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Date: Thu, 20 Nov 2003 09:54:09 -0700
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To: Christian.Schaller@siemens.com
Cc: caml-list@inria.fr
In-reply-to: <A1364BC6814D92479D4EED572D6F6FD806953C@uranus.certw2k.net>
	(Christian.Schaller@siemens.com)
Subject: Re: [Caml-list] closing file descriptors and channels
References:  <A1364BC6814D92479D4EED572D6F6FD806953C@uranus.certw2k.net>
Message-Id: <20031120175959.81A7F4BED8@lobus.fungible.com>
From: tim@fungible.com (Tim Freeman)
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From: "Christian Schaller" <Christian.Schaller@siemens.com>
>Yup, I've seen this one already.  This one makes it complicated, since
>for propagating the exception, I have to duplicate the closings:
>
>    .
>    .
>    .
>  with          
>  End_of_file -> output_string oc (line ^ "\n");
>  Found line -> (close_out oc; close_in ic; raise Found line);
>  close_out oc;
>  close_in ic

Java's "try ... finally" is useful and can be recreated in ML.  Here's
a definition:

let finally (body: unit -> 'a) (handler: unit -> unit): 'a =
  let result: 'a option ref = ref None
  in begin
      begin
        try begin
          result := Some (body ());
        end with e -> begin
          handler ();
          raise e;
        end;
      end;
      handler ();
      match !result with
          Some r -> r
        | _ -> failwith "Confused in finally";
    end;;

and here's how to use it for your example:

   let ic = "open the input file somehow" in
   finally (fun () ->
      let oc = "open the output file somehow" in
      finally (fun () ->
         .
	 .  (sometimes raise Found somewhere in here)
	 .
	 with 
	 End_of_file -> output_string oc "whatever")
      (fun () -> close_out oc))
   (fun () -> close_in ic)

I have to admit Java's syntax for try...finally is better.
-- 
Tim Freeman                                                  tim@fungible.com
GPG public key fingerprint ECDF 46F8 3B80 BB9E 575D  7180 76DF FE00 34B1 5C78 
Your levity is good.  It relieves tension and fear of death.  -- Terminator 3

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