XXX cache map inputs

Is this correct? Is looking at the length along sufficient?

lang/fancyfunc/fancy_func.go

@@ -64,7 +64,11 @@ func (obj *FuncValue) Cmp(val types.Value) error {
 		return errwrap.Wrapf(err, "cannot cmp types")
 	}
 
-	return fmt.Errorf("cannot cmp funcs") // TODO: can we ?
+	if obj != val { // best we can do
+		return fmt.Errorf("different pointers")
+	}
+
+	return nil
 }
 
 // Copy returns a copy of this value.

lang/funcs/core/iter/map_func.go

@@ -60,6 +60,7 @@ type MapFunc struct {
 	RType *types.Type // this is the type of the elements in our output list
 
 	init *interfaces.Init
+	last types.Value // last value received to use for diff
 
 	lastFuncValue       *fancyfunc.FuncValue // remember the last function value
 	lastInputListLength int                  // remember the last input list length
@@ -627,6 +628,12 @@ func (obj *MapFunc) Stream(ctx context.Context) error {
 				continue
 			}
 
+			// XXX: double check this passes through function changes
+			if obj.last != nil && input.Cmp(obj.last) == nil {
+				continue // value didn't change, skip it
+			}
+			obj.last = input // store for next
+
 			value, exists := input.Struct()[mapArgNameFunction]
 			if !exists {
 				return fmt.Errorf("programming error, can't find edge")