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Message-ID: <46920328.2080509@ed.ac.uk>
Date: Mon, 09 Jul 2007 10:43:04 +0100
From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
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To: Vincent Hanquez <tab@snarc.org>
Cc: Jon Harrop <jon@ffconsultancy.com>, caml-list@yquem.inria.fr
Subject: Re: [Caml-list] try .. finally using new camlp4
References: <200707080114.42213.jon@ffconsultancy.com> <20070709090131.GA17462@snarc.org>
In-Reply-To: <20070709090131.GA17462@snarc.org>
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X-Spam: no; 0.00; camlp:01 expr:01 expr:01 val:01 exn:01 val:01 exn:01 camlp:01 endline:01 endline:01 polymorphic:01 wrote:01 avoids:01 caml-list:01 int:01 

Vincent Hanquez wrote:
>>   EXTEND Gram
>>     expr: LEVEL "top"
>>     [[ "try"; f=expr; "finally"; g=expr ->
>> 	 <:expr<
>> 	   ((function
>> 	     | `Val v, g -> g(); v
>> 	     | `Exn e, g -> g(); raise e)
>> 	      ((try `Val($f$) with e -> `Exn e), (fun () -> $g$)))
>> 	 >>]];
>>   END
> 
> I don't know camlp4, but why don't you use a more straightforward
> (let r = try f() with e -> g(); raise e in g(); r) construct
> instead of wrapping/unwrapping the thing into polymorphic variant ?

Jon's version avoids name capture.  If the user writes

    let r = 1 in
      try
        2
      finally
        print_endline (string_of_int r)

then your version (modulo the unit arguments) expands into

    let r = 1 in
       let r = try 2
               with e ->
                  print_endline (string_of_int r);
                  raise e in
            print_endline (string_of_int r);
            r

which prints "2" instead of "1".

Jeremy.