- Exceptions made easy
- 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 easily 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.
- current-exception-handler #!optional new-handlerparameter
chicken's handler parameter reexported.
- exceptions #!optional symprocedure
- exception? xprprocedure
type predicate. Note, that each exception is a condition of kind exn.
- exn? xprprocedure
alias to exception?
- 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? kind-keyprocedure
alias to exception-of?
- 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 msg #!rest kind-keysprocedure
alias to make-exception
- location exnprocedure
returns the location property of its exception argument.
- message exnprocedure
returns the message property of its exception argument.
- arguments exnprocedure
returns the arguments property of its exception argument.
- raise exnprocedure
raises its argument, i.e. calls (current-exception-handler) with exn. In essence chicken's abort.
- 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 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.
- 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
The following routines are outsourced to the checks egg with slightly modified syntax: assert*, <<, <<<, >>, >>>, named-lambda, true?, false?
(import simple-exceptions) (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 02, 2019
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Copyright (c) 2014-2019, Juergen Lorenz All rights reserved.
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- <<< and friends outsourced to checks
- <<< and >>> with additional optional argument
- zero argument case added to << and (<<< loc)
- port of chicken-4's version 0.6