engine: Transform the send/recv init functions into helpers

Since we'll want to use them elsewhere, we should make these helper
functions. It also makes the code look a lot neater. Unfortunately, it
adds a bit more indirection, but this isn't a critical flaw here.

engine/graph/state.go

@@ -169,26 +169,9 @@ func (obj *State) Init() error {
 			}
 			return res.Refresh()
 		},
-		Send: func(st interface{}) error {
-			res, ok := obj.Vertex.(engine.SendableRes)
-			if !ok {
-				panic("res does not support the Sendable trait")
-			}
-			// XXX: type check this
-			//expected := res.Sends()
-			//if err := XXX_TYPE_CHECK(expected, st); err != nil {
-			//	return err
-			//}
 
-			return res.Send(st) // send the struct
-		},
-		Recv: func() map[string]*engine.Send { // TODO: change this API?
-			res, ok := obj.Vertex.(engine.RecvableRes)
-			if !ok {
-				panic("res does not support the Recvable trait")
-			}
-			return res.Recv()
-		},
+		Send: engine.GenerateSendFunc(res),
+		Recv: engine.GenerateRecvFunc(res),
 
 		// FIXME: pass in a safe, limited query func instead?
 		// TODO: not implemented, use FilteredGraph

engine/sendrecv.go

@@ -33,7 +33,9 @@ type SendableRes interface {
 	Sends() interface{}
 
 	// Send is used in CheckApply to send the desired data. It returns an
-	// error if the data is malformed or doesn't type check.
+	// error if the data is malformed or doesn't type check. You should use
+	// the GenerateSendFunc helper function to build this function for use
+	// in the resource internal state handle.
 	Send(st interface{}) error
 
 	// Sent returns the most recently sent data. This is used by the engine.
@@ -54,7 +56,8 @@ type RecvableRes interface {
 
 	// Recv is used by the resource to get information on changes. This data
 	// can be used to invalidate caches, restart watches, or it can be
-	// ignored entirely.
+	// ignored entirely. You should use the GenerateRecvFunc helper function
+	// to build this function for use in the resource internal state handle.
 	Recv() map[string]*Send
 }
 
@@ -65,3 +68,35 @@ type Send struct {
 
 	Changed bool // set to true if this key was updated, read only!
 }
+
+// GenerateSendFunc generates the Send function using the resource of our
+// choice for use in the resource internal state handle.
+func GenerateSendFunc(res Res) func(interface{}) error {
+	return func(st interface{}) error {
+		//fmt.Printf("from: %+v\n", res)
+		//fmt.Printf("send: %+v\n", st)
+		r, ok := res.(SendableRes)
+		if !ok {
+			panic("res does not support the Sendable trait")
+		}
+		// XXX: type check this
+		//expected := r.Sends()
+		//if err := XXX_TYPE_CHECK(expected, st); err != nil {
+		//	return err
+		//}
+
+		return r.Send(st) // send the struct
+	}
+}
+
+// GenerateRecvFunc generates the Recv function using the resource of our
+// choice for use in the resource internal state handle.
+func GenerateRecvFunc(res Res) func() map[string]*Send {
+	return func() map[string]*Send { // TODO: change this API?
+		r, ok := res.(RecvableRes)
+		if !ok {
+			panic("res does not support the Recvable trait")
+		}
+		return r.Recv()
+	}
+}