parse let expressions, handle syntax errors

parser/parser_test.go

@@ -16,6 +16,8 @@ let foobar = 838383;
 	p := New(l)
 
 	program := p.ParseProgram()
+	checkParserErrors(t, p)
+
 	if program == nil {
 		t.Fatalf("ParseProgram() returned nil")
 	}
@@ -40,6 +42,19 @@ let foobar = 838383;
 	}
 }
 
+func checkParserErrors(t *testing.T, p *Parser) {
+	errors := p.Errors()
+	if len(errors) == 0 {
+		return
+	}
+
+	t.Errorf("parser has %d errors", len(errors))
+	for _, msg := range errors {
+		t.Errorf("parser error: %q", msg)
+	}
+	t.FailNow()
+}
+
 func testLetStatement(t *testing.T, s ast.Statement, name string) bool {
 	if s.TokenLiteral() != "let" {
 		t.Errorf("s.TokenLiteral not 'let'. got=%q", s.TokenLiteral())