reuse zero arg data constructors + correct semantics with forwarding

interpreter/code.c

@@ -274,8 +274,16 @@ void exec(Code* expr, int frame_ptr, int root_frame_ptr)
         }
         case CT_THUNK:
         {
-            // TODO: set forward
-            set_return(root_frame_ptr, &((ThunkEntry*) expr)->thunk);
+            Thunk* target = get_dst(root_frame_ptr);
+            Thunk* thunk = &((ThunkEntry*) expr)->thunk;
+            
+            if(target != NULL)
+            {
+                target->desc = (Desc*) __FORWARD_PTR__;
+                target->_forward_ptr = thunk;                
+            }
+            
+            set_return(root_frame_ptr, thunk);
             destroy_stack_frame(root_frame_ptr);
             return;               
         }

interpreter/parse.c

@@ -312,15 +312,13 @@ Code* parseApp(char **ptr, bool dynamic) {
         Desc* desc = parseFunName(ptr); // can fail 
         if(desc != NULL) desc = get_slice(desc, nrArgs);
 
-        /*
-        if(desc->type == FT_ADT || desc->type == FT_SLICE)
+        if(desc != NULL && (desc->type == FT_ADT || desc->type == FT_SLICE))
         {
             struct ThunkEntry* entry = (ThunkEntry*) alloc_code(sizeof (ThunkEntry));
             entry->base.type = CT_THUNK;
             entry->thunk.desc = desc;
             return (Code*) entry;
         }
-        */
         
         entry = (AppEntry*) alloc_code(sizeof (AppEntry));        
         entry->f = desc;