Print a parse tree even if syntax error occur

A parser can construct a parse tree even if syntax error occur.
When there is a parse tree, print it.

2 files changed, 18 insertions, 4 deletions

diff --git a/cmd/vartan/parse.go b/cmd/vartan/parse.go
index 9b15251..14412bf 100644
--- a/cmd/vartan/parse.go
+++ b/cmd/vartan/parse.go
@@ -141,14 +141,22 @@ func runParse(cmd *cobra.Command, args []string) (retErr error) {
 		fmt.Fprintf(os.Stderr, "\n")
 	}
 
-	if len(synErrs) == 0 && !*parseFlags.onlyParse {
+	if !*parseFlags.onlyParse {
+		// A parser can construct a parse tree even if syntax errors occur.
+		// When therer is a parse tree, print it.
+
 		var tree *driver.Node
 		if *parseFlags.cst {
 			tree = treeAct.CST()
 		} else {
 			tree = treeAct.AST()
 		}
-		driver.PrintTree(os.Stdout, tree)
+		if tree != nil {
+			if len(synErrs) > 0 {
+				fmt.Println("")
+			}
+			driver.PrintTree(os.Stdout, tree)
+		}
 	}
 
 	return nil
diff --git a/driver/semantic_action.go b/driver/semantic_action.go
index d88d5fa..61b00f0 100644
--- a/driver/semantic_action.go
+++ b/driver/semantic_action.go
@@ -37,6 +37,7 @@ type Node struct {
 	Row      int
 	Col      int
 	Children []*Node
+	Error    bool
 }
 
 func PrintTree(w io.Writer, node *Node) {
@@ -48,9 +49,12 @@ func printTree(w io.Writer, node *Node, ruledLine string, childRuledLinePrefix s
 		return
 	}
 
-	if node.Text != "" {
+	switch {
+	case node.Error:
+		fmt.Fprintf(w, "%v!%v\n", ruledLine, node.KindName)
+	case node.Text != "":
 		fmt.Fprintf(w, "%v%v %#v\n", ruledLine, node.KindName, node.Text)
-	} else {
+	default:
 		fmt.Fprintf(w, "%v%v\n", ruledLine, node.KindName)
 	}
 
@@ -212,11 +216,13 @@ func (a *SyntaxTreeActionSet) TrapAndShiftError(cause VToken, popped int) {
 	if a.makeAST {
 		ast = &Node{
 			KindName: a.gram.Terminal(a.gram.Error()),
+			Error:    true,
 		}
 	}
 	if a.makeCST {
 		cst = &Node{
 			KindName: a.gram.Terminal(a.gram.Error()),
+			Error:    true,
 		}
 	}