Simplify Nexter interface

This change allows a Nexter to be used in the same manner as a scanner
using a for graph.Next(it) {} construction.

It is important that graph.Next(it) and any associated it.Result() calls
operate on the same iterator.

graph/leveldb/all_iterator.go

@@ -101,10 +101,10 @@ func (it *AllIterator) Clone() graph.Iterator {
 	return out
 }
 
-func (it *AllIterator) Next() (graph.Value, bool) {
+func (it *AllIterator) Next() bool {
 	if !it.open {
 		it.result = nil
-		return nil, false
+		return false
 	}
 	var out []byte
 	out = make([]byte, len(it.iter.Key()))
@@ -115,10 +115,10 @@ func (it *AllIterator) Next() (graph.Value, bool) {
 	}
 	if !bytes.HasPrefix(out, it.prefix) {
 		it.Close()
-		return nil, false
+		return false
 	}
 	it.result = out
-	return out, true
+	return true
 }
 
 func (it *AllIterator) ResultTree() *graph.ResultTree {

graph/leveldb/iterator.go

@@ -117,19 +117,19 @@ func (it *Iterator) Close() {
 	}
 }
 
-func (it *Iterator) Next() (graph.Value, bool) {
+func (it *Iterator) Next() bool {
 	if it.iter == nil {
 		it.result = nil
-		return nil, false
+		return false
 	}
 	if !it.open {
 		it.result = nil
-		return nil, false
+		return false
 	}
 	if !it.iter.Valid() {
 		it.result = nil
 		it.Close()
-		return nil, false
+		return false
 	}
 	if bytes.HasPrefix(it.iter.Key(), it.nextPrefix) {
 		out := make([]byte, len(it.iter.Key()))
@@ -139,11 +139,11 @@ func (it *Iterator) Next() (graph.Value, bool) {
 		if !ok {
 			it.Close()
 		}
-		return out, true
+		return true
 	}
 	it.Close()
 	it.result = nil
-	return nil, false
+	return false
 }
 
 func (it *Iterator) ResultTree() *graph.ResultTree {

graph/leveldb/leveldb_test.go

@@ -45,12 +45,8 @@ func makeTripleSet() []*quad.Quad {
 
 func iteratedTriples(qs graph.TripleStore, it graph.Iterator) []*quad.Quad {
 	var res ordered
-	for {
-		val, ok := graph.Next(it)
-		if !ok {
-			break
-		}
-		res = append(res, qs.Quad(val))
+	for graph.Next(it) {
+		res = append(res, qs.Quad(it.Result()))
 	}
 	sort.Sort(res)
 	return res
@@ -85,12 +81,8 @@ func (o ordered) Swap(i, j int) { o[i], o[j] = o[j], o[i] }
 
 func iteratedNames(qs graph.TripleStore, it graph.Iterator) []string {
 	var res []string
-	for {
-		val, ok := graph.Next(it)
-		if !ok {
-			break
-		}
-		res = append(res, qs.NameOf(val))
+	for graph.Next(it) {
+		res = append(res, qs.NameOf(it.Result()))
 	}
 	sort.Strings(res)
 	return res
@@ -266,8 +258,8 @@ func TestIterator(t *testing.T) {
 	it.Reset()
 
 	it = qs.TriplesAllIterator()
-	edge, _ := graph.Next(it)
-	triple := qs.Quad(edge)
+	graph.Next(it)
+	triple := qs.Quad(it.Result())
 	set := makeTripleSet()
 	var ok bool
 	for _, t := range set {

graph/leveldb/triplestore_iterator_optimize.go

@@ -37,10 +37,10 @@ func (ts *TripleStore) optimizeLinksTo(it *iterator.LinksTo) (graph.Iterator, bo
 	if primary.Type() == graph.Fixed {
 		size, _ := primary.Size()
 		if size == 1 {
-			val, ok := graph.Next(primary)
-			if !ok {
-				panic("Sizes lie")
+			if !graph.Next(primary) {
+				panic("unexpected size during optimize")
 			}
+			val := primary.Result()
 			newIt := ts.TripleIterator(it.Direction(), val)
 			nt := newIt.Tagger()
 			nt.CopyFrom(it)