A common requirement is to be able to show as much useful context as
possible when a Scheme program hits an error. The most immediate
information about an error is the kind of error that it is – such as
“division by zero” – and any parameters that the code which signalled
the error chose explicitly to provide. This information originates with
raise-exception call (or their C code
equivalents, if the error is detected by C code) that signals the error,
and is passed automatically to the handler procedure of the innermost
applicable exception handler.
Therefore, to catch errors that occur within a chunk of Scheme code, and
to intercept basic information about those errors, you need to execute
that code inside the dynamic context of a
or the equivalent in C.
For example, to print out a message and return #f when an error occurs, you might use:
(define (catch-all thunk) (with-exception-handler (lambda (exn) (format (current-error-port) "Uncaught exception: ~s\n" exn) #f) thunk #:unwind? #t)) (catch-all (lambda () (error "Not a vegetable: tomato"))) -| Uncaught exception: #<&exception-with-kind-and-args ...> ⇒ #f
See Exceptions, for full details.