"ocaml_beginners"::[] Question on exceptions

Jon Harrop wrote:
> On Sunday 18 June 2006 13:24, Till Crueger wrote:
>> let apply func =
>>     init ();
>>     let retVal =
>>        begin
>>           try
>>              func ();
>>           with a ->
>>              cleanup ();
>>              raise a
>>         end
>>     cleanup ();
>>     retVal
>>
>> but here I have th cleanup step twice. Is there a
simple way to eliminate
>> one of the cleanups and still have the same
behaviour?
> 
> Yes. You just factor this functionality out into a
higher-order function that 
> accepts the cleanup function as an argument. This
higher-order function is 
> typically called "unwind protect":
> 
> let unwind_protect body cleanup =
>   try body(); cleanup() with e -> cleanup(); raise e

A minor point: while not as "spare" as this,
Jonathan's solution does 
not have the problem this one will if there is an exception
in the 
*cleanup* function.

Robert Roessler
robertrrftp.com
http://www.rftp.com


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> A minor point: while not as "spare" as
this, Jonathan's solution does
> not have the problem this one will if there is an
exception in the
> *cleanup* function.

If you wanted a HOF for this, you could factor out my
solution too:

let use_resource init cleanup f x =
  init ();
  let res = try `Result (f x) with e -> `Exn e in
  cleanup ();
  match res with
  | `Result x -> x
  | `Exn e -> raise e;;

To be really neat, init & cleanup could be optional
arguments, in
which you'd need:

let may opt x = match opt with
| None -> ()
| Some f -> f x
(* val may : ('a -> unit) option -> unit *)

and replace init(); with may init (); and similarly for
cleanup --
this way, if you don't need one or the other, then you
don't specify
them in the function call.
http://caml.inria.fr/pub/docs/manual-ocaml/manual006.ht
ml has info on
labels & optional arguments (optionals are a special
case of labels).

Then it's more flexible in that you can use any init &
cleanup
functions you want.

Jonathan


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On Monday 19 June 2006 00:46, Robert Roessler wrote:
> > let unwind_protect body cleanup =
> >   try body(); cleanup() with e -> cleanup();
raise e
>
> A minor point: while not as "spare" as
this, Jonathan's solution does
> not have the problem this one will if there is an
exception in the
> *cleanup* function.

Will this preserve those semantics:

let unwind_protect body cleanup =
  (try body() with e -> cleanup(); raise e); cleanup()

Of course, you're in problems anyway if your cleanup
function is also raising 
exceptions. Do you really want to ignore the exception from
"body"?

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scient
ists


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