better (thread) error handling

ml.cpp

@@ -22,6 +22,7 @@
 #include <ctime>
 #include <chrono>
 #include <thread>
+#include <mutex>
 
 
 #define TOO_FEW_ARGS "too few arguments to function"
@@ -624,12 +625,16 @@ MlError::~MlError() {
 	delete cause;
 }
 
-std::string MlError::description() {
+std::string MlError::description() const {
 	// return "error: the expression `" + cause->debug() + "` failed in scope " + to_string(env) + " with message \"" + msg + "\"";
     return MlPerfMon::instance().callstack() +
            "error: the expression `" + cause->debug() + "` with message \"" + msg + "\"";
 }
 
+const char* MlError::what() const noexcept {
+    return ("error: the expression `" + cause->debug() + "` with message \"" + msg + "\"").c_str();
+}
+
 void MlEnvironment::combine(MlEnvironment const &other) {
 	// Normally, I would use the `insert` method of the `map` class,
 	// but it doesn't overwrite previously declared values for keys.
@@ -1753,8 +1758,12 @@ namespace builtin {
 
     MlValue thread_create(std::vector<MlValue> args, MlEnvironment &env) {
 		auto functor = [](std::vector<MlValue> args, MlEnvironment &env) -> void {
+			try {
 				for (size_t i = 0; i < args.size(); i++)
 					MlValue acc = args[i].eval(env);
+			} catch (std::exception &e) {
+				std::cerr << "thread exception: " << e.what() << std::endl;
+			}
 		};
 
         std::thread th(functor, args, std::ref(env));
@@ -1777,9 +1786,7 @@ namespace builtin {
 		if (args[0].as_string() != "ilock")
            throw MlError(MlValue("thread_under_lock", thread_under_lock), env, UNKNOWN_ERROR);
 
-
         std::lock_guard<std::mutex> lockGuard(interpreter_mutex);
-
 		MlValue acc;
 		for (size_t i = 0; i < args.size(); i++)
 			acc = args[i].eval(env);
@@ -1791,6 +1798,7 @@ namespace builtin {
  		if (args.size() != 0)
            throw MlError(MlValue("threads_join", threads_join), env, TOO_MANY_ARGS);
 
+		std::lock_guard<std::mutex> lockGuard(register_mutex);
 		for (auto &th : threads_register)
 		    if (th.joinable()) th.join();
   		
@@ -1841,7 +1849,7 @@ void repl(MlEnvironment &env) {
 			} catch (MlError &e) {
 				std::cerr << e.description() << std::endl;
 				MlPerfMon::instance().clear_callstack();
-			} catch (std::runtime_error &e) {
+			} catch (std::exception &e) {
 				std::cerr << e.what() << std::endl;
 			}
 		}
@@ -2065,8 +2073,8 @@ int main(int argc, char *argv[]) {
 		return 0;
 		
 	} catch (MlError &e) {
-		std::cerr << e.description() << std::endl;
-	} catch (std::runtime_error &e) {
+		std::cerr << e.what() << std::endl;
+	} catch (const std::exception &e) {
 		std::cerr << MlPerfMon::instance().callstack() << e.what() << std::endl;
 		MlPerfMon::instance().clear_callstack();
 	}

ml.h

@@ -67,8 +67,10 @@ private:
 
 
 // An exception thrown by the lisp
-class MlError {
+class MlError : public std::exception {
 public:
+    MlError() = delete;
+
     // Create an error with the value that caused the error,
     // the scope where the error was found, and the message.
     MlError(const MlValue &v, MlEnvironment const &env, const char *msg);
@@ -79,7 +81,10 @@ public:
     ~MlError();
 
     // Get the printable error description.
-    std::string description();
+    std::string description() const;
+
+    // inherited from std::exception 
+    virtual const char* what() const noexcept;
 
 private:
     MlValue *cause;