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This is an egg for CHICKEN 4, the unsupported old release. You're almost certainly looking for the CHICKEN 5 version of this egg, if it exists.

If it does not exist, there may be equivalent functionality provided by another egg; have a look at the egg index. Otherwise, please consider porting this egg to the current version of CHICKEN.

Chicken's condition system is versatile and flexible, but apart from the two high level exception handlers, handle-exceptions and condition-case, not very user friendly. In particular the low-level exception handler, with-exception-handler, must be used with extreme care to avoid non-terminating loops. Moreover creating and inspecting condition-objects is cumbersome.

So this library introduces some procedures and macros to facilitate matters. Exceptions are introduced as special conditions, to be more precise, [composite] conditions of kind exn, which all have properties message, location and arguments, which can easyly be accessed by procedures of that very name. They are constructed by separating the general property, message, and the general kinds, from the special properties, location and arguments, the latter being considered as parameters of an exception.

In other words, one can define a global exception-variable of a special kind, e.g.

(define assert-exn (make-exception "assertion violated" 'assert))

which can then be used within the definition of a procedure, foo say, as follows

(raise (assert-exn 'foo 'xpr))

Here, raise is a version of abort with improved error messages.

The exception raised can now be handled in the usual way, preferably

(condition-case (foo arg) ((exn assert) ...))

(handle-exceptions exn (if ((exception-of? 'assert) exn) ...) (foo arg))

but other handlers are provided as well, in particular, guard of R6RS and R7RS, and with-exn-handler, which has the same syntax as Chicken's with-exception-handler, but avoids its subtleties. For example, the follwing code does what you expect, it returns 0.

(with-exn-handler (lambda (exn) 0) (lambda () (car '())))

without looping forewer, what with-exception-handler would have done.

Another useful macro is assert* which -- contrary to assert -- allows many expressions to be tested and demands a location argument, which is used to produce a meaningful error-message.


(use simple-exceptions)

((named-lambda ! (n)
   (if (zero? n)
     1
     (* n (! (- n 1)))))
 5) ; -> 120
(<< 5 integer? odd? (named-lambda 5<= (x) (<= 5 x))) ; -> 5
(condition-case
       (>> 5 integer? even?)
       ((exn results) #f)) ; -> #f 
(<< ((lambda () #f)) boolean?) ; -> #f

(define foo-exn (make-exception "foo-msg"))
(define bar-exn (make-exception "bar-msg" 'bar))
(exception? (foo-exn 'nowhere))

(bar-exn 'nowhere)
(exception? (bar-exn 'nowhere)) ; -> #t

((exception-of? 'bar) (bar-exn 'nowhere)) ; -> #t
((exception-of? 'bar) (foo-exn 'nowhere)) ; -> #f

(arguments ((make-exception "msg" 'baz) 'nowhere "bar"))
  ; ,> (list "bar")

((exception-of? 'key)
 ((make-exception "msg" 'key) 'nowhere))
  ; -> #t

(define list-empty-exn
  (make-exception "argument list empty" 'list-empty))

(define (try-car lst)
  (if (null? lst)
    (raise (list-empty-exn 'try-car lst))
    (car lst)))

;; exception handler procedure
(with-exn-handler
  (lambda (exn)
    (if ((exception-of? 'list-empty) exn)
      #f
      #t))
  (lambda () (try-car '())))
  ; -> #f

(with-exn-handler (lambda (e) 0) (lambda () (/ 5 0))) ; -> 0

(guard
  (exn (((exception-of? 'foo) exn)
        (location exn))
       ((exception? exn)
        (message exn))
       (else
         (arguments exn)))
  (raise ((make-exception "msg" 'foo)
           'location-unknown)))
  ; -> 'location-unknown)

Copyright (c) 2014-2017, Juergen Lorenz
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.

Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
Neither the name of the author nor the names of its contributors may be
used to endorse or promote products derived from this software without
specific prior written permission. 
  
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Version History

0.6: bug in <<< and >>> fixed
0.5: <<< and >>> added, pre- and post-exceptions renamed argument- and result-exn, some aliases added
0.4: named-lambda added
0.3.1: << and >> now procedures
0.3: <<, >>, pre-exception and post-exception from bindings
0.2: with-handler renamed with-exn-handler
0.1: initial import

Outdated egg!

Exceptions made easy

The interface

current-exception-handler

exceptions

exception?

exn?

exception-of?

exn-of?

make-exception

make-exn

location

message

arguments

raise

with-exn-handler

condition-case

handle-exceptions

guard

assert*

argument-exception

argument-exn

result-exception

result-exn

named-lambda

<<

>>

true?

false?

Examples

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