Add support for Fiber.try(_) (#835)

* Add support for Fiber.try(_)
* Add documentation for Fiber.try(_)
* Add another test for Fiber.try(_)

doc/site/modules/core/fiber.markdown

@@ -174,6 +174,24 @@ System.print("Caught error: " + error)
 
 If the called fiber raises an error, it can no longer be used.
 
+### **try**(value)
+Tries to run the fiber. If a runtime error occurs
+in the called fiber, the error is captured and is returned as a string.
+If the fiber is being
+started for the first time, and its function takes a parameter, `value` is
+passed to it.
+
+<pre class="snippet">
+var fiber = Fiber.new {|value|
+  value.badMethod
+}
+
+var error = fiber.try("just a string")
+System.print("Caught error: " + error)
+</pre>
+
+If the called fiber raises an error, it can no longer be used.
+
 ### **transfer**()
 
 **TODO**

src/vm/wren_core.c

@@ -191,6 +191,15 @@ DEF_PRIMITIVE(fiber_try)
   return false;
 }
 
+DEF_PRIMITIVE(fiber_try1)
+{
+  runFiber(vm, AS_FIBER(args[0]), args, true, true, "try");
+  
+  // If we're switching to a valid fiber to try, remember that we're trying it.
+  if (!wrenHasError(vm->fiber)) vm->fiber->state = FIBER_TRY;
+  return false;
+}
+
 DEF_PRIMITIVE(fiber_yield)
 {
   ObjFiber* current = vm->fiber;
@@ -1235,6 +1244,7 @@ void wrenInitializeCore(WrenVM* vm)
   PRIMITIVE(vm->fiberClass, "transfer(_)", fiber_transfer1);
   PRIMITIVE(vm->fiberClass, "transferError(_)", fiber_transferError);
   PRIMITIVE(vm->fiberClass, "try()", fiber_try);
+  PRIMITIVE(vm->fiberClass, "try(_)", fiber_try1);
 
   vm->fnClass = AS_CLASS(wrenFindVariable(vm, coreModule, "Fn"));
   PRIMITIVE(vm->fnClass->obj.classObj, "new(_)", fn_new);

test/core/fiber/try_value.wren

@@ -0,0 +1,12 @@
+var fiber = Fiber.new {|v|
+  System.print("before")
+  System.print(v)
+  true.unknownMethod
+  System.print("after")
+}
+
+System.print(fiber.try("value"))
+// expect: before
+// expect: value
+// expect: Bool does not implement 'unknownMethod'.
+System.print("after try") // expect: after try

test/core/fiber/try_value_yield.wren

@@ -0,0 +1,16 @@
+var fiber = Fiber.new {|v|
+  System.print("before")
+  System.print(v)
+  v = Fiber.yield()
+  System.print(v)
+  true.unknownMethod
+  System.print("after")
+}
+
+fiber.try("value1")
+// expect: before
+// expect: value1
+System.print(fiber.try("value2"))
+// expect: value2
+// expect: Bool does not implement 'unknownMethod'.
+System.print("after try") // expect: after try