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<!DOCTYPE HTML>
<html>
<!--
SPDX-FileCopyrightText: 1999 Marc Feeley
SPDX-FileCopyrightText: 1999 Matthew Flatt
SPDX-FileCopyrightText: 1999 R. Kent Dybvig
SPDX-FileCopyrightText: 1999 William D Clinger <will@ccs.neu.edu>
SPDX-License-Identifier: MIT
-->
<head>
<title>SRFI 12: Exception Handling</title>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<link rel="stylesheet" href="/srfi.css" type="text/css" />
</head>
<body>
<H1>Title</H1>
Exception Handling
<H1>Authors</H1>
William Clinger, R. Kent Dybvig, Matthew Flatt, and Marc Feeley
<H1>Status</H1>
<p>This SRFI is currently in <em>withdrawn</em> status. Here is <a href="https://srfi.schemers.org/srfi-process.html">an explanation</a> of each status that a SRFI can hold. To provide input on this SRFI, please send email to <code><a href="mailto:srfi+minus+12+at+srfi+dotschemers+dot+org">srfi-12@<span class="antispam">nospam</span>srfi.schemers.org</a></code>. To subscribe to the list, follow <a href="https://srfi.schemers.org/srfi-list-subscribe.html">these instructions</a>. You can access previous messages via the mailing list <a href="https://srfi-email.schemers.org/srfi-12">archive</a>.</p>
<UL>
<LI>Received: 1999-10-08</LI>
<LI>Draft: 1999-10-10--1999-12-09</LI>
<LI>Withdrawn: 2000-01-22</LI>
</UL>
<H1>Abstract</H1>
The SRFI defines exception-handling constructs for Scheme, including
<UL>
<LI> the CURRENT-EXCEPTION-HANDLER procedure for obtaining the
current exception-handling procedure,
<LI> the WITH-EXCEPTION-HANDLER procedure and HANDLE-EXCEPTIONS form
for installing an exception-handling procedure,
<LI> the ABORT and SIGNAL procedures for raising exceptions, and
<LI> constructs for creating and inspecting <EM>condition</EM>
values, which encapsulate information about an exception.
</UL>
<P>
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.
<H1>Issues</H1>
None.<P>
<H1>Rationale</H1>
The goals of the exception mechanism are
<UL>
<LI> to support programs that catch and handle exceptions raised by a
Scheme implementation,
<LI> to help programmers to share code that relies on exception
handling, and
<LI> to be easily added to most existing Scheme systems.
</UL>
To this end, the SRFI defines an exception system whose essence can be
implemented with CALL-WITH-CURRENT-CONTINUATION. The SRFI also defines
a minimal set of constructs for creating and inspecting condition
values, which will allow programmers to at least catch primitive
exceptions and log exception messages, and to create new libraries
that take advantage of exception handling.
<H1>Specification</H1>
A Scheme implementation ("the system") raises 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. A program may also explicitly raise an exception.
<P>
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.
<P>
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.
<P>
The initial current exception handler is implementation-dependent.
<H2>Exception Handlers</H2>
(CURRENT-EXCEPTION-HANDLER) <b>[procedure]</b>
<P>
Returns the current exception handler.
<P>
(WITH-EXCEPTION-HANDLER <I>handler</I> <I>thunk</I>) <b>[procedure]</b>
<P>
Returns the result(s) of invoking <I>thunk</I>. The <I>handler</I> procedure is
installed as the current exception handler in the dynamic context of
invoking <I>thunk</I>.
<P>
Example:
<PRE>
(call-with-current-continuation
(lambda (k)
(WITH-EXCEPTION-HANDLER (lambda (x) (k '()))
(lambda () (car '())))))
; = '()
</PRE>
<P>
(HANDLE-EXCEPTIONS <I>var</I> <I>handle-expr</I> <I>expr1</I> <I>expr2</I> ...) <b>[syntax]</b>
<P>
Evaluates the body expressions <I>expr1</I>, <I>expr2</I>, ... in sequence
with an exception handler constructed from <I>var</I> and
<I>handle-expr</I>. Assuming no exception is raised, the result(s) of
the last body expression is(are) the result(s) of the
HANDLE-EXCEPTIONS expression.
<P>
The exception handler created by HANDLE-EXCEPTIONS restores the
dynamic context (continuation, exception handler, etc.) of the
HANDLE-EXCEPTIONS expression, and then evaluates <I>handle-expr</I> with
<I>var</I> bound to the value provided to the handler.
<P>
Examples:
<PRE>
(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")
</PRE>
<P>
The HANDLE-EXCEPTIONS form can be implemented with DEFINE-SYNTAX as
shown in the Implementation section of this SRFI.
<H2>Raising Exceptions</H2>
(ABORT <I>obj</I>) <b>[procedure]</b>
<P>
Raises a non-continuable exception represented by <I>obj</I>. The ABORT
procedure can be implemented as follows:
<PRE>
(define (abort obj)
((current-exception-handler) obj)
(abort (make-property-condition
'exn
'message
"Exception handler returned")))
</PRE>
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.
<P>
(SIGNAL <I>obj</I>) <b>[procedure]</b>
<P>
Raises a continuable exception represented by <I>obj</I>. The SIGNAL
procedure can be implemented as follows:
<PRE>
(define (signal exn)
((current-exception-handler) exn))
</PRE>
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.
<P>
<H2>Condition Objects</H2>
<P>
(CONDITION? <I>obj</I>) <b>[procedure]</b>
<P>
Returns #t if <I>obj</I> is a condition, otherwise returns #f. If any
of the predicates listed in Section 3.2 of the R5RS is true of <I>obj</I>,
then CONDITION? is false of <I>obj</I>.
<P>
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.
<P>
(MAKE-PROPERTY-CONDITION <I>kind-key</I> <I>prop-key</I> <I>value</I> ...) <b>[procedure]</b>
<P>
This procedure accepts any even number of arguments after
<I>kind-key</I>, which are regarded as a sequence of alternating
<I>prop-key</I> and <I>value</I> objects. Each <I>prop-key</I> is
regarded as the name of a property, and each <I>value</I> is regarded
as the value associated with the <I>key</I> that precedes it. Returns
a <I>kind-key</I> condition that associates the given <I>prop-key</I>s
with the given <I>value</I>s.
<P>
(MAKE-COMPOSITE-CONDITION <I>condition</I> ...) <b>[procedure]</b>
<P>
Returns a newly-allocated condition whose components correspond to the
the given <I>condition</I>s. 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.
<P>
(CONDITION-PREDICATE <I>kind-key</I>) <b>[procedure]</b>
<P>
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 <I>kind-key</I> 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.
<P>
(CONDITION-PROPERTY-ACCESSOR <I>kind-key</I> <I>prop-key</I>) <b>[procedure]</b>
<P>
Returns a procedure that can be called with any condition that
satisfies (CONDITION-PREDICATE <I>kind-key</I>). Given a condition
that was created by MAKE-PROPERTY-CONDITION and <I>kind-key</I>, the
procedure returns the value that is associated with <I>prop-key</I>.
Given a composite condition created with MAKE-COMPOSITE-CONDITION, the
procedure returns the value that is associated with <I>prop-key</I> in
one of the components that satisfies (CONDITION-PREDICATE
<I>kind-key</I>).
<PRE>
(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
</PRE>
<H3>Standard Condition Kind and Property</H3>
When the system raises an exception, the condition it passes to the
exception handler must include the <CODE>'exn</CODE> kind with the
<CODE>'message</CODE> property.
Thus, if <I>exn</I> is a condition representing a system
exception, then
<PRE>
((condition-property-accessor 'exn 'message) <I>exn</I>)
</PRE>
extracts the error message from <I>exn</I>.
Example:
<PRE>
(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"
</PRE>
<H1>Implementation</H1>
This SRFI cannot be fully implemented in R5RS because it requires
special support from the system, which must 1) call the current
exception handler for any system error, 2) define the initial
exception handler, and 3) provide a class of condition values disjoint
from other Scheme values. However, the functionality can be roughly
implemented as follows for a single-threaded system. (For simplicity,
we assume that none of the variables defined below will be redefined
by a program.)
<PRE>
(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))))))
</PRE>
<H2>More Examples</H2>
<PRE>
(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"
</PRE>
<H1>Copyright</H1>
<p>Copyright (C) William Clinger, R. Kent Dybvig, Matthew Flatt, and Marc Feeley (1999). All Rights Reserved. </p>
<p>
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
</p>
<p>
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
</p>
<p>
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
</p>
<hr>
<address>Editor: <a href="mailto:srfi+minus+editors+at+srfi+dot+schemers+dot+org">Dave Mason</a></address>
<!-- Created: Tue Sep 29 19:20:08 EDT 1998 -->
<!-- hhmts start -->
Last modified: Sun Jan 28 13:40:27 MET 2007
<!-- hhmts end -->
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