Exception-like error handling in Software AG's Natural

Error handling in Software AG’s Natural can be done in a way that resembles Exception handling in object-oriented languages like Java.

throw

Instead of throwing an Exception, you raise an error simply by assigning a value to the system variable *ERROR-NR. As soon as a statement like the following is executed, the current program flow is interrupted and the nearest ON ERROR block is executed.

*ERROR-NR := 1234

In fact, we use exactly this feature for raising assertion errors in NatUnit.

catch

You can handle a Natural error in an ON ERROR block anywhere inside your code. Just like an Exception travels up through the call stack to get caught in the nearest try-catch block, a Natural error is handled in the nearest ON ERROR block.

Here’s an example of an ON ERROR block that exits the current module and marks the error as handled:

ON ERROR
    /* do something about it */
    ESCAPE MODULE
END-ERROR

catch (SpecificException e)

You can only define a single ON ERROR block in each Natural module. So if you need to handle specific errors in a different way, you need to have some kind of distinction logic like this:

ON ERROR
    IF *ERROR-NR EQ 1234
        /* do something about it */
        ESCAPE MODULE
    END-IF
END-ERROR

Or if you need to distinguish between multiple errors:

ON ERROR
    DECIDE ON FIRST VALUE OF *ERROR-NR
        VALUE 1234
            /* do something about it */
            ESCAPE MODULE
        VALUE 1235
            /* do something about it */
            ESCAPE MODULE
        NONE IGNORE
    END-DECIDE
END-ERROR

re-throw

If you can’t handle the error in an ON ERROR block, but you want to log it or do something else with it before letting the next ON ERROR block handle it, you don’t need to do anything at all, because that’s the default behaviour.

However, if you exit the ON ERROR block with any statement from the following list, the error is marked as handled and the normal control flow (in the calling module of the module containing the ON ERROR block) is continued. So be sure not to exit the block with any of these statements.

Exiting from an ON ERROR Block:
An ON ERROR block may be exited by using a FETCH, STOP, TERMINATE, RETRY, ESCAPE ROUTINE or ESCAPE MODULE statement. If the block is not exited using one of these statements, standard error message processing is performed and program execution is terminated.

Here’s an example of such a “re-throw”:

ON ERROR
    IF *ERROR-NR EQ 1234
        /* log the error */
        /* DON'T exit with `FETCH`, `STOP`, `TERMINATE`, `RETRY`, `ESCAPE ROUTINE` or `ESCAPE MODULE` */
    END-IF
END-ERROR

Checking which error occurred

Even if you “handle” the Natural error in an ON ERROR block, e.g. by using ESCAPE MODULE, the system variable *ERROR-NR isn’t reset to 0. You need to do that yourself, if you need to. If you don’t, the variable can be used in the calling module to check whether an error (that was handled) occured. By the way, the system variable *ERROR-LINE contains the line number of the statement that raised the error.

CALLER

CALLNAT 'CALLEE'
IF *ERROR-NR NE 0
    WRITE 'Error' *ERROR-NR 'occurred in line' *ERROR-LINE 'while calling CALLEE'
    /* prints: "Error     1234 occurred while calling CALLEE" */
END-IF
END

CALLEE

*ERROR-NR := 1234
ON ERROR
    ESCAPE MODULE
END-ERROR
END

If you don’t want any caller to know that an error occurred, simply reset *ERROR-NR:

ON ERROR
    RESET *ERROR-NR
    ESCAPE MODULE
END-ERROR

Global error handler (like a try-catch in main())

You can define a global error handler by setting the system variable *ERROR-TA to the name of a Natural module. In case of an error, Natural automatically calls this module (which has to be a program) and puts information about the error on the stack. The system variables *ERROR-NR and *ERROR-LINE will be reset at this point, so the error handler has to read the information from the stack with INPUT.

CALLER

*ERROR-TA := 'HANDLER'
CALLNAT 'CALLEE'
END

CALLEE

*ERROR-NR := 1234
END

HANDLER

DEFINE DATA LOCAL
1 #ERROR-NR           (N5)
1 #LINE               (N4)
1 #STATUS-CODE        (A1)
1 #PROGRAM            (A8)
1 #LEVEL              (N2)
1 #LEVELI4            (I4)
1 #POSITION-IN-LINE   (N3)
1 #LENGTH-OF-ITEM     (N3)
END-DEFINE

/* read error information from stack */
INPUT #ERROR-NR #LINE #STATUS-CODE #PROGRAM #LEVEL #LEVELI4

WRITE #ERROR-NR #LINE #STATUS-CODE #PROGRAM #LEVEL #LEVELI4 #STATUS-CODE
WRITE *ERROR-NR *ERROR-LINE

END

Output:

1234    10 O CALLEE     2           0 O
     0     0

For more information about the error information on the stack take a look at the section Using an Error Transaction Program in the Natural documentation.

Getting more information about errors programmatically

If you need to find out more about the current error, e.g. in your ON ERROR block, there are quite a few User Exits that deal with errors:

USR0040N: Get type of last error
USR1016N: Get error level for error in nested copycodes
USR2001N: Get information on last error
USR2006N: Get information from error message collector
USR2007N: Get or set data for RPC default server
USR2010N: Get error information on last database call
USR2026N: Get TECH information
USR2030N: Get dynamic error message parts from the last error
USR3320N: Find user short error message (including steplibs search)
USR4214N: Get program level information

Exception-like error handling in Software AG’s Natural