Trapping Functions and Statements


 Trapping errors while a program is running
 ------------------------------------------
 ON ERROR GOTO line
 At the module level, causes a program to branch to line, where line refers
 to either a line number or line label. Branching takes place whenever an
 error occurs during execution.

 ON LOCAL ERROR
 GOTO line
 At the procedure level, causes a program to branch to line, where line
 refers to either a line number or line label. Branching takes place whenever
 an error occurs during execution of the procedure.

 ON ERROR RESUME NEXT
 At the module level, causes an implicit error trap, then RESUME NEXT.
 Returns the error number in ERR.

 ON LOCAL ERROR RESUME NEXT
 At the procedure level, causes an implicit error trap, then RESUME NEXT.
 Returns the error number in ERR.

 RESUME
 Returns control to the program after executing an error-handling routine.
 The program resumes at either the statement causing the error (RESUME), the
 statement after the one causing the error (RESUME NEXT), or the line
 identified by line (RESUME line)


 Getting error-status data
 -------------------------
 ERR Function
 Returns the code for an error that occurs at run time.

 ERR Statement
 Sets ERR to an integer.

 ERL
 Returns the number of the line on which an error occurred (if program has
 line numbers).

 ERDEV
 Returns a device-specific error code for the last device (such as a printer)
 for which the system detected an error.

 ERDEV$
 Returns the name of the last device for which the system detected an error.


 Defining your own error codes
 -----------------------------
 ERROR
 Simulates the occurrence of a BASIC error; can also be used to define an
 error not trapped by BASIC.


 Trapping events while a program is running
 ------------------------------------------
 ON event GOSUB line
 Causes a branch to the subroutine starting with line, where line refers
 either to a line number or line label, whenever event occurs during
 execution.

 event ON
 Enables trapping of event.

 event OFF
 Disables trapping of event.

 event STOP
 Suspends trapping of event.

 EVENT ON
 Enables event trapping.

 EVENT OFF
 Disables event trapping.

 RETURN
 Returns control to the program after executing an event-handling subroutine.
 The program resumes at either the statement immediately following the
 statement that called the subroutine (RETURN), or the line that is
 identified by line (RETURN line).