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From: net@opal.cs.tu-berlin.de (Oliver Laumann)
Newsgroups: comp.lang.scheme
Subject: Re: open-input-file [topics from hell, part 2]
Message-ID: <3158@kraftbus.cs.tu-berlin.de>
Date: 25 Apr 91 09:17:39 GMT
Article-I.D.: kraftbus.3158
References: <9104211907.AA25632@august> <CARLTON.91Apr23155155@scws1.harvard.edu> <5387@goanna.cs.rmit.oz.au>
Organization: Technical University of Berlin, Germany
Lines: 35

ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
> The only Scheme implementation I actually understand is Elk, and it
> would be very easy to add this to Elk.

In Elk one can easily write a function that calls open-input-file
with a file name and, if the file cannot be opened, invokes a
user-supplied error handler like this:

(define (open-input-file-with-error-handler file handler)
  (call/cc
    (lambda (return)
      (fluid-let
        ((error-handler (lambda args (apply handler return args))))
	   (open-input-file file)))))

In this example, the user-supplied error handler is invoked with a
continuation, which, when called, returns a value to the caller
of open-input-file-with-error-handler and an error message (a symbol
identifying the function that failed, a format string, and optional
arguments).

The function could be used like this:

(define (my-handler return . error-message)
  ;; do something (e.g. print error message)
  (return #f))

(let ((port (open-input-file-with-error-handler '/etc/passwd my-handler)))
  (case port
    (#f   ;; file could not be opened
    (else ;; do something with the port

In case the user-supplied error handler terminates normally, Elk would
invoke the standard error handler supplied by the top level (which
prints the error message in a standard way and resets to the top level).