IDL users may find that all their variables have seemingly disappeared after an error occurs inside a procedure or function. The misunderstood subtlety is that after the error occurs, IDL's context is inside the called procedure, not in the main level. All variables in procedures and functions, with the exception of parameters and common variables, are local in scope. Typing RETURN or RETALL will make the lost variables reappear.
RETALL is best suited for use when an error is detected in a procedure and it is desired to return immediately to the main program level despite nested procedure calls. RETALL issues RETURN commands until the main program level is reached.
The HELP command can be used to see the current call stack (i.e., which program unit IDL is in and which program unit called it). For more information, see HELP.