WHENEVER instruction

Use the WHENEVER instruction to define how exceptions must be handled for the rest of the module.

Syntax

WHENEVER exception-class
 exception-action

where exception-class is one of:

{ [ANY] ERROR
| [ANY]  SQLERROR
|  NOT FOUND
|  WARNING
}

and exception-action is one of:

{ CONTINUE
| STOP
| CALL [module.]function 
| RAISE
| GOTO label
}

function can be any function name defined in the program.
module is the name of a module imported with IMPORT FGL.
label must be a label defined in the current program block (main, function or report routine).

Usage

The WHENEVER instruction defines the exception handling by associating an exception class with an exception action.

The WHENEVER instruction is similar to a C preprocessor macro: Its effect is local to the module and defines the error handling for the rest of the module, unless a new WHENEVER instruction is encountered by the compiler, or a TRY/CATCH block is used. A WHENEVER instruction defines only the exception handling for that module and therefore does not affect callers. Use the instruction WHENEVER ERROR RAISE to propagate exceptions occuring in the module.

If no WHENEVER instruction is used, the default is WHENEVER ERROR STOP. Stopping the program in case of error is the recommended default. However, this default does not catch expression errors like type conversion errors. Consider using the fglrun.mapAnyErrorToError FGLPROFILE entry, to catch conversion errors. For more details, see Default exception handling.

This code example shows a typical WHENEVER instruction usage:

WHENEVER ERROR CONTINUE
DROP TABLE mytable -- SQL error will be ignored 
CREATE TABLE mytable ( k INT, c VARCHAR(20) )
WHENEVER ERROR STOP
IF sqlca.sqlcode != 0 THEN
   ERROR "Could not create the table..."
END IF

Unlike the TRY/CATCH block, a WHENEVER ERROR CONTINUE exception handler does not stop the evaluation of an expression. For example, in the expression [(Pi() / 0) + Pi()] (where Pi() is a user function returning the number Pi), the function is called twice, even if the division by zero produces error -1202. This behavior exists to be compatible with Informix 4GL legacy code.

If an exception is thrown by a method or function that can be used in an expression (an typically returns an object), this exception cannot be handled with WHENEVER ERROR CALL / CONTINUE. In such case, only a TRY/CATCH block can be used to handle exceptions.

Exception classes ERROR and SQLERROR are synonyms: In the previous example it is also possible to use WHENEVER SQLERROR instead of WHENEVER ERROR.

Actions for classes ERROR, WARNING and NOT FOUND can be set independently:

WHENEVER ERROR STOP
WHENEVER WARNING CONTINUE
WHENEVER NOT FOUND GOTO not_found_handler 
...

The effect of the WHENEVER instruction is local to the current module, and applies to all source lines following that instruction (cross-function definitions). In the next example, module1.4gl uses a WHENEVER instruction that takes effect in the second function, but does not affect the calling module main.4gl:

$ head module1.4gl main.4gl
==> module1.4gl <==
FUNCTION function1()
    WHENEVER ANY ERROR CONTINUE -- applies to subsequent lines
END FUNCTION

FUNCTION function2()
    DEFINE x INTEGER
    LET x = "aaa"
    DISPLAY "function2: x = ", x, " status = ", status
END FUNCTION

==> main.4gl <==
IMPORT FGL module1
MAIN
    DEFINE x INTEGER
    WHENEVER ANY ERROR STOP
    CALL module1.function2()
    LET x = "aaa"
END MAIN

$ fglcomp main.4gl && fglrun main.42m
function2: x =             status =       -1213
Program stopped at 'main.4gl', line number 6.
FORMS statement error number -1213.
A character to numeric conversion process failed.

In the above example, the error -1213 in line 6 of main.4gl is expected, because the WHENEVER instruction of the main.4gl module applies.

When using the WHENEVER ... CALL function instruction, the program flow will go to the specified function and return to the code block where the exception occurred:

MAIN
    DEFINE x INTEGER
    WHENEVER ANY ERROR CALL error_handler
    -- WHENEVER handler takes effect
    LET x = 1/0
    DISPLAY "Back in MAIN..."
END MAIN

FUNCTION error_handler()
    DISPLAY "error_handler: ", status
END FUNCTION

-- output:

error_handler:       -1202
Back in MAIN...

In a WHENEVER ... CALL instruction, do not specify parentheses after the function name.

A TRY/CATCH block takes precedence over the last WHENEVER instruction, see the following example:

MAIN
    DEFINE x INTEGER
    WHENEVER ANY ERROR CONTINUE
    -- WHENEVER handler takes effect
    LET x = 1/0
    DISPLAY "WHENEVER: ", status
    -- WHENEVER handler is hidden by TRY/CATCH block
    TRY
        LET x = 1/0
    CATCH
        DISPLAY "CATCH   : ", status
    END TRY
    -- WHENEVER handler takes again effect
    CALL func()
END MAIN

FUNCTION func()
    DEFINE x INTEGER
    LET x = 1/0
    DISPLAY "WHENEVER: ", status
END FUNCTION

-- Output:

WHENEVER:       -1202
CATCH   :       -1202
WHENEVER:       -1202

The RAISE option can be used to propagate exceptions to the caller, which typically traps the error in a TRY/CATCH block:

-- main.4gl
IMPORT FGL myutils
MAIN
    TRY
       -- Pass a NULL form name to get error -1110
       CALL myutils.open_form(NULL)
    CATCH
       DISPLAY "Error: ", status
    END TRY
END MAIN

-- myutils.4gl
FUNCTION open_form(fn)
    DEFINE fn STRING
    WHENEVER ERROR RAISE -- Propagate exceptions to caller
    OPEN FORM f1 FROM fn
END FUNCTION

Note that WHENEVER [ANY] ERROR RAISE is not supported in a REPORT routine.