'Throw' and 'Catch' functions

Hi Mallikarjun,

throw and catch are a means to define your own exceptions and handle them.
A throw statement must always occur within a catch block that reflects the name of the throw.
What happens is that a throw statement makes your code jump to the end of the containing catch
block rather than returning from the entire function were the exception occured.
Consider the code below - it's a function that expects a list of positive numeric elements, then
iterates over that list end performs a division of 1 by that element:

(defun testIt (ll)
    (catch 'noList
        (unless (listp ll)
            (throw    'noList
                        (warn "%L is not a list\n" ll)))
        (foreach el ll
            (catch t
                (catch 'zeroElem
                    (when el<0
                        (throw    'zeroElem
                                    (printf "illegal negative element %L\n" el)))
                    (println 1.0/el)
                )
            )
        )
    )
)

In the outer catch block (catch 'noList...) it checkes whether the incoming parameter is a list in
the first place - if not it throws a 'noList exception, issues a warning statement and jumps to
the end of the outer catch block. So the first argument to throw() is always the name of the
exception, the second one a piece of SKILL code to be executed once the exception is thrown.
In the foreach loop we have two nested catch blocks - the inner one catches negative list elements,
the outer one catches all potential otherwise fatal exceptions. So neither negative elements nor
elements being zero (illegal division by zero) stop the foreach loop from going. So (catch t...) is
a little special because it catches everything (like illegal divisions by zero), not only user defined exceptions.

Hope that helps a little
max

Hi Max,

Thanks a lot.

The example is very helpful.

Got an understanding on 'throw' and 'catch' now.

 Mallikarjun  

Hi Max,

Thanks a lot.

The example is very helpful.

Got an understanding on 'throw' and 'catch' now.

 Mallikarjun