Basic Characteristics RAPID

Basic Characteristics RAPID Error Recovery
8 Error Recovery
An execution error is an abnormal situation, related to the execution of a specific piece
of a program. An error makes further execution impossible (or at least hazardous).
“Overflow” and “division by zero” are examples of errors. Errors are identified by their
unique error number and are always recognized by the robot. The occurrence of an
error causes suspension of the normal program execution and the control is passed to
an error handler. The concept of error handlers makes it possible to respond to and,
possibly, recover from errors that arise during program execution. If further execution
is not possible, the error handler can at least assure that the program is given a graceful
abortion.
8.1 Error handlers
Any routine may include an error handler. The error handler is really a part of the routine,
and the scope of any routine data also comprises the error handler of the routine.
If an error occurs during the execution of the routine, control is transferred to its error
handler.
Example: FUNC num safediv( num x, num y)
RETURN x / y;
ERROR
IF ERRNO = ERR_DIVZERO THEN
TPWrite "The number cannot be equal 0";
RETURN x;
ENDIF
!All other errors are propagated RAISE;
ENDFUNC
The system variable ERRNO contains the error number of the (most recent) error and
can be used by the error handler to identify that error. After any necessary actions have
been taken, the error handler can:
- Resume execution, starting with the instruction in which the error occurred.
This is done using the RETRY instruction. If this instruction causes the same
error again, up to five error recoveries will take place; after that execution will
stop.
- Resume execution, starting with the instruction following the instruction in
which the error occurred. This is done using the TRYNEXT instruction.
- Return control to the caller of the routine using the RETURN instruction. If the
routine is a function, the RETURN instruction must specify an appropriate
return value.
- Propagate the error to the caller of the routine using the RAISE instruction.
An error handler should normally end with RAISE or EXIT, in order to take care of
errors that cannot be handled.
In a chain of routine calls, each routine may have its own error handler. If an error
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