aster-core/src/vm/compiler.rs

@@ -23,7 +23,11 @@ pub struct FunctionProto {
     pub arity: u8,
     pub code: Vec<u8>,
     pub constants: Vec<Value>,
+    /// Nested function protos (for closures and function declarations)
+    pub protos: Vec<Rc<FunctionProto>>,
     pub upvalue_count: u8,
+    /// Upvalue descriptors for this function (for Closure opcode emission)
+    pub upvalues: Vec<(bool, u8)>, // (is_local, index)
     pub lines: Vec<(usize, usize)>, // (bytecode_offset, source_line)
 }
 
@@ -34,7 +38,9 @@ impl FunctionProto {
             arity: 0,
             code: Vec::new(),
             constants: Vec::new(),
+            protos: Vec::new(),
             upvalue_count: 0,
+            upvalues: Vec::new(),
             lines: Vec::new(),
         }
     }
@@ -66,6 +72,7 @@ fn values_eq(a: &Value, b: &Value) -> bool {
 // Compiler
 // ============================================================================
 
+#[derive(Clone)]
 struct Local {
     name: String,
     depth: u8,       // scope depth where declared; 0 = uninitialized
@@ -73,6 +80,7 @@ struct Local {
     is_const: bool,
 }
 
+#[derive(Clone)]
 struct Upvalue {
     index: u8,
     is_local: bool,  // true = captured from enclosing fn's local; false = from upvalue
@@ -89,10 +97,12 @@ pub struct Compiler {
     function: FunctionProto,
     locals: Vec<Local>,
     upvalues: Vec<Upvalue>,
+    /// Snapshot of enclosing compiler's locals (for upvalue resolution)
+    enclosing_locals: Option<Vec<Local>>,
+    /// Snapshot of enclosing compiler's upvalues (for transitive upvalues)
+    enclosing_upvalues: Option<Vec<Upvalue>>,
     scope_depth: u8,
     loop_stack: Vec<LoopContext>,
-    /// Index into an outer compiler array for upvalue resolution
-    enclosing_idx: Option<usize>,
 }
 
 impl Compiler {
@@ -101,9 +111,10 @@ impl Compiler {
             function: FunctionProto::new(name),
             locals: Vec::new(),
             upvalues: Vec::new(),
+            enclosing_locals: None,
+            enclosing_upvalues: None,
             scope_depth: 0,
             loop_stack: Vec::new(),
-            enclosing_idx: None,
         }
     }
 
@@ -159,6 +170,8 @@ impl Compiler {
             }
             Stmt::Function { name, params, body } => {
                 self.compile_function_decl(name, params, body)?;
+                // DefineGlobal pushes the value back; pop it as this is a statement
+                self.emit_op(OpCode::Pop);
             }
             Stmt::Return(expr) => {
                 if let Some(e) = expr {
@@ -181,11 +194,11 @@ impl Compiler {
     fn compile_let(&mut self, name: &str, initializer: &Expr, mutable: bool) -> Result<(), RuntimeError> {
         self.compile_expr(initializer)?;
         if self.scope_depth == 0 {
-            // Global variable
             let name_idx = self.add_string_constant(name);
             emit_u16(&mut self.function.code, OpCode::DefineGlobal, name_idx);
+            // DefineGlobal pushes value back; pop it for statement-level let
+            self.emit_op(OpCode::Pop);
         } else {
-            // Local variable
             let slot = self.locals.len() as u8;
             self.locals.push(Local {
                 name: name.to_string(),
@@ -193,6 +206,7 @@ impl Compiler {
                 is_captured: false,
                 is_const: !mutable,
             });
+            // StoreLocal PEEKS the value — it stays on stack as the local
             emit_u8(&mut self.function.code, OpCode::StoreLocal, slot);
         }
         Ok(())
@@ -304,6 +318,8 @@ impl Compiler {
             is_const: false,
         });
         emit_u8(&mut self.function.code, OpCode::StoreLocal, loop_var_slot);
+        // Pop the ForInNext value from stack top (it's stored in the local)
+        self.emit_op(OpCode::Pop);
 
         let start_ip = self.function.code.len();
 
@@ -324,7 +340,7 @@ impl Compiler {
         }
 
         // Jump back to ForInNext
-        self.emit_loop_jump(start_ip - 3); // jump back to the ForInNext instruction
+        self.emit_loop_jump(exit_jump);
         let exit_ip = self.function.code.len();
         self.patch_jump_to(exit_jump, exit_ip);
 
@@ -344,20 +360,9 @@ impl Compiler {
 
     fn compile_function_decl(&mut self, name: &str, params: &[String], body: &[Stmt]) -> Result<(), RuntimeError> {
         let proto = self.compile_nested_function(Some(name.to_string()), params, body)?;
-        let const_idx = self.function.add_constant(Value::Function(Rc::new(
+        let upvalues = proto.upvalues.clone();
-            crate::interpreter::Function {
-                params: params.to_vec(),
-                body: body.to_vec(),
-                env: Rc::new(RefCell::new(crate::interpreter::Env::new(None))),
-                name: Some(name.to_string()),
-            }
-        )));
-        // For now, we also need to store the proto for the VM to use.
-        // We'll store it as a special constant.
         let proto_idx = self.add_function_proto_constant(proto);
-
+        self.emit_closure(proto_idx, &upvalues);
-        // Emit Closure opcode with upvalues
-        self.emit_closure(proto_idx);
 
         // Bind to name
         if self.scope_depth == 0 {
@@ -447,17 +452,8 @@ impl Compiler {
         }
         // Try to resolve as upvalue
         if let Some(upvalue_idx) = self.resolve_upvalue(name) {
-            // Upvalue access: LoadUpvalue is LoadLocal with slot = upvalue-local-marker
+            emit_u8(&mut self.function.code, OpCode::LoadUpvalue, upvalue_idx);
-            // For simplicity, we use a convention: upvalues are locals with special marking.
+            return Ok(());
-            // Actually, we need a dedicated LoadUpvalue opcode. Let's add it...
-            // For now, store upvalues at "local slots" offset by 256.
-            emit_u16(&mut self.function.code, OpCode::LoadConst, 0xFFFF); // placeholder
-            // We'll handle upvalues properly when we have LoadUpvalue
-            // TODO: Add LoadUpvalue opcode
-            return Err(RuntimeError::RuntimeError {
-                message: format!("Upvalue '{}' not yet supported", name),
-                token: None,
-            });
         }
         // Fall back to global
         let name_idx = self.add_string_constant(name);
@@ -471,6 +467,8 @@ impl Compiler {
             self.compile_expr(value)?;
             if let Some(slot) = self.resolve_local(name) {
                 emit_u8(&mut self.function.code, OpCode::StoreLocal, slot);
+            } else if let Some(uv_idx) = self.resolve_upvalue(name) {
+                emit_u8(&mut self.function.code, OpCode::StoreUpvalue, uv_idx);
             } else {
                 let name_idx = self.add_string_constant(name);
                 emit_u16(&mut self.function.code, OpCode::StoreGlobal, name_idx);
@@ -489,6 +487,14 @@ impl Compiler {
             if let Some(slot) = self.resolve_local(name) {
                 emit_u8(&mut self.function.code, OpCode::CompoundAssignLocal, slot);
                 self.function.code.push(compound_op as u8);
+            } else if self.resolve_upvalue(name).is_some() {
+                // Compound assign on upvalue: load upvalue, compound op, store
+                // For simplicity, use a workaround: re-emit this as a separate step
+                // TODO: add CompoundAssignUpvalue opcode
+                return Err(RuntimeError::RuntimeError {
+                    message: "Compound assignment on upvalue not yet supported".into(),
+                    token: None,
+                });
             } else {
                 let name_idx = self.add_string_constant(name);
                 emit_u16(&mut self.function.code, OpCode::CompoundAssignProp, name_idx);
@@ -643,27 +649,27 @@ impl Compiler {
 
     fn compile_lambda(&mut self, params: &[String], body: &[Stmt]) -> Result<(), RuntimeError> {
         let proto = self.compile_nested_function(None, params, body)?;
+        let upvalues = proto.upvalues.clone();
         let proto_idx = self.add_function_proto_constant(proto);
-        self.emit_closure(proto_idx);
+        self.emit_closure(proto_idx, &upvalues);
         Ok(())
     }
 
     /// Compile a nested function (lambda or function declaration) and return its proto.
     fn compile_nested_function(&mut self, name: Option<String>, params: &[String], body: &[Stmt]) -> Result<FunctionProto, RuntimeError> {
         let mut child = Compiler::new(name);
-        child.enclosing_idx = Some(0); // placeholder — we handle upvalues differently
+        // Pass enclosing state for upvalue resolution
+        child.enclosing_locals = Some(self.locals.clone());
+        child.enclosing_upvalues = Some(self.upvalues.clone());
 
-        // Add params as locals
+        // Add params as locals without StoreLocal — they're already on stack from Call
         for param in params {
-            let slot = child.locals.len() as u8;
             child.locals.push(Local {
                 name: param.clone(),
                 depth: 1, // params are at scope depth 1 (function body)
                 is_captured: false,
                 is_const: false,
             });
-            // Params are already on stack from Call; StoreLocal just peeks
-            emit_u8(&mut child.function.code, OpCode::StoreLocal, slot);
         }
         child.function.arity = params.len() as u8;
 
@@ -676,11 +682,17 @@ impl Compiler {
         child.emit_op(OpCode::LoadNil);
         child.emit_op(OpCode::Return);
 
-        // Resolve upvalues: for each variable reference in the child that wasn't
+        // Mark captured locals in the parent compiler
-        // resolved locally, check if it exists in the parent's locals/upvalues.
+        for uv in &child.upvalues {
-        // (We handle this lazily in compile_variable for now — if not local, try upvalue.)
+            if uv.is_local {
+                if let Some(local) = self.locals.get_mut(uv.index as usize) {
+                    local.is_captured = true;
+                }
+            }
+        }
 
         child.function.upvalue_count = child.upvalues.len() as u8;
+        child.function.upvalues = child.upvalues.iter().map(|uv| (uv.is_local, uv.index)).collect();
         Ok(child.function)
     }
 
@@ -694,16 +706,18 @@ impl Compiler {
 
     fn end_scope(&mut self) {
         self.scope_depth -= 1;
-        // Pop locals that are going out of scope
+        // Pop locals that are going out of scope (except captured ones)
         let mut pop_count = 0u8;
         while let Some(local) = self.locals.last() {
             if local.depth > self.scope_depth {
                 if local.is_captured {
-                    // Close upvalue instead of pop
+                    // Don't pop — the upvalue still needs it on the stack
-                    // (For now, just pop — upvalue closing handled in VM)
+                    // The VM will close the upvalue when the function returns
-                }
+                    self.locals.pop();
+                } else {
                     self.locals.pop();
                     pop_count += 1;
+                }
             } else {
                 break;
             }
@@ -728,9 +742,24 @@ impl Compiler {
     }
 
     /// Try to resolve a variable as an upvalue from enclosing functions.
+    /// Returns the upvalue index in this function's upvalues list.
     fn resolve_upvalue(&mut self, name: &str) -> Option<u8> {
-        // For now, we don't have enclosing compiler access.
+        if let Some(ref enclosing_locals) = self.enclosing_locals {
-        // Upvalues will be fully implemented in a follow-up.
+            for (i, local) in enclosing_locals.iter().enumerate().rev() {
+                if local.name == name && local.depth > 0 {
+                    // Check if we already captured this upvalue
+                    for (j, uv) in self.upvalues.iter().enumerate() {
+                        if uv.is_local && uv.index == i as u8 {
+                            return Some(j as u8);
+                        }
+                    }
+                    let idx = self.upvalues.len() as u8;
+                    self.upvalues.push(Upvalue { index: i as u8, is_local: true });
+                    return Some(idx);
+                }
+            }
+        }
+        // Transitive upvalues (from enclosing's upvalues) not yet implemented
         None
     }
 
@@ -759,13 +788,17 @@ impl Compiler {
         emit_i16(&mut self.function.code, OpCode::Jump, offset as i16);
     }
 
-    fn emit_closure(&mut self, proto_idx: u16) {
+    fn emit_closure(&mut self, proto_idx: u16, upvalues: &[(bool, u8)]) {
         let code = &mut self.function.code;
         code.push(OpCode::Closure as u8);
         code.push((proto_idx & 0xFF) as u8);
         code.push(((proto_idx >> 8) & 0xFF) as u8);
-        // No upvalues yet
+        let upvalue_count = upvalues.len() as u8;
-        code.push(0u8); // upvalue count = 0 for now
+        code.push(upvalue_count);
+        for &(is_local, index) in upvalues {
+            code.push(if is_local { 1u8 } else { 0u8 });
+            code.push(index);
+        }
     }
 
     fn patch_jump(&mut self, jump_loc: usize) {
@@ -791,20 +824,10 @@ impl Compiler {
     }
 
     fn add_function_proto_constant(&mut self, proto: FunctionProto) -> u16 {
-        // Store compiled proto as a special marker.
+        let idx = self.function.protos.len() as u16;
-        // We use Value::Nil as placeholder since FunctionProto isn't a Value.
+        self.function.protos.push(Rc::new(proto));
-        // The VM will look up the proto from a separate table.
+        // Store proto index as a sentinel value in constants
-        // For now, store the proto's index in a side table.
+        self.function.constants.push(Value::Number(f64::from_bits(idx as u64 | 0x_F000_0000_0000_0000)));
-        // Actually, let's store it as a string tag that the VM can recognize.
-        // We'll use a dedicated proto storage: just append to a Vec.
-        // But FunctionProto isn't a Value... let's store it inline.
-        // HACK: store proto in constants with a special Object wrapping
-        let idx = self.function.constants.len() as u16;
-        // Use a Value::Object with a special marker
-        // The VM will need to handle this
-        let mut map = std::collections::HashMap::new();
-        map.insert("__proto__".to_string(), Value::String(format!("proto_{}", idx)));
-        self.function.constants.push(Value::Object(Rc::new(RefCell::new(map))));
         idx
     }
 }

aster-core/src/vm/opcode.rs

@@ -20,6 +20,8 @@ pub enum OpCode {
     // --- Local variables (operand: u8 slot index) ---
     LoadLocal = 6,
     StoreLocal = 7,
+    LoadUpvalue = 42,   // operand: u8 upvalue index
+    StoreUpvalue = 43,  // operand: u8 upvalue index
 
     // --- Global variables (operand: u16 index into constant pool for name) ---
     LoadGlobal = 8,
@@ -80,16 +82,16 @@ pub enum OpCode {
 
 impl OpCode {
     pub fn from_u8(byte: u8) -> Option<Self> {
-        if byte <= 41 {
+        match byte {
-            // SAFETY: OpCode is #[repr(u8)] with contiguous values 0..=41
+            0..=41 => Some(unsafe { std::mem::transmute::<u8, OpCode>(byte) }),
-            Some(unsafe { std::mem::transmute::<u8, OpCode>(byte) })
+            42 => Some(OpCode::LoadUpvalue),
-        } else {
+            43 => Some(OpCode::StoreUpvalue),
-            None
+            _ => None,
         }
     }
 
     /// Total number of distinct opcodes
-    pub const COUNT: usize = 42;
+    pub const COUNT: usize = 44;
 }
 
 /// Compound assignment operator tags (used as operand byte after

aster-core/src/vm/vm.rs

@@ -63,8 +63,8 @@ pub struct Vm {
     /// Open upvalues (tracked so closures share the same upvalue object)
     pub open_upvalues: Vec<Rc<RefCell<UpvalueObj>>>,
 
-    /// Allocated function protos (indexed by the compiler's constant pool index)
+    /// VM-compiled closures: maps Function Rc pointer → Closure data
-    protos: Vec<Rc<FunctionProto>>,
+    closures: RefCell<HashMap<*const crate::interpreter::Function, Rc<Closure>>>,
 }
 
 impl Vm {
@@ -85,7 +85,7 @@ impl Vm {
                 .map(|p| p.to_string_lossy().to_string())
                 .unwrap_or_else(|_| ".".to_string()),
             open_upvalues: Vec::new(),
-            protos: Vec::new(),
+            closures: RefCell::new(HashMap::new()),
         }
     }
 
@@ -133,12 +133,13 @@ impl Vm {
 
             if ip >= code_len {
                 // End of function — implicit return nil
-                self.frames.pop();
+                let frame = self.frames.pop().unwrap();
+                self.close_upvalues(frame.stack_base);
                 if self.frames.is_empty() {
                     return Ok(());
                 }
-                let base = self.frames.last().unwrap().stack_base;
+                // Remove callee + args + function locals, push nil result
-                self.stack.truncate(base);
+                self.stack.truncate(frame.stack_base.saturating_sub(1));
                 self.stack.push(Value::Nil);
                 continue;
             }
@@ -193,15 +194,51 @@ impl Vm {
                 // --- Locals ---
                 OpCode::LoadLocal => {
                     let slot = read_u8(code, ip) as usize;
-                    let val = self.stack[self.frame().stack_base + slot].clone();
+                    let idx = self.frames.last().unwrap().stack_base + slot;
+                    if idx >= self.stack.len() {
+                        return Err(RuntimeError::RuntimeError {
+                            message: format!("LoadLocal: slot {} uninitialized", slot),
+                            token: None,
+                        });
+                    }
+                    let val = self.stack[idx].clone();
                     self.stack.push(val);
                     self.advance_ip(SIZE_U8);
                 }
                 OpCode::StoreLocal => {
                     let slot = read_u8(code, ip) as usize;
-                    let val = self.stack.last().unwrap().clone();
+                    let val = self.stack.last().unwrap().clone(); // peek
                     let base = self.frames.last().unwrap().stack_base;
-                    self.stack[base + slot] = val;
+                    let idx = base + slot;
+                    if idx >= self.stack.len() {
+                        self.stack.resize(idx + 1, Value::Nil);
+                    }
+                    self.stack[idx] = val;
+                    self.advance_ip(SIZE_U8);
+                }
+                OpCode::LoadUpvalue => {
+                    let idx = read_u8(code, ip) as usize;
+                    let uv = Rc::clone(&self.frames.last().unwrap().closure.upvalues[idx]);
+                    let uv_ref = uv.borrow();
+                    let val = if let Some(ref closed) = uv_ref.closed {
+                        closed.clone()
+                    } else {
+                        self.stack[uv_ref.location].clone()
+                    };
+                    drop(uv_ref);
+                    self.stack.push(val);
+                    self.advance_ip(SIZE_U8);
+                }
+                OpCode::StoreUpvalue => {
+                    let idx = read_u8(code, ip) as usize;
+                    let val = self.stack.last().unwrap().clone(); // peek
+                    let uv = Rc::clone(&self.frames.last().unwrap().closure.upvalues[idx]);
+                    let mut uv_ref = uv.borrow_mut();
+                    if let Some(ref mut closed) = uv_ref.closed {
+                        *closed = val;
+                    } else {
+                        self.stack[uv_ref.location] = val;
+                    }
                     self.advance_ip(SIZE_U8);
                 }
 
@@ -402,31 +439,34 @@ impl Vm {
                     let result = self.stack.pop().unwrap();
                     let frame = self.frames.pop().unwrap();
                     self.close_upvalues(frame.stack_base);
-                    self.stack.truncate(frame.stack_base);
+                    // Remove callee + args + function locals, push return value
+                    self.stack.truncate(frame.stack_base.saturating_sub(1));
                     self.stack.push(result);
                     if self.frames.is_empty() {
                         return Ok(());
                     }
-                    // IP of caller is unchanged (already at next instruction)
                 }
                 OpCode::Closure => {
                     let proto_idx = read_u16(code, ip) as usize;
                     let upvalue_count = code[ip + 3] as usize;
-                    let proto = Rc::clone(&self.protos[proto_idx]);
+                    let proto = proto.protos.get(proto_idx).ok_or_else(|| RuntimeError::RuntimeError {
+                        message: format!("Closure proto index {} out of bounds", proto_idx),
+                        token: None,
+                    })?.clone();
 
-                    // Collect upvalue capture info from bytecode first
+                    // Collect upvalue capture info
                     struct UpCapture { is_local: bool, index: usize }
                     let mut captures: Vec<UpCapture> = Vec::new();
-                    let mut offset = ip + 4;
+                    let mut off = ip + 4;
                     for _ in 0..upvalue_count {
-                        let is_local = code[offset] != 0;
+                        let is_local = code[off] != 0;
-                        offset += 1;
+                        off += 1;
-                        let index = code[offset] as usize;
+                        let index = code[off] as usize;
-                        offset += 1;
+                        off += 1;
                         captures.push(UpCapture { is_local, index });
                     }
 
-                    // Now do the actual captures (no conflicting borrows)
+                    // Do the actual captures
                     let base = self.frames.last().unwrap().stack_base;
                     let parent_upvalues = self.frames.last().unwrap().closure.upvalues.clone();
                     let mut upvalues = Vec::new();
@@ -439,17 +479,17 @@ impl Vm {
                         }
                     }
 
-                    // Store closure (stub for now)
+                    let closure = Rc::new(Closure { proto, upvalues });
-                    let _closure = Rc::new(Closure { proto, upvalues });
+                    let func = Rc::new(crate::interpreter::Function {
-                    self.stack.push(Value::Function(Rc::new(
-                        crate::interpreter::Function {
                         params: Vec::new(),
                         body: Vec::new(),
                         env: Rc::new(RefCell::new(crate::interpreter::Env::new(None))),
                         name: None,
-                        }
+                    });
-                    )));
+                    let key = Rc::as_ptr(&func) as *const crate::interpreter::Function;
-                    self.advance_ip_to(offset);
+                    self.closures.borrow_mut().insert(key, closure);
+                    self.stack.push(Value::Function(func));
+                    self.advance_ip_to(off);
                 }
 
                 // --- Object/Array ---
@@ -507,9 +547,16 @@ impl Vm {
                     let compound_op = CompoundOp::from_u8(op_tag).unwrap();
                     let rhs = self.stack.pop().unwrap();
                     let base = self.frames.last().unwrap().stack_base;
-                    let lhs = self.stack[base + slot].clone();
+                    let idx = base + slot;
+                    if idx >= self.stack.len() {
+                        return Err(RuntimeError::RuntimeError {
+                            message: format!("CompoundAssignLocal to uninitialized local {}", slot),
+                            token: None,
+                        });
+                    }
+                    let lhs = self.stack[idx].clone();
                     let result = self.apply_compound_op(lhs, rhs, compound_op)?;
-                    self.stack[base + slot] = result.clone();
+                    self.stack[idx] = result.clone();
                     self.stack.push(result);
                     self.advance_ip(SIZE_U8 + 1);
                 }
@@ -570,8 +617,13 @@ impl Vm {
         let callee_idx = self.stack.len() - 1 - arg_count;
         let callee = self.stack[callee_idx].clone();
 
-        match callee {
+        match &callee {
-            Value::NativeFunction(native_fn) => {
+            Value::NativeFunction(_) => {
+                // Get the native function
+                let native_fn = match self.stack[callee_idx].clone() {
+                    Value::NativeFunction(f) => f,
+                    _ => unreachable!(),
+                };
                 // Pop arguments from stack
                 let mut args = Vec::new();
                 for _ in 0..arg_count {
@@ -583,27 +635,27 @@ impl Vm {
                 self.stack.push(result);
                 self.advance_ip(SIZE_U8);
             }
-            Value::Function(f) => {
+            Value::Function(_) => {
-                // User-defined function: create new call frame
+                // Look up the VM-compiled closure
-                let base = callee_idx; // where the new frame's locals start
+                let func_ptr = match &self.stack[callee_idx] {
-                let new_closure = Rc::new(Closure {
+                    Value::Function(f) => Rc::as_ptr(f) as *const crate::interpreter::Function,
-                    proto: Rc::new(FunctionProto::new(f.name.clone())),
+                    _ => unreachable!(),
-                    upvalues: Vec::new(),
+                };
-                });
+                let closure = self.closures.borrow().get(&func_ptr).cloned().ok_or_else(|| RuntimeError::RuntimeError {
+                    message: "VM: call to non-VM function (tree-walker function not supported in VM)".into(),
+                    token: None,
+                })?;
+
+                // Advance caller's IP past the Call instruction before pushing new frame
+                self.frame_mut().ip += SIZE_U8;
+
+                // Callee is at callee_idx, args start at callee_idx+1
+                let base = callee_idx + 1; // first param is here
                 self.frames.push(CallFrame {
-                    closure: new_closure,
+                    closure,
                     ip: 0,
                     stack_base: base,
                 });
-                // Arguments are already on the stack at the right position
-                // (they become locals 0..arg_count in the new frame)
-                // The callee itself becomes slot 0 (TODO: handle properly)
-                // For now, this is a stub — full function support needs
-                // proper closure/proto integration
-                return Err(RuntimeError::RuntimeError {
-                    message: "VM user-defined function calls not yet fully implemented".into(),
-                    token: None,
-                });
             }
             _ => {
                 return Err(RuntimeError::RuntimeError {