Error Handling

This section describes how errors are handled in HDevelopEVO programs. When an error occurs, the default behavior of HDevelopEVO is to stop the program execution and display an error message. While this is certainly beneficial at the time the program is developed, it is usually not desired when the program is actually deployed. A finished program should react to errors itself. This is of particular importance if the program interacts with the user.

There are two approaches to error handling in HDevelopEVO:

Tracking the return value (error code) of operator calls: This method handles errors inside the procedure in which they occur.

Exception handling: This method allows errors to work their way up in the call stack until they are finally dealt with.

Tracking the Return Value of Operator Calls

The operator dev_set_check specifies if error message boxes are displayed at all.

To turn message boxes off, use

dev_set_check('~give_error')

HDevelopEVO will then ignore any errors in the program. Consequently, the programmer has to take care of the error handling.

Every operator call provides a return value (or error code), which signals success or failure of its execution. This error code can be accessed through a designated error variable:

dev_error_var(ErrorCode, 1)

This operator call instantiates the variable ErrorCode. It stores the error code of the last executed operator. Using this error code, the program can make its further flow dependent on the success of an operation.

...
if (ErrorCode != H_MSG_TRUE)
      * react to error
endif
* continue with program
...

The error message related to a given error code can be obtained with the operator get_error_text. This is useful when reporting errors back to the user of the program.

To handle the error in a calling procedure, add an appropriate output control variable to the interface of each participating procedure, or define the error variable as a global variable (see the section about variable scopes).

global tuple ErrorCode
dev_error_var(ErrorCode, 1)
...

Exception Handling

HDevelopEVO supports dynamic exception handling, which is comparable to the exception handling in C++ and C#.

A block of program lines is watched for run-time errors. If an error occurs, an exception is raised and an associated exception handler is called. An exception handler is just another block of program lines, which is invisible to the program flow unless an error occurs. The exception handler can directly act on the error, or it can pass the associated information (the exception) on to a parent exception handler. This is also known as rethrowing an exception.

In contrast to the tracking method described in the previous section, the exception handling requires HDevelopEVO to be set up to stop on errors. This is the default behavior. It can also be turned on explicitly:

dev_set_check('give_error')

An HDevelopEVO exception is a tuple containing data related to a specific error. It always contains the error code as the first item. The operator dev_get_exception_data provides access to the elements of an exception tuple.

HDevelopEVO exception handling is applied in the following way:

...
try
  * start block of watched program lines
  ...
catch(Exception)
  * get error code
  ErrorCode := Exception[0]
  * react to error
endtry
* program continues normally
...