diff --git a/src/structs.h b/src/structs.h
index dd80215..b4bafa5 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1583,10 +1583,11 @@
 
 // values used for "uf_def_status"
 typedef enum {
-    UF_NOT_COMPILED,
-    UF_TO_BE_COMPILED,
-    UF_COMPILING,
-    UF_COMPILED
+    UF_NOT_COMPILED,	    // executed with interpreter
+    UF_TO_BE_COMPILED,	    // to be compiled before execution
+    UF_COMPILING,	    // in compile_def_function()
+    UF_COMPILED,	    // successfully compiled
+    UF_COMPILE_ERROR	    // compilation error, cannot execute
 } def_status_T;
 
 /*