ON ERROR PROCname

ON ERROR OFF

Installs or removes a runtime error handler.

ON ERROR PROCname redirects errors to the named procedure.
The handler is called once for the next error; if you want it to handle further errors you must reinstall it inside the handler.
ON ERROR OFF removes any installed handler (no-op if none is installed).

When an error occurs:

Built-ins are set: ERR$ to the message, ERR to 1, ERRLINE to the line where it occurred.
Your handler PROCname is invoked.
After the handler returns, execution continues at the statement after the one that failed, unless the handler terminates the program (for example with END).

Note: Only the two forms above are supported. There is no ON ERROR GOTO, RESUME, or similar.; The handler is invoked as a one-off to prevent recursion.; If you want ongoing handling, call ON ERROR PROCname again inside your handler before it returns.; Pressing CTRL+ESC generates a break-like error that is delivered to your installed handler.; Without a handler, the program terminates.

Example: handle and continue

ON ERROR PROChandle
 
PRINT "About to raise an error"
ERROR "example failure"
PRINT "Continuing after the error"
 
END
 
DEF PROChandle
    PRINT "Handled error: "; ERR$; " at line "; ERRLINE
    ON ERROR PROChandle
ENDPROC

Example: temporarily disable handling

ON ERROR PROChandle
 
PRINT "Phase 1 (handled)"
ERROR "first failure"
 
ON ERROR OFF
PRINT "Phase 2 (unhandled)"
ERROR "this will terminate"
END
 
DEF PROChandle
    PRINT "Handler saw: "; ERR$
    ON ERROR PROChandle
ENDPROC

Behaviour

If another error occurs while calling your handler, the built-ins ERR$, ERR, and ERRLINE are set for that error, and normal error handling rules apply.
Handlers are ordinary procedures: they can call other PROCs or FNs, examine variables, and decide whether to END or to reinstall and continue.
If you are using line-numbered code, you may use your usual control flow after the handler returns, but prefer structured constructs in new code.

See also:
ERROR · DEF · PROC · Builtin Variables