fix #38 MustCompile method will return nopQuery object instead of…

… `nil` when input expr is an invalid XPath expression.

query.go

@@ -22,6 +22,17 @@ type query interface {
 	Clone() query
 }
 
+// nopQuery is an empty query that always return nil for any query.
+type nopQuery struct {
+	query
+}
+
+func (nopQuery) Select(iterator) NodeNavigator { return nil }
+
+func (nopQuery) Evaluate(iterator) interface{} { return nil }
+
+func (nopQuery) Clone() query { return nopQuery{} }
+
 // contextQuery is returns current node on the iterator object query.
 type contextQuery struct {
 	count int

xpath.go

@@ -151,7 +151,7 @@ func Compile(expr string) (*Expr, error) {
 func MustCompile(expr string) *Expr {
 	exp, err := Compile(expr)
 	if err != nil {
-		return nil
+		return &Expr{s: expr, q: nopQuery{}}
 	}
 	return exp
 }

xpath_test.go

@@ -30,6 +30,22 @@ func TestCompile(t *testing.T) {
 		t.Fatalf("/a/b/(c, .[not(c)]) should be correct but got error %s", err)
 	}
 }
+
+func TestMustCompile(t *testing.T) {
+	expr := MustCompile("//")
+	if expr == nil {
+		t.Fatal("// should be compiled but got nil object")
+	}
+
+	if wanted := (nopQuery{}); expr.q != wanted {
+		t.Fatalf("wanted nopQuery object but got %s", expr)
+	}
+	iter := expr.Select(createNavigator(html))
+	if iter.MoveNext() {
+		t.Fatal("should be an empty node list but got one")
+	}
+}
+
 func TestSelf(t *testing.T) {
 	testXPath(t, html, ".", "html")
 	testXPath(t, html.FirstChild, ".", "head")