[racket] with-handlers and exception structure

From: Greg Hendershott (greghendershott at gmail.com)
Date: Mon Sep 3 17:44:05 EDT 2012

To add to what Matthew and Danny said, in addition to using
`exn-message' you can show a "stack trace" if you wish. For example:

(define (exn->string exn)
  (string-append "Exception: " (exn-message exn)))

(define (exn+stack->string exn)
  (string-append (exn->string exn) "\n"
                 (stack-trace-string exn)))

(define (stack-trace-string exn)
   (map (lambda (x)
          (format "'~a' ~a ~a"
                  (if (car x) (car x) "")
                  (if (cdr x) (srcloc-source (cdr x)) "")
                  (if (cdr x) (srcloc-line (cdr x)) "")))
        (continuation-mark-set->context (exn-continuation-marks exn)))

;; Example usage:
(with-handlers ([exn:fail? (compose1 displayln exn+stack->string)])
  (/ 1 0))

On Mon, Sep 3, 2012 at 10:56 AM, Danny Yoo <dyoo at hashcollision.org> wrote:
> On Monday, September 3, 2012, Gregory Woodhouse wrote:
>> Dumb question: If an exception is handled by (lambda (e) ... ) is it
>> possible to recover exception details such as any message that may have been
>> used in a raise or related statement?
> Hi Gregory,
> The value raised by exceptions should be structured; in most cases, the
> exception value will be an instance of the exn:fail struct.  Take a look at
> http://docs.racket-lang.org/reference/exns.html#(part._.Built-in_.Exception_.Types)
> for the definition of the exn struct.  The exn-message function, in
> particular, should let you select out the string message associated to the
> raised exception.
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