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From: jinx@zurich.ai.mit.edu (Guillermo J. Rozas)
Newsgroups: comp.lang.scheme
Subject: Re: open-input-file [topics from hell, part 2]
Message-ID: <JINX.91Apr23230905@chamarti.ai.mit.edu>
Date: 24 Apr 91 03:09:05 GMT
References: <CARLTON.91Apr20004133@husc10.harvard.edu> <9104211907.AA25632@august>
	<CARLTON.91Apr23155155@scws1.harvard.edu>
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In-reply-to: carlton@scws1.harvard.edu's message of 23 Apr 91 19:51:55 GMT


    Horror?  Good grief...  But I do agree that finer control over such
    matters is preferable.  I don't think that taking care of it with an
    exception mechanism is the right way to do this - I would much rather
    write code that looks like

    (let ((port (open-input-port filename)))
      (if (not port)
	  (do-something)
	  (proceed-with-one's-business)))

    rather than

    (let ((port (begin (install-exception-handler-to-do-something))
		       (open-input-port filename)))
      (proceed-with-one's-business))

    Matter of taste, I suppose, but I would rather reserve exception
    handlers for unexpected sorts of circumstances that programs normally
    would die on, such as functions being applied to arguments of the
    wrong type or the user hitting the interrupt key, rather than what I
    think is probably a fairly common circumstance and one which any
    program that does I/O should be prepared to handle, namely the
    non-existence of files.

The non-existence of an expected file sounds like an unexpected sort
of circumstance to me. :-)

Clearly both solutions are equipotent.  Given control over exceptions,
you can

(define (my-open-input-port filename)
  (install-exception-handler-return-false)
  (open-input-port filename))

although install-exception-handler would probably be a wrapper to
limit the extent of the action.
The converse is straightforward.

The language could choose either mechanism and you could use whichever
one you preferred in your code.

Personally, I am tired of fixing C programs that do not check the
return status of system calls and fail with segmentation violations
later on.  To avoid having the same thing happen in Scheme, I'd much
rather have Scheme's primitives error, and have a reasonable condition
system that allows me to handle those errors when needed.