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Re: [PATCH 3/4] Work towards a more complete implementation of `(rnrs io

From: Andreas Rottmann
Subject: Re: [PATCH 3/4] Work towards a more complete implementation of `(rnrs io ports)'
Date: Wed, 24 Nov 2010 00:44:54 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (gnu/linux)

address@hidden (Ludovic Courtès) writes:

> Hi Andreas,
> Looks good, but...
> Andreas Rottmann <address@hidden> writes:
>>   (call-with-port): Don't use `dynamic-wind', as it is against its
>>   specification in R6RS 8.2.6.
> [...]
>>  (define (call-with-port port proc)
>>    "Call @var{proc}, passing it @var{port} and closing @var{port} upon exit 
>> of
>>  @var{proc}.  Return the return values of @var{proc}."
>> -  (dynamic-wind
>> -      (lambda ()
>> -        #t)
>> -      (lambda ()
>> -        (proc port))
>> -      (lambda ()
>> -        (close-port port))))
>> +  (call-with-values
>> +      (lambda () (proc port))
>> +    (lambda vals
>> +      (close-port port)
>> +      (apply values vals))))
> I don’t see what in §8.2.6 would be against ‘dynamic-wind’, 
To quote that paragraph about `call-with-port':

| Proc must accept one argument. The call-with-port procedure calls proc
| with port as an argument. If proc returns, port is closed automatically
| and the values returned by proc are returned. If proc does not return,
| port is not closed automatically, except perhaps when it is possible to
| prove that port will never again be used for an input or output
| operation.

I interpreted that the way I did based on the question of what should
happen when `proc' throws an exception (or otherwise invokes a
continuation that makes it leave its current dynamic extent).  To me, it
seems that in this case, `proc' does not _return_, and hence the port
should not be closed.  Just to clarify, by a procedure "returning", I
understand delivering a value (or values) to its continuation, which
does not happen if the procedure invokes some arbitrary (other)

FWIW, `call-with-port' is implemented without `dynamic-wind' in Ikarus,
Ypsilon and Larceny.

> and <> seems to
> suggest that it’s OK.
Hmm, to me, the answer given to that comment does not really indicate
whether the proposed implementation using `dynamic-wind' is acceptable
or not (which is unfortunate).

Regards, Rotty
Andreas Rottmann -- <>

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