no methods or fields.
 
 ## Methods
 
-Once we've made a unicorn, to let it do stuff, we need to give it methods.
+To let our unicorn do stuff, we need to give it methods.
 
     :::dart
     class Unicorn {
@@ -35,7 +35,7 @@ This defines a `prance` method that takes no arguments. To support parameters, a
       }
     }
 
-Unlike most other dynamically-typed languages, in Wren you can have multiple methods in a class with the same name, as long as they take a different number of parameters. In technical terms, you can overload by *arity*. So this class is fine:
+Unlike most other dynamically-typed languages, in Wren you can have multiple methods in a class with the same name, as long as they take a different number of parameters. In technical terms, you can *overload by arity*. So this class is fine:
 
     :::dart
     class Unicorn {
@@ -118,7 +118,6 @@ Operator overloading is really useful for types like vectors and complex numbers
 
 ## Setters
 
-[Assignment](variables.html) *cannot* be overloaded. It isn't an operator, and its semantics are built right into the language.
 
 **TODO: ...**
 
@@ -143,8 +142,29 @@ This declares a new class `Pegasus` that inherits from `Unicorn`.
 
 The metaclass hierarchy does *not* parallel the regular class hierarchy. So, if `Pegasus` inherits from `Unicorn`, `Pegasus`'s metaclass will not inherit from `Unicorn`'s metaclass. In more prosaic terms, this means that static methods are not inherited.
 
+    :::dart
+    class Unicorn {
+      // Unicorns cannot fly. :(
+      static canFly { false }
+    }
+
+    class Pegasus is Unicorn {}
+
+    Pegasus.canFly // ERROR: Static methods are not inherited.
+
 Constructors, however, initialize the instance *after* it has been created. They are defined as instance methods on the class and not on the metaclass. That means that constructors *are* inherited.
 
+    :::dart
+    class Unicorn {
+      new(name) {
+        IO.print("My name is " + name + ".")
+      }
+    }
+
+    class Pegasus is Unicorn {}
+
+    new Pegasus("Fred") // Prints "My name is Fred.".
+
 ## Superclass method calls
 
 **TODO**

src/wren_core.c

@@ -269,10 +269,17 @@ DEF_NATIVE(fiber_call1)
   return PRIM_RUN_FIBER;
 }
 
+DEF_NATIVE(fiber_error)
+{
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+  if (runFiber->error == NULL) RETURN_NULL;
+  RETURN_OBJ(runFiber->error);
+}
+
 DEF_NATIVE(fiber_isDone)
 {
   ObjFiber* runFiber = AS_FIBER(args[0]);
-  RETURN_BOOL(runFiber->numFrames == 0);
+  RETURN_BOOL(runFiber->numFrames == 0 || runFiber->error != NULL);
 }
 
 DEF_NATIVE(fiber_run)
@@ -320,12 +327,33 @@ DEF_NATIVE(fiber_run1)
   return PRIM_RUN_FIBER;
 }
 
+DEF_NATIVE(fiber_try)
+{
+  ObjFiber* runFiber = AS_FIBER(args[0]);
+
+  if (runFiber->numFrames == 0) RETURN_ERROR("Cannot try a finished fiber.");
+  if (runFiber->caller != NULL) RETURN_ERROR("Fiber has already been called.");
+
+  // Remember who ran it.
+  runFiber->caller = fiber;
+  runFiber->callerIsTrying = true;
+
+  // If the fiber was yielded, make the yield call return null.
+  if (runFiber->stackSize > 0)
+  {
+    runFiber->stack[runFiber->stackSize - 1] = NULL_VAL;
+  }
+
+  return PRIM_RUN_FIBER;
+}
+
 DEF_NATIVE(fiber_yield)
 {
   if (fiber->caller == NULL) RETURN_ERROR("No fiber to yield to.");
 
   ObjFiber* caller = fiber->caller;
   fiber->caller = NULL;
+  fiber->callerIsTrying = false;
 
   // Make the caller's run method return null.
   caller->stack[caller->stackSize - 1] = NULL_VAL;
@@ -341,6 +369,7 @@ DEF_NATIVE(fiber_yield1)
 
   ObjFiber* caller = fiber->caller;
   fiber->caller = NULL;
+  fiber->callerIsTrying = false;
 
   // Make the caller's run method return the argument passed to yield.
   caller->stack[caller->stackSize - 1] = args[1];
@@ -1027,9 +1056,11 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->fiberClass->metaclass, "yield ", fiber_yield1);
   NATIVE(vm->fiberClass, "call", fiber_call);
   NATIVE(vm->fiberClass, "call ", fiber_call1);
+  NATIVE(vm->fiberClass, "error", fiber_error);
   NATIVE(vm->fiberClass, "isDone", fiber_isDone);
   NATIVE(vm->fiberClass, "run", fiber_run);
   NATIVE(vm->fiberClass, "run ", fiber_run1);
+  NATIVE(vm->fiberClass, "try", fiber_try);
 
   vm->fnClass = defineClass(vm, "Fn");
 

src/wren_debug.c

@@ -4,7 +4,7 @@
 
 void wrenDebugPrintStackTrace(WrenVM* vm, ObjFiber* fiber)
 {
-  fprintf(stderr, "%s\n", fiber->error);
+  fprintf(stderr, "%s\n", fiber->error->value);
 
   for (int i = fiber->numFrames - 1; i >= 0; i--)
   {

src/wren_value.c

@@ -140,6 +140,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn)
   fiber->openUpvalues = NULL;
   fiber->caller = NULL;
   fiber->error = NULL;
+  fiber->callerIsTrying = false;
 
   CallFrame* frame = &fiber->frames[0];
   frame->fn = fn;
@@ -512,6 +513,8 @@ static void markFiber(WrenVM* vm, ObjFiber* fiber)
   // The caller.
   if (fiber->caller != NULL) markFiber(vm, fiber->caller);
 
+  if (fiber->error != NULL) markString(vm, fiber->error);
+
   // Keep track of how much memory is still in use.
   vm->bytesAllocated += sizeof(ObjFiber);
   // TODO: Count size of error message buffer.
@@ -586,13 +589,6 @@ void wrenFreeObj(WrenVM* vm, Obj* obj)
       wrenMethodBufferClear(vm, &((ObjClass*)obj)->methods);
       break;
 
-    case OBJ_FIBER:
-    {
-      ObjFiber* fiber = ((ObjFiber*)obj);
-      if (fiber->error != NULL) wrenReallocate(vm, fiber->error, 0, 0);
-      break;
-    }
-
     case OBJ_FN:
     {
       ObjFn* fn = (ObjFn*)obj;
@@ -613,6 +609,7 @@ void wrenFreeObj(WrenVM* vm, Obj* obj)
       break;
 
     case OBJ_CLOSURE:
+    case OBJ_FIBER:
     case OBJ_INSTANCE:
     case OBJ_RANGE:
     case OBJ_UPVALUE:

src/wren_value.h

@@ -94,6 +94,14 @@ typedef struct
 
 DECLARE_BUFFER(Value, Value);
 
+typedef struct
+{
+  Obj obj;
+  char* value;
+
+  // TODO: Flexible array.
+} ObjString;
+
 // The dynamically allocated data structure for a variable that has been used
 // by a closure. Whenever a function accesses a variable declared in an
 // enclosing function, it will get to it through this.
@@ -160,7 +168,12 @@ typedef struct sObjFiber
 
   // If the fiber failed because of a runtime error, this will contain the
   // error message. Otherwise, it will be NULL.
-  char* error;
+  ObjString* error;
+
+  // This will be true if the caller that called this fiber did so using "try".
+  // In that case, if this fiber fails with an error, the error will be given
+  // to the caller.
+  bool callerIsTrying;
 } ObjFiber;
 
 typedef enum
@@ -179,14 +192,6 @@ typedef enum
 
 } PrimitiveResult;
 
-typedef struct
-{
-  Obj obj;
-  char* value;
-
-  // TODO: Flexible array.
-} ObjString;
-
 typedef PrimitiveResult (*Primitive)(WrenVM* vm, ObjFiber* fiber, Value* args);
 
 // TODO: See if it's actually a perf improvement to have this in a separate

src/wren_vm.c

@@ -303,19 +303,30 @@ static void callForeign(WrenVM* vm, ObjFiber* fiber,
 }
 
 // Puts [fiber] into a runtime failed state because of [error].
-static void runtimeError(WrenVM* vm, ObjFiber* fiber, const char* error)
+//
+// Returns the fiber that should receive the error or `NULL` if no fiber
+// caught it.
+static ObjFiber* runtimeError(WrenVM* vm, ObjFiber* fiber, const char* error)
 {
-  // Copy the error onto the heap.
-  size_t length = strlen(error) + 1;
-  char* heapError = wrenReallocate(vm, NULL, 0, length);
-  strncpy(heapError, error, length);
-
   ASSERT(fiber->error == NULL, "Can only fail once.");
-  fiber->error = heapError;
 
-  // TODO: If the calling fiber is going to handle the error, we shouldn't dump
-  // a stack trace.
+  // Store the error in the fiber so it can be accessed later.
+  fiber->error = AS_STRING(wrenNewString(vm, error, strlen(error)));
+
+  // If the caller ran this fiber using "try", give it the error.
+  if (fiber->callerIsTrying)
+  {
+    ObjFiber* caller = fiber->caller;
+
+    // Make the caller's try method return the error message.
+    caller->stack[caller->stackSize - 1] = OBJ_VAL(fiber->error);
+
+    return caller;
+  }
+
+  // If we got here, nothing caught the error, so show the stack trace.
   wrenDebugPrintStackTrace(vm, fiber);
+  return NULL;
 }
 
 // Pushes [function] onto [fiber]'s callstack and invokes it. Expects [numArgs]
@@ -384,11 +395,16 @@ static bool runInterpreter(WrenVM* vm)
         upvalues = ((ObjClosure*)frame->fn)->upvalues; \
       }
 
+  // Terminates the current fiber with error string [error]. If another calling
+  // fiber is willing to catch the error, transfers control to it, otherwise
+  // exits the interpreter.
   #define RUNTIME_ERROR(error)                         \
       do {                                             \
         STORE_FRAME();                                 \
-        runtimeError(vm, fiber, error);                \
-        return false;                                  \
+        fiber = runtimeError(vm, fiber, error);        \
+        if (fiber == NULL) return false;               \
+        LOAD_FRAME();                                  \
+        DISPATCH();                                    \
       }                                                \
       while (false)
 
@@ -953,6 +969,11 @@ static bool runInterpreter(WrenVM* vm)
 
       ObjClass* classObj = wrenNewClass(vm, superclass, numFields, name);
 
+      // Don't pop the superclass and name off the stack until the subclass is
+      // done being created, to make sure it doesn't get collected.
+      DROP();
+      DROP();
+
       // Now that we know the total number of fields, make sure we don't
       // overflow.
       if (superclass->numFields + numFields > MAX_FIELDS)
@@ -964,11 +985,6 @@ static bool runInterpreter(WrenVM* vm)
         RUNTIME_ERROR(message);
       }
 
-      // Don't pop the superclass and name off the stack until the subclass is
-      // done being created, to make sure it doesn't get collected.
-      DROP();
-      DROP();
-
       PUSH(OBJ_VAL(classObj));
       DISPATCH();
     }

test/fiber/error.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+IO.print(fiber.error) // expect: null
+IO.print(fiber.try)   // expect: String does not implement method 'unknown'.
+IO.print(fiber.error) // expect: String does not implement method 'unknown'.

test/fiber/is_done_after_error.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+fiber.try
+IO.print(fiber.isDone) // expect: true

test/fiber/try.wren

@@ -0,0 +1,10 @@
+var fiber = new Fiber {
+  IO.print("before")
+  true.unknownMethod
+  IO.print("after")
+}
+
+IO.print(fiber.try)
+// expect: before
+// expect: Bool does not implement method 'unknownMethod'.
+IO.print("after try") // expect: after try

test/fiber/try_direct_reenter.wren

@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.try // expect runtime error: Fiber has already been called.
+}
+
+fiber.call

test/fiber/try_indirect_reenter.wren

@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.try // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call
+}
+
+b.call

test/fiber/try_when_done.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("try")
+}
+
+fiber.try // expect: try
+fiber.try // expect runtime error: Cannot try a finished fiber.

test/fiber/try_without_error.wren

@@ -0,0 +1,8 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before")  // expect: before
+IO.print(fiber.try) // expect: fiber
+                    // expect: null
+IO.print("after")   // expect: after