Support the underscore symbol matching any symbols in vartan-test com…

…mand

README.md

@@ -150,6 +150,8 @@ Passed test.txt
 
 When you specify a directory as the 2nd argument of `vartan test` command, it will run all test cases in the directory.
 
+The underscore `_` allows you to match any symbols. Thus `(expr (expr (id)) (_) (expr (id)))` matches `a + b`, `a - b`, and so on.
+
 ### 5. Generate a parser
 
 Using `vartan-go` command, you can generate a source code of a parser to recognize your grammar.

spec/test/parser.go

@@ -89,7 +89,8 @@ func DiffTree(expected, actual *Tree) []*TreeDiff {
 	if expected == nil && actual == nil {
 		return nil
 	}
-	if actual.Kind != expected.Kind {
+	// _ matches any symbols.
+	if expected.Kind != "_" && actual.Kind != expected.Kind {
 		msg := fmt.Sprintf("unexpected kind: expected '%v' but got '%v'", expected.Kind, actual.Kind)
 		return []*TreeDiff{
 			newTreeDiff(expected, actual, msg),
@@ -103,7 +104,7 @@ func DiffTree(expected, actual *Tree) []*TreeDiff {
 	}
 	var diffs []*TreeDiff
 	for i, exp := range expected.Children {
-		if ds := DiffTree(actual.Children[i], exp); len(ds) > 0 {
+		if ds := DiffTree(exp, actual.Children[i]); len(ds) > 0 {
 			diffs = append(diffs, ds...)
 		}
 	}

spec/test/parser_test.go

@@ -74,6 +74,26 @@ func TestDiffTree(t *testing.T) {
 				),
 			),
 		},
+		{
+			t1: NewTree("_"),
+			t2: NewTree("a"),
+		},
+		{
+			t1: NewTree("a",
+				NewTree("_"),
+			),
+			t2: NewTree("a",
+				NewTree("b"),
+			),
+		},
+		{
+			t1: NewTree("_",
+				NewTree("b"),
+			),
+			t2: NewTree("a",
+				NewTree("b"),
+			),
+		},
 		{
 			t1:        NewTree("a"),
 			t2:        NewTree("b"),
@@ -139,6 +159,35 @@ func TestDiffTree(t *testing.T) {
 			),
 			different: true,
 		},
+		{
+			t1: NewTree("a",
+				NewTree("_"),
+				NewTree("c"),
+			),
+			t2: NewTree("a",
+				NewTree("b"),
+				NewTree("x"),
+			),
+			different: true,
+		},
+		{
+			t1: NewTree("_"),
+			t2: NewTree("a",
+				NewTree("b"),
+			),
+			different: true,
+		},
+		{
+			t1: NewTree("a",
+				NewTree("_"),
+			),
+			t2: NewTree("a",
+				NewTree("b",
+					NewTree("c"),
+				),
+			),
+			different: true,
+		},
 	}
 	for i, tt := range tests {
 		t.Run(fmt.Sprintf("#%v", i), func(t *testing.T) {

tester/tester.go

@@ -152,7 +152,7 @@ func runTest(g *gspec.CompiledGrammar, c *TestCaseWithMetadata) *TestResult {
 	}
 
 	// When a parse tree exists, the test continues regardless of whether or not syntax errors occurred.
-	diffs := tspec.DiffTree(ConvertSyntaxTreeToTestableTree(tb.Tree()).Fill(), c.TestCase.Output)
+	diffs := tspec.DiffTree(c.TestCase.Output, ConvertSyntaxTreeToTestableTree(tb.Tree()).Fill())
 	if len(diffs) > 0 {
 		return &TestResult{
 			TestCasePath: c.FilePath,