Error Handling

Overview

Within Carapace, all error handling is done by throwing and catching errors. If a function or method wishes to raise an error, it constructs an Error object and then throws it. The script evaluation then unwinds until it gets to a point where this particular type of error is caught.

The common alternative for error handling is for functions to return an error and for the caller to deal with this in the correct way. You can of course do this with Carapace, since you can return an Error object in the same way as you can return any other object from a function. The advantage of the throw/catch style is that the caller does not need to do the error checking -- the called function either is successful and returns the appropriate value, or it is throws an error. In this error case, the caller never sees the return of the function.

Wherever an error is caught, Carapace requires there is a local variable to be in existence called err of type Error. This variable gets instantiated with the caught error.

The error-handling functions are as follows:

The example shows the use of try, throw and catch.

try

Show the intention to catch errors generated in the following expression. The example shows the use of try, throw and catch.

Arguments:

Return type: the empty List () is returned.

catch

Catch a given class of errors. Each Error object has an Integer class. A catch-statement either catches just a single class (eg. class 5) or all errors as follows:

    (catch -1

        (printAux "The error caught was " err.string)
    )

Arguments:

Return type: the empty List () is returned.

throw

Throw an error - so it can be caught elsewhere. The effect of throwing an error is to unwind the call stack back to the place where the error is caught. The effect is to jump out of the current script location to the first appropriate catch statement which may well be in a different function.

Arguments:

Return type: the empty List () is returned.