Added a flow control struct

flow/flow.go

@@ -0,0 +1,31 @@
+package flow
+
+// Flow represents any multi-stage process that relies on callbacks to advance
+// to other stages. It allows for such a process to be laid flat instead of
+// nested.
+type Flow struct {
+	// Transition specifies a function to call before moving on to another
+	// stage, such as clearing out a container so that new elements may be
+	// added to it.
+	Transition func ()
+
+	// Stages is a map that pairs stage names with stage functions.
+	Stages map [string] func ()
+	
+	stage string
+}
+
+// Switch transitions the flow to a different stage, running the specified
+// transition callback first.
+func (flow Flow) Switch (stage string) {
+	stageCallback := flow.Stages[stage]
+	if stageCallback == nil { return }
+	if flow.Transition != nil { flow.Transition() }
+	flow.stage = stage
+	stageCallback()
+}
+
+// Stage returns the name of the current stage the flow is on.
+func (flow Flow) Stage () (name string) {
+	return flow.stage
+}