- Exceptions made easy
- The interface
- Last update
- Version History
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.
- (current-exception-handler [new-handler]) parameter
chicken's handler parameter reexported.
- (exceptions [sym]) procedure
- (exception? xpr) procedure
type predicate. Note, that each exception is a condition of kind exn.
- (exn? xpr) procedure
alias to exception?
- (exception-of? kind-key) procedure
returns a unary predicate, which checks, if its argument expression is an exception of kind kind-key, a symbol.
- (exn-of? kind-key) procedure
alias to exception-of?
- (make-exception msg . kind-keys) procedure
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 msg . kind-keys) procedure
alias to make-exception
- (location exn) procedure
returns the location property of its exception argument.
- (message exn) procedure
returns the message property of its exception argument.
- (arguments exn) procedure
returns the arguments property of its exception argument.
- (raise exn) procedure
raises its argument, i.e. calls (current-exception-handler) with exn. In essence chicken's abort.
- (with-exn-handler handler thunk) procedure
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 xpr ([var] (kind ...) body) . other-clauses))) syntax
Chicken's highest level exception-handler reexported.
- (handle-exceptions exn handle-xpr xpr . xprs) syntax
Chicken's high level exception handler reexported.
- (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* 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 loc arg ...) procedure
raised when a precondition is violated. Can be catched with (exn argument)
- (argument-exn loc arg ...) procedure
alias to argument-exception
- (result-exception loc arg ...) procedure
raised when a postcondition is violated. Can be catched with (exn result)
- (result-exn loc arg ...) procedure
alias to result-exception
- (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? xpr) procedure
returns always #t
- (false? xpr) procedure
returns always #f
(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)
Feb 17, 2017
Copyright (c) 2014-2017, Juergen Lorenz All rights reserved.
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- bug in <<< and >>> fixed
- <<< and >>> added, pre- and post-exceptions renamed argument- and result-exn, some aliases added
- named-lambda added
- << and >> now procedures
- <<, >>, pre-exception and post-exception from bindings
- with-handler renamed with-exn-handler
- initial import