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This SRFI requires a Scheme implementation to raise an exception whenever an error is to be signaled or whenever the system determines that evaluation cannot proceed in a manner consistent with the semantics of Scheme. However, this SRFI does not define the information to be supplied by an implementation for each possible kind of exception; such a specification is left open for future SRFIs.
Whenever the system raises an exception, it invokes the current exception handler with a condition object (encapsulating information about the exception) as its only argument. Any procedure accepting one argument may serve as an exception handler. When a program explicitly raises an exception, it may supply any object to the exception handler.
An exception is either continuable or non-continuable. When the current exception handler is invoked for a continuable exception, the continuation uses the handler's result(s) in an exception-specific way to continue. When an exception handler is invoked for a non-continuable exception, the continuation raises a non-continuable exception indicating that the exception handler returned.
The initial current exception handler is implementation-dependent.
Returns the current exception handler.
(WITH-EXCEPTION-HANDLER handler thunk) [procedure]
Returns the result(s) of invoking thunk. The handler procedure is installed as the current exception handler in the dynamic context of invoking thunk.
(call-with-current-continuation (lambda (k) (WITH-EXCEPTION-HANDLER (lambda (x) (k '())) (lambda () (car '()))))) ; = '()
(HANDLE-EXCEPTIONS var handle-expr expr1 expr2 ...) [syntax]
Evaluates the body expressions expr1, expr2, ... in sequence with an exception handler constructed from var and handle-expr. Assuming no exception is raised, the result(s) of the last body expression is(are) the result(s) of the HANDLE-EXCEPTIONS expression.
The exception handler created by HANDLE-EXCEPTIONS restores the dynamic context (continuation, exception handler, etc.) of the HANDLE-EXCEPTIONS expression, and then evaluates handle-expr with var bound to the value provided to the handler.
(HANDLE-EXCEPTIONS exn (begin (display "Went wrong") (newline)) (car '())) ; displays "Went wrong" (HANDLE-EXCEPTIONS exn (cond ((eq? exn 'one) 1) (else (ABORT exn))) (case (random-number) [(0) 'zero] [(1) (ABORT 'one)] [else (ABORT "Something else")])) ; = 'zero, 1, or (ABORT "Something else")
The HANDLE-EXCEPTIONS form can be implemented with DEFINE-SYNTAX as shown in the Implementation section of this SRFI.
Raises a non-continuable exception represented by obj. The ABORT procedure can be implemented as follows:
(define (abort obj) ((current-exception-handler) obj) (abort (make-property-condition 'exn 'message "Exception handler returned")))The ABORT procedure does not ensure that its argument is a condition. If its argument is a condition, ABORT does not ensure that the condition indicates a non-continuable exception.
(SIGNAL obj) [procedure]
Raises a continuable exception represented by obj. The SIGNAL procedure can be implemented as follows:
(define (signal exn) ((current-exception-handler) exn))The SIGNAL procedure does not ensure that its argument is a condition. If its argument is a condition, SIGNAL does not ensure that the condition indicates a continuable exception.
(CONDITION? obj) [procedure]
Returns #t if obj is a condition, otherwise returns #f. If any of the predicates listed in Section 3.2 of the R5RS is true of obj, then CONDITION? is false of obj.
Rationale: Any Scheme object may be passed to an exception handler. This would cause ambiguity if conditions were not disjoint from all of Scheme's standard types.
(MAKE-PROPERTY-CONDITION kind-key prop-key value ...) [procedure]
This procedure accepts any even number of arguments after kind-key, which are regarded as a sequence of alternating prop-key and value objects. Each prop-key is regarded as the name of a property, and each value is regarded as the value associated with the key that precedes it. Returns a kind-key condition that associates the given prop-keys with the given values.
(MAKE-COMPOSITE-CONDITION condition ...) [procedure]
Returns a newly-allocated condition whose components correspond to the the given conditions. A predicate created by CONDITION-PREDICATE returns true for the new condition if and only if it returns true for one or more of its component conditions.
(CONDITION-PREDICATE kind-key) [procedure]
Returns a predicate that can be called with any object as its argument. Given a condition that was created by MAKE-PROPERTY-CONDITION, the predicate returns #t if and only if kind-key is EQV? to the kind key that was passed to MAKE-PROPERTY-CONDITION. Given a composite condition created with MAKE-COMPOSITE-CONDITION, the predicate returns #t if and only if the predicate returns #t for at least one of its components.
(CONDITION-PROPERTY-ACCESSOR kind-key prop-key) [procedure]
Returns a procedure that can be called with any condition that satisfies (CONDITION-PREDICATE kind-key). Given a condition that was created by MAKE-PROPERTY-CONDITION and kind-key, the procedure returns the value that is associated with prop-key. Given a composite condition created with MAKE-COMPOSITE-CONDITION, the procedure returns the value that is associated with prop-key in one of the components that satisfies (CONDITION-PREDICATE kind-key).
(let* ((cs-key (list 'color-scheme)) (bg-key (list 'background)) (color-scheme? (condition-predicate cs-key)) (color-scheme-background (condition-property-accessor cs-key bg-key)) (condition1 (make-property-condition cs-key bg-key 'green)) (condition2 (make-property-condition cs-key bg-key 'blue)) (condition3 (make-composite-condition condition1 condition2))) (and (color-scheme? condition1) (color-scheme? condition2) (color-scheme? condition3) (color-scheme-background condition3))) ; = 'green or 'blue
((condition-property-accessor 'exn 'message) exn)extracts the error message from exn. Example:
(HANDLE-EXCEPTIONS exn (begin (display "Went wrong: ") (display ((condition-property-accessor 'exn 'message) exn)) (newline)) (car '())) ; displays something like "Went wrong: can't take car of nil"
(define *current-exn-handler* ...) ; implementation-dependent (define (CURRENT-EXCEPTION-HANDLER) *current-exn-handler*) (define (WITH-EXCEPTION-HANDLER handler thunk) (let ((old #f)) (dynamic-wind (lambda () (set! old *current-exn-handler*) (set! *current-exn-handler* handler)) thunk (lambda () (set! *current-exn-handler* old))))) (define (ABORT obj) ((CURRENT-EXCEPTION-HANDLER) obj) (ABORT (make-property-condition 'exn 'message "Exception handler returned"))) (define (SIGNAL exn) ((CURRENT-EXCEPTION-HANDLER) exn)) (define-syntax HANDLE-EXCEPTIONS (syntax-rules () ((_ var handle-body e1 e2 ...) ((call-with-current-continuation (lambda (k) (with-exception-handler (lambda (var) (k (lambda () handle-body))) (lambda () (call-with-values (lambda () e1 e2 ...) (lambda args (k (lambda () (apply values args))))))))))))) ; The following is an approximate implementation of conditions that uses lists, ; instead of a disjoint class of values (define (CONDITION? obj) ; A condition is represented as a pair where the first value of the ; pair is this function. A program could forge conditions, and they're ; not disjoint from Scheme pairs. (and (pair? obj) (eq? CONDITION? (car obj)))) (define (MAKE-PROPERTY-CONDITION kind-key . prop-vals) (cons CONDITION? (list (cons kind-key prop-vals)))) (define (MAKE-COMPOSITE-CONDITION . conditions) (cons CONDITION? (apply append (map cdr conditions)))) (define (CONDITION-PREDICATE kind-key) (lambda (exn) (if (CONDITION? exn) (assq kind-key (cdr exn)) #f))) (define (CONDITION-PROPERTY-ACCESSOR kind-key prop-key) (lambda (exn) (let ((p ((CONDITION-PREDICATE kind-key) exn))) ; either cadr or cdr could fail; should check arguments for ; better error reporting: (cadr (memq prop-key (cdr p))))))
(define (try-car v) (let ((orig (current-exception-handler))) (WITH-EXCEPTION-HANDLER (lambda (exn) (orig (make-composite-condition (make-property-condition 'not-a-pair 'value v) exn))) (lambda () (car v))))) (try-car '(1)) ; = 1 (HANDLE-EXCEPTIONS exn (if ((condition-predicate 'not-a-pair) exn) (begin (display "Not a pair: ") (display ((condition-property-accessor 'not-a-pair 'value) exn)) (newline)) (ABORT exn)) (try-car 0)) ; displays "Not a pair: 0"
Copyright (C) William Clinger, R. Kent Dybvig, Matthew Flatt, and Marc Feeley (1999). All Rights Reserved.
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