Enable truncation of exception output

[Daniel Llorens <daniel.llorens@bluewin.ch>]

(I've messed up with git send-email, the patch with this email is
at http://lists.gnu.org/archive/html/guile-devel/2017-02/msg00008.html).

Some objects have large printed representations and trying to print
them can kill the terminal. This is a serious problem when handling
e.g. big arrays.

For repl values there's (repl-default-option-set! 'print (lambda (val)
...)) which lets you override the default printing routine. But for
exceptions there's no equivalent. The default exception printers are
in (ice-9 boot-9) and are difficult to override. Besides, the format
of the exception output is often chosen by arguments to scm-error and
not by those printers.

The patch to master (ice-9 boot-9) lets you override the (format) used
internally by the exception printers. Then I rewrite the format string
in user code, see below. It's a bit hacky, maybe someone has a better
solution.

See also the other patch I've posted to the list to support arrays in
(truncated-print). The patches are independent, but they make each
other useful so to speak.

Regards

        Daniel

(In the code below for .guile, replace [AT] by the at symbol).

; Truncate output on exceptions. Requires exception-format in ice-9/boot.scm.
; FIXME doesn't handle e.g. "x~~~s" -> "x~~~[AT]y"
(define (rewrite-fmt fmt)
  (let loop ((f "") (b 0))
    (let ((next (string-contains-ci fmt "~s" b)))
      (if next
        (loop
         (if (or (zero? next) (not (char=? #\~ (string-ref fmt (- next 1)))))
           (string-append f (substring fmt 0 next) "~[AT]y")
           fmt)
         (+ b next 2))
        (string-append f (substring fmt b))))))

(when (defined? 'exception-format)
  (set! exception-format
        (lambda (port fmt . args)
          (apply format port (rewrite-fmt fmt) args))))