fail on parsing errors

.clang-format

@@ -81,6 +81,8 @@ IfMacros:
   - KJ_IF_MAYBE
 IncludeBlocks:   Regroup
 IncludeCategories:
+  - Regex:           '^<BaseErrorListener\.h>$'
+    Priority:        3
   - Regex:           '^<xlang.*\.h>$'
     Priority:        3
   - Regex:           '^<sys/.*\.h>$'

bootstrap/error.cc

@@ -0,0 +1,23 @@
+#include <error.hh>
+#include <iostream>
+
+namespace xlang {
+
+ErrorListener::ErrorListener(std::string_view inputfile)
+    : file{inputfile}, has_error{false} {}
+
+bool ErrorListener::hasError() {
+  return has_error;
+}
+
+void ErrorListener::syntaxError([[maybe_unused]] antlr4::Recognizer *recognizer,
+                                [[maybe_unused]] antlr4::Token *offendingSymbol,
+                                size_t line, size_t charPositionInLine,
+                                const std::string &msg,
+                                [[maybe_unused]] std::exception_ptr e) {
+  std::cerr << file << ":" << line << ":" << charPositionInLine << ": " << msg
+            << std::endl;
+  has_error = true;
+}
+
+}  // namespace xlang

bootstrap/error.hh

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <BaseErrorListener.h>
+#include <string_view>
+
+namespace xlang {
+
+class ErrorListener : public antlr4::BaseErrorListener {
+  std::string_view file;
+  bool has_error;
+
+ public:
+  ErrorListener(std::string_view inputfile);
+  bool hasError();
+  void syntaxError(antlr4::Recognizer *recognizer,
+                   antlr4::Token *offendingSymbol, size_t line,
+                   size_t charPositionInLine, const std::string &msg,
+                   std::exception_ptr e) override;
+};
+
+}  // namespace xlang

bootstrap/main.cc

@@ -2,6 +2,7 @@
 #include <unistd.h>
 
 #include <emit.hh>
+#include <error.hh>
 #include <iostream>
 #include <string>
 #include <xlangLexer.h>
@@ -41,7 +42,14 @@ int main(int argc, char **argv) {
   xlang::xlangLexer lexer{&input};
   antlr4::CommonTokenStream tokens{&lexer};
   xlang::xlangParser parser{&tokens};
+  xlang::ErrorListener errorlistener{inputfile};
+
+  parser.removeErrorListeners();
+  parser.addErrorListener(&errorlistener);
   auto *tree = parser.file();
+  if (errorlistener.hasError()) {
+    exit(1);
+  }
 
   xlang::EmitVisitor emit{outputfile};
   emit.visitFile(tree);

bootstrap/meson.build

@@ -2,6 +2,7 @@ xc_exe = executable('xc',
   sources : [
       'main.cc',
       'emit.cc',
+      'error.cc',
       antlr4.process('xlang.g4'),
     ],
   dependencies : [