Start working on functions

src/types.h

@@ -12,7 +12,7 @@ typedef enum GroundInstType {
 } GroundInstType;
 
 typedef enum GroundValueType {
-    INT, DOUBLE, STRING, CHAR, BOOL, LIST, CUSTOM
+    INT, DOUBLE, STRING, CHAR, BOOL, LIST, CUSTOM, NONE
 } GroundValueType;
 
 typedef enum GroundArgType {
@@ -159,4 +159,4 @@ ListAccess getListAt(List* list, int idx);
 // Sets an item in list (list) at index (idx) to GroundValue (value).
 ListAccessStatus setListAt(List* list, int idx, GroundValue value);
 
-#endif
+#endif