better error messages

src/vm/include/Handle.h

@@ -44,23 +44,33 @@ public:
 
     const Handle car() const {
         if (!_target) return Handle(nullptr);
+        if (_target->_type != CellType::CONS) throw std::invalid_argument("Expected cons cell, got something else");
         return dynamic_cast<ConsCell &>(*_target)._car.load();
     }
     const Handle cdr() const {
         if (!_target) return Handle(nullptr);
+        if (_target->_type != CellType::CONS) throw std::invalid_argument("Expected cons cell, got something else");
         return dynamic_cast<ConsCell &>(*_target)._cdr.load();
     }
     Handle car() {
         if (!_target) return Handle(nullptr);
+        if (_target->_type != CellType::CONS) throw std::invalid_argument("Expected cons cell, got something else");
         return dynamic_cast<ConsCell &>(*_target)._car.load();
     }
     Handle cdr() {
         if (!_target) return Handle(nullptr);
+        if (_target->_type != CellType::CONS) throw std::invalid_argument("Expected cons cell, got something else");
         return dynamic_cast<ConsCell &>(*_target)._cdr.load();
     }
 
-    CellValType val() const { return dynamic_cast<NumAtomCell &>(*_target)._val; }
+    CellValType val() const {
-    std::string_view strval() const { return dynamic_cast<StrAtomCell &>(*_target)._val; }
+        if (_target->_type != CellType::NUMATOM) throw std::invalid_argument("Expected number cell, got something else");
+        return dynamic_cast<NumAtomCell &>(*_target)._val;
+    }
+    std::string_view strval() const {
+        if (_target->_type != CellType::STRATOM) throw std::invalid_argument("Expected string cell, got something else");
+        return dynamic_cast<StrAtomCell &>(*_target)._val;
+    }
 
     CellType type() const {
         if (!_target) return CellType::CONS;

src/vm/src/Handle.cpp

@@ -68,6 +68,7 @@ Handle Handle::makeNumCell(int64_t val) { return MemoryContext::get().create_cel
 Handle Handle::makeStrCell(std::string val) { return MemoryContext::get().create_cell<StrAtomCell>(std::move(val)); }
 
 void Handle::setcar(const Handle &car) {
+    if (_target->_type != CellType::CONS) throw std::invalid_argument("Expected cons cell, got something else");
     MemoryContext::get().run_dirty<void>([&](std::function<void(Cell *)> lost) -> void {
         lost(dynamic_cast<ConsCell &>(*_target)._car);
         dynamic_cast<ConsCell &>(*_target)._car = car.get();
@@ -75,6 +76,7 @@ void Handle::setcar(const Handle &car) {
 }
 
 void Handle::setcdr(const Handle &cdr) {
+    if (_target->_type != CellType::CONS) throw std::invalid_argument("Expected cons cell, got something else");
     MemoryContext::get().run_dirty<void>([&](std::function<void(Cell *)> lost) -> void {
         lost(dynamic_cast<ConsCell &>(*_target)._cdr);
         dynamic_cast<ConsCell &>(*_target)._cdr = cdr.get();

src/vm/src/Parser.cpp

@@ -64,7 +64,10 @@ std::string Parser::Tokenizer::getNext() {
     return ret;
 }
 
-std::string_view Parser::Tokenizer::peek() const { return _tokens.front(); }
+std::string_view Parser::Tokenizer::peek() const {
+    if (_tokens.empty()) throw std::invalid_argument("Unexpected end of input (or maybe missing \")\"?");
+    return _tokens.front();
+}
 
 void Parser::Tokenizer::load(std::string_view input) {
     std::string_view::size_type curpos = input.find_first_not_of(std::string{' ', '\n', '\r'});

src/vm/src/VM.cpp

@@ -242,7 +242,7 @@ void VM::step() {
         std::getline(_instream, read);
         _s.push(Parser::parse_str(read));
     } else {
-        assert(false);
+        throw std::invalid_argument("Unexpected end of program");
     }