6 files changed, 18 insertions, 21 deletions

diff --git a/Makefile b/Makefile
index 0664a27..ea9d9a9 100644
--- a/Makefile
+++ b/Makefile
@@ -10,3 +10,6 @@ clean:
 
 db:
 	gdb main
+
+cdb:
+	cgdb main
diff --git a/TODO.rst b/TODO.rst
index 99dccee..4650ec7 100644
--- a/TODO.rst
+++ b/TODO.rst
@@ -1,3 +1,2 @@
-- Better judgement of continuation exit
 - Better inherit relationship between Cons and EvalObj
 - Replace many `dynamic_cast` with `static_cast`
diff --git a/builtin.cpp b/builtin.cpp
index 19516bd..75f470e 100644
--- a/builtin.cpp
+++ b/builtin.cpp
@@ -195,8 +195,8 @@ Cons *SpecialOptSet::call(ArgList *args, Environment * &envt,
     Cons *ret_addr = dynamic_cast<RetAddr*>(*top_ptr)->addr;
     Cons *pc = dynamic_cast<Cons*>(ret_addr->car);
     SymObj *id = dynamic_cast<SymObj*>(pc->cdr->car);
-    if (envt->has_obj(id))
-        envt->add_binding(id, args->cdr->car);
+    bool flag = envt->add_binding(id, args->cdr->car, false);
+    // TODO: throw an exc "unbound variable"
     *top_ptr++ = new UnspecObj();
     return ret_addr->next;
 }
diff --git a/eval.cpp b/eval.cpp
index 07ea00e..44569c1 100644
--- a/eval.cpp
+++ b/eval.cpp
@@ -22,7 +22,7 @@ void Evaluator::add_builtin_routines() {
     ADD_ENTRY("if", new SpecialOptIf());
     ADD_ENTRY("lambda", new SpecialOptLambda());
     ADD_ENTRY("define", new SpecialOptDefine());
-    ADD_ENTRY("set", new SpecialOptSet());
+    ADD_ENTRY("set!", new SpecialOptSet());
 }
 
 Evaluator::Evaluator() {
diff --git a/model.cpp b/model.cpp
index ee578f2..95f70c6 100644
--- a/model.cpp
+++ b/model.cpp
@@ -131,8 +131,11 @@ string BuiltinProcObj::_debug_repr() { return ext_repr(); }
 
 Environment::Environment(Environment *_prev_envt) : prev_envt(_prev_envt) {}
 
-void Environment::add_binding(SymObj *sym_obj, EvalObj *eval_obj) {
+bool Environment::add_binding(SymObj *sym_obj, EvalObj *eval_obj, bool def) {
+    bool has_key = binding.count(sym_obj->val);
+    if (!def && !has_key) return false;
     binding[sym_obj->val] = eval_obj;
+    return true;
 }
 
 EvalObj *Environment::get_obj(EvalObj *obj) {
@@ -145,15 +148,7 @@ EvalObj *Environment::get_obj(EvalObj *obj) {
         bool has_key = ptr->binding.count(name);
         if (has_key) return ptr->binding[name];
     }
-    //TODO: exc key not found
-}
-
-bool Environment::has_obj(SymObj *sym_obj) {
-    string name(sym_obj->val);
-    for (Environment *ptr = this; ptr; ptr = ptr->prev_envt)
-        if (ptr->binding.count(name))
-            return true;
-    return false;
+    return NULL;                    // Object not found
 }
 
 Continuation::Continuation(Environment *_envt, Cons *_pc, 
diff --git a/model.h b/model.h
index 13a5af2..ee0e8ae 100644
--- a/model.h
+++ b/model.h
@@ -253,19 +253,19 @@ class Environment {
     public:
         /** Create an runtime environment
          * @param prev_envt the outer environment
-         * */
+         */
         Environment(Environment *prev_envt);
-        /** Add a binding entry which binds sym_obj to eval_obj */
-        void add_binding(SymObj *sym_obj, EvalObj *eval_obj);  
+        /** Add a binding entry which binds sym_obj to eval_obj 
+         * @param def true to force the assignment
+         * @return when def is set to false, this return value is true iff. the
+         * assignment carried out successfully
+         */
+        bool add_binding(SymObj *sym_obj, EvalObj *eval_obj, bool def = true);  
         /** Extract the corresponding EvalObj if obj is a SymObj, or just
          * simply return obj as it is 
          * @param obj the object as request
          * */
         EvalObj *get_obj(EvalObj *obj);
-        /** Check if the desired obj exists
-         * @return true for yes
-         */
-        bool has_obj(SymObj *);
 };
 
 /** @class Continuation