simple-exceptions | chickadee

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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.

The interface

current-exception-handler

current-exception-handler #!optional new-handlerparameter: chicken's handler parameter reexported.

exceptions

exceptions #!optional symprocedure: documentation procedure.

exception?

exception? xprprocedure: type predicate. Note, that each exception is a condition of kind exn.

exn?

exn? xprprocedure: alias to exception?

exception-of?

exception-of? kind-keyprocedure: returns a unary predicate, which checks, if its argument expression is an exception of kind kind-key, a symbol.

exn-of?

exn-of? kind-keyprocedure: alias to exception-of?

make-exception

make-exception msg #!rest kind-keysprocedure: Returns a procedure of at least one argument, a symbol describing the location of its call, and an optional other argument, a list of arguments, which creates an exception of kinds kind-keys, message msg, as well as the given location and arguments.

make-exn

make-exn msg #!rest kind-keysprocedure: alias to make-exception

location

location exnprocedure: returns the location property of its exception argument.

message

message exnprocedure: returns the message property of its exception argument.

arguments

arguments exnprocedure: returns the arguments property of its exception argument.

raise

raise exnprocedure: raises its argument, i.e. calls (current-exception-handler) with exn. In essence chicken's abort.

with-exn-handler

with-exn-handler handler thunkprocedure: A save version of chicken's low-level with-exception-handler. Sets the current-exception-handler to handler for the dynamic extent of its call and executes thunk in this context.

condition-case

(condition-case xpr ([var] (kind ...) body) . other-clauses)))syntax: Chicken's highest level exception-handler reexported.

handle-exceptions

(handle-exceptions exn handle-xpr xpr . xprs)syntax: Chicken's high level exception handler reexported.

guard

(guard (exn cond-clause . cond-clauses) xpr . xprs)syntax: The high level exception handler of R6RS and R7RS. Sets the current-exception-handler to an exception-handler created from exn und the supplied cond-clauses for the dynamic extent of its call and executes the body xpr . xprs in this context.

assert*

(assert* loc xpr . xprs)syntax: checks the expressions xpr . xprs in sequence and raises an exception for the first failing expression with location property loc and arguments property the failing expression quoted.

argument-exception

argument-exception loc arg ...procedure: raised when a precondition is violated. Can be catched with (exn argument)

argument-exn

argument-exn loc arg ...procedure: alias to argument-exception

result-exception

result-exception loc arg ...procedure: raised when a postcondition is violated. Can be catched with (exn result)

result-exn

result-exn loc arg ...procedure: alias to result-exception

named-lambda

(named-lambda name args xpr . xprs)syntax: can replace anonymous procedures and can be used recursively.

<<

<< x loc .. x? ...procedure: precondition test: passes x unchanged only if all predicates x? return #t on it, raises an argument-exception at location loc, if given, or '<< otherwise

>>

>> x loc .. x? ...procedure: postcondition test: passes x unchanged only if all predicates x? return #t on it raises a result-exception at location loc, if given, or '>> otherwise

true?

true? xprprocedure: returns always #t

false?

false? xprprocedure: returns always #f

Examples

(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
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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

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