Add invalid input node in testdata

go/dev/interactive.go

@@ -43,12 +43,11 @@ func main() {
 		in = strings.TrimSpace(in)
 		running, err = en.Exec([]byte(in), ctx)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "execution terminated: %v\n", err)
+			fmt.Fprintf(os.Stderr, "unexpected termination: %v\n", err)
 			os.Exit(1)
 		}
 		b := bytes.NewBuffer(nil)
 		en.WriteResult(b)
 		fmt.Println(b.String())
 	}
-
 }

go/testdata/testdata.go

@@ -86,13 +86,24 @@ func baz() error {
 	return out("baz", b, tpl)
 }
 
+func defaultCatch() error {
+	b := []byte{}
+	b = vm.NewLine(b, vm.MOUT, []string{"0", "back"}, nil, nil)
+	b = vm.NewLine(b, vm.HALT, nil, nil, nil)
+	b = vm.NewLine(b, vm.MOVE, []string{"_"}, nil, nil)
+
+	tpl := "invalid input"
+
+	return out("_catch", b, tpl)
+}
+
 func generate() error {
 	err := os.MkdirAll(DataDir, 0755)
 	if err != nil {
 		return err
 	}
 
-	fns := []genFunc{root, foo, bar, baz}
+	fns := []genFunc{root, foo, bar, baz, defaultCatch}
 	for _, fn := range fns {
 		err = fn()
 		if err != nil {

go/vm/runner.go

@@ -94,8 +94,6 @@ func RunDeadCheck(b []byte, st *state.State, rs resource.Resource, ctx context.C
 		return b, fmt.Errorf("dead runner with no current location")
 	}
 	b = NewLine(nil, MOVE, []string{"_catch"}, nil, nil)
-	b = NewLine(b, HALT, nil, nil, nil)
-	b = NewLine(b, MOVE, []string{location}, nil, nil)
 	log.Printf("code is now %x", b)
 	return b, nil
 }
@@ -180,7 +178,12 @@ func RunMove(b []byte, st *state.State, rs resource.Resource, ctx context.Contex
 	if err != nil {
 		return b, err
 	}
+	if sym == "_" {
+		st.Up()
+		sym = st.Where()
+	} else {
 		st.Down(sym)
+	}
 	code, err := rs.GetCode(sym)
 	if err != nil {
 		return b, err