Re: [Chicken-users] catching exceptions

[F. Wittenberger]

Am Dienstag, den 29.07.2008, 03:21 -0700 schrieb Elf:
> #;1> (use srfi-34)
> ; loading /usr/lib/chicken/3/srfi-34.scm ...
> ; loading /usr/lib/chicken/3/syntax-case.so ...
> ; loading /usr/lib/chicken/3/syntax-case-chicken-macros.scm ...
> ; loading library srfi-18 ...
> #;2> (print (guard (ex (else 'success)) (with-input-from-string ")" read)))
> success
> #;3> (condition-case (with-input-from-string ")" read) (val () "no prob"))
> "no prob"
> #;4> (condition-case (call-with-input-string ")" read) (val () "no prob"))
> "no prob"
> #;5>
> 
> wasnt srfi-34.

True.  Was the old version of srfi-34, which I could not get replaced
because of some network errors and probably some confusion on my part.

Those - and my workaround - revealed some confusion wrt. chicken related
resources: I fetched manually
http://www.call-with-current-continuation.org/eggs/srfi-34.egg
This is still version 0.1 at this time.

chicken-setup - which refused to download last time (yesterday I think)
- now finds version 0.2 as changed by Elf recently.

However version 0.2 seems non-functional too.  Though it catches
exceptions, which the 0.1 version prevented to catch, it failes on
several tests from the SRFi-34 examples.

(print (guard (ex (else 'success)) (call-with-input-string ")" read)))
=> success

OK

1st example from srfi-34:

(call-with-current-continuation
 (lambda (k)
   (with-exception-handler (lambda (x)
                             (display "condition: ")
                             (write x)
                             (newline)
                             (k 'exception))
     (lambda ()
       (+ 1 (raise 'an-error))))))
PRINTS: condition: an-error
=> exception

chicken prints: condition: #<condition: (exn)>

5th example:

(call-with-current-continuation
 (lambda (k)
   (with-exception-handler (lambda (x)
                             (display "reraised ") (write x) (newline)
                             (k 'zero))
     (lambda ()
       (guard (condition
                ((positive? condition) 'positive)
                ((negative? condition) 'negative))
        (raise -1))))))
=> positive

chicken: reraised #<condition: (exn type)>

Ergo: raise from SRFI-18 - i.e., ##sys#signal from library.scm is not
compatible with SRFI-34 (btw: this is obvious from the source) and needs
to be replaced.

But I'm not entirely clear what the problem actually is.  To me it seems
to be related to my disability to change some of chickens variables as I
want to and apparently the chicken library.scm happens to have the same
problem.

Let's see:

(condition-case
 (with-exception-handler
  (lambda (ex) (print "Toplevelhandler: " ex))
  (lambda ()
    ((current-exception-handler) 1)))
 (var () (print "Backstop: " var)))
prints: Toplevelhandler: 1

(condition-case
 (with-exception-handler
  (lambda (ex) (print "Toplevelhandler: " ex))
  (lambda ()
    (raise 1)))
 (var () (print "Backstop: " var)))

Loops printing: Toplevelhandler: #<condition: (exn)>

Assembling the relevant definitions from chicken's source we find:

srfi-18.scm:

(define raise ##sys#signal)

library.scm:

(define (##sys#signal x)
  (##sys#current-exception-handler x) )

(define (with-exception-handler handler thunk)
  (let ([oldh ##sys#current-exception-handler])
    (##sys#dynamic-wind 
        (lambda () (set! ##sys#current-exception-handler handler))
        thunk
        (lambda () (set! ##sys#current-exception-handler oldh)) ) ) )

(define (current-exception-handler) ##sys#current-exception-handler)

This is where I can't follow.  To my understanding (raise 1) and
((current-exception-handler) 1) are supposed to be equivalent.
Apparently I'm wrong on that one, since they are not.

Judging from the source, I tried:

(with-exception-handler
  (lambda (ex) (print "Toplevelhandler: " ex))
  (lambda ()
    (raise (make-property-condition 'testcondition))))

since I'd expect that to go unchanged though the
##sys#current-exception-handler but it doesn't help and I don#t know
where my testcondition get's converted into #<condition: (exn)> - which
is what happens.

I wan't be able to fix that one, since I do not understand what's going
on.
Let's use

(define (raise obj)
  ((current-exception-handler) obj))

for the time being and continue to test for srfi-34 compliance.  Example
#7:

(with-exception-handler
 (lambda (ex) (print "Toplevelhandler:" ex))
 (lambda ()
   (call-with-current-continuation
    (lambda (k)
      (with-exception-handler
       (lambda (x)
         (display "reraised ") (write x) (newline)
         (k 'zero))
       (lambda ()
         (guard (condition
                 ((positive? condition) 'positive)
                 ((negative? condition) 'negative))
                (raise 0))))))))
PRINTS: reraised 0
=> zero

chicken: does not print anything, hangs in a tight loop (since the
exceptionhandler in effect while evaluating the guard handler is the
same as during guard's body, which is wrong.

Summary: I'm afraid srfi-34.egg is not yet ready for the prime time.

I appreciate either a fixed version or *any* help and pointers to get me
going to fix it.

best regards

/Jörg