added scribble-exn->string

original commit: a29f37f18b19e8a87f01fb4da147c8c30eee788b

collects/scribble/eval.rkt

@@ -30,6 +30,7 @@
          make-eval-factory
          close-eval
 
+         scribble-exn->string
          scribble-eval-handler)
 
 (define scribble-eval-handler
@@ -242,9 +243,7 @@
   (define (do-ev s)
     (with-handlers ([(lambda (x) (not (exn:break? x)))
                      (lambda (e)
-                       (cons (if (exn? e)
+                       (cons ((scribble-exn->string) e)
-                               (exn-message e)
-                               (format "uncaught exception: ~s" e))
                              (get-outputs)))])
       (cons (render-value (do-plain-eval ev s #t)) (get-outputs))))
   (define (do-ev/expect s expect)
@@ -264,6 +263,13 @@
               (list (list (void)) "" "")
               (do-ev/expect s expect))))))
 
+(define scribble-exn->string
+  (make-parameter
+   (λ (e)
+     (if (exn? e)
+         (exn-message e)
+         (format "uncaught exception: ~s" e)))))
+
 ;; Since we evaluate everything in an interaction before we typeset,
 ;;  copy each value to avoid side-effects.
 (define (copy-value v ht)

collects/scribblings/scribble/eval.scrbl

@@ -196,3 +196,12 @@ is supplied as the first argument to the parameter's value, and the
 second argument is the form to evaluate. The last argument is
 @racket[#t] if exceptions are being captured (to display exception
 results), @racket[#f] otherwise.}
+
+@defparam[scribble-exn->string handler (-> (or/c exn? any/c) string?)]{
+  A parameter that controls how exceptions are rendered by 
+  @racket[interaction]. Defaults to
+  @racketblock[(λ (e)
+                 (if (exn? e)
+                     (exn-message e)
+                     (format "uncaught exception: ~s" e)))]
+}