feat: add EmptyPathSegment, distinct from PathSegment{}

datamodel/path.go

@@ -65,6 +65,9 @@ type Path struct {
 	segments []PathSegment
 }
 
+// EmptyPath is the Path with no segments.
+var EmptyPath = Path{}
+
 // NewPath returns a Path composed of the given segments.
 //
 // This constructor function does a defensive copy,
@@ -195,7 +198,7 @@ func (p Path) AppendSegmentInt(ps int64) Path {
 // the zero path if it's already empty).
 func (p Path) Parent() Path {
 	if len(p.segments) == 0 {
-		return Path{}
+		return EmptyPath
 	}
 	return Path{p.segments[0 : len(p.segments)-1]}
 }
@@ -208,15 +211,15 @@ func (p Path) Truncate(i int) Path {
 // Last returns the trailing segment of the path.
 func (p Path) Last() PathSegment {
 	if len(p.segments) < 1 {
-		return PathSegment{}
+		return EmptyPathSegment
 	}
 	return p.segments[len(p.segments)-1]
 }
 
 // Pop returns a path with all segments except the last.
 func (p Path) Pop() Path {
 	if len(p.segments) < 1 {
-		return Path{}
+		return EmptyPath
 	}
 	return Path{p.segments[0 : len(p.segments)-1]}
 }
@@ -225,7 +228,7 @@ func (p Path) Pop() Path {
 // If applied to a zero-length path, it returns an empty segment and the same zero-length path.
 func (p Path) Shift() (PathSegment, Path) {
 	if len(p.segments) < 1 {
-		return PathSegment{}, Path{}
+		return EmptyPathSegment, EmptyPath
 	}
 	return p.segments[0], Path{p.segments[1:]}
 }

datamodel/pathSegment.go

@@ -69,6 +69,12 @@ type PathSegment struct {
 	i int64
 }
 
+// EmptyPathSegment is the PathSegment with no value. This is not the same as
+// PathSegment{}, which will be interpreted to mean "0", as a list index.
+// EmptyPathSegment is equivalent to ParsePathSegment("") or
+// PathSegmentOfString("").
+var EmptyPathSegment = PathSegment{i: -1}
+
 // ParsePathSegment parses a string into a PathSegment,
 // handling any escaping if present.
 // (Note: there is currently no escaping specified for PathSegments,

datamodel/path_test.go

@@ -36,4 +36,8 @@ func TestPathSegmentZeroValue(t *testing.T) {
 	i, err := PathSegment{}.Index()
 	qt.Check(t, err, qt.IsNil)
 	qt.Check(t, i, qt.Equals, int64(0))
+
+	qt.Check(t, EmptyPathSegment.String(), qt.Equals, "")
+	_, err = EmptyPathSegment.Index()
+	qt.Check(t, err, qt.IsNotNil)
 }

traversal/walk.go

@@ -318,7 +318,7 @@ func (prog Progress) reify(n datamodel.Node, s selector.Selector) (datamodel.Nod
 		}
 
 		// explore into the `InterpretAs` clause to the child selector.
-		s, err = s.Explore(n, datamodel.PathSegment{})
+		s, err = s.Explore(n, datamodel.EmptyPathSegment)
 		if err != nil {
 			return nil, nil, err
 		}