fixed mutability

vm/vm.go

@@ -157,22 +157,23 @@ func (vm *VM) Run() error {
 			vm.currentFrame().ip += 2
 
 			ref := vm.pop()
-			if mutable, ok := ref.(object.Mutable); ok {
-				vm.globals[globalIndex] = mutable.Clone()
+			if immutable, ok := ref.(object.Immutable); ok {
+				vm.globals[globalIndex] = immutable.Clone()
 			} else {
 				vm.globals[globalIndex] = ref
 			}
 
-		case code.OpAssign:
-			ident := vm.pop()
-			val := vm.pop()
+		case code.OpAssignGlobal:
+			globalIndex := code.ReadUint16(ins[ip+1:])
+			vm.currentFrame().ip += 2
+			vm.globals[globalIndex] = vm.pop()
 
-			obj, ok := ident.(object.Mutable)
-			if !ok {
-				return fmt.Errorf("cannot assign to %s", ident.Type())
-			}
+		case code.OpAssignLocal:
+			localIndex := code.ReadUint8(ins[ip+1:])
+			vm.currentFrame().ip += 1
 
-			obj.Set(val)
+			frame := vm.currentFrame()
+			vm.stack[frame.basePointer+int(localIndex)] = vm.pop()
 
 		case code.OpGetGlobal:
 			globalIndex := code.ReadUint16(ins[ip+1:])