Exceptions and errors

Kawa supports the exception framework and forms from R6RS and R7RS. See Exception handling for details.

Native exception handling

You can also work with native Java exceptions at a low level.

The primitive-throw procedure throws a Throwable value. It is implemented just like Java’s throw.

(primitive-throw (java.lang.IndexOutOfBoundsException "bad index"))

You can catch an exception with the try-catch syntax. For example:

(try-catch
  (do-a-bunch-of-stuff)
  (ex java.lang.Throwable
    (format #f "caught ~a~%~!" ex)
    (exit)))

A try-finally does the obvious:

(define (call-with-port port proc)
  (try-finally
   (proc port)
   (close-port port)))

Both try-catch and try-finally are expression forms that can return values, while the corresponding Java forms are statements that cannot return values.