> Yeah. I was thinking that that might be avoided. Something on the line
> of putting the file descriptor into a local module so ocaml would
> complain about it escaping its scope or something.
You can use the "ST monad" trick to track escape of values or effects:
"Region-based resource management", Oleg
http://okmij.org/ftp/Haskell/regions.html
However, this requires writing the resource-manipulating operations in
monadic style.
module ST : sig
type ('s, 'a) st
val return : 'a -> ('s, 'a) st
val bind : ('a -> ('s, 'b) st) -> ('s, 'a) st -> ('s, 'b) st
type ('s, 'a) st_ref
val st_ref : 'a -> ('s, ('s, 'a) st_ref) st
val get : ('s, 'a) st_ref -> ('s, 'a) st
val set : ('s, 'a) st_ref -> 'a -> ('s, unit) st
type 'a secretive = { run : 's . unit -> ('s, 'a) st }
val run_st : 'a secretive -> 'a
end = struct
type ('s, 'a) st = 'a
let return x = x
let bind f m = f m
type ('s, 'a) st_ref = 'a ref
let st_ref, get, set = ref, (!), (:=)
type 'a secretive = { run : 's . unit -> ('s, 'a) st }
let run_st st = st.run ()
end
let (>>=) m f = ST.bind f m
# let test =
ST.run_st { ST.run = fun () ->
ST.st_ref 1 >>= fun r ->
ST.get r >>= fun before ->
ST.set r 2 >>= fun () ->
ST.get r >>= fun after ->
ST.return (before + after)
};;
val test : int = 3
# let test_escape_1 =
ST.run_st { ST.run = fun () ->
ST.st_ref 1 >>= fun r ->
ST.return r
};;
Error: This field value has type unit -> ('a, ('a, int) ST.st_ref) ST.st
which is less general than 'b. unit -> ('b, 'c) ST.st
# let test_escape_2 =
let hole = ref None in
ST.run_st { ST.run = fun () ->
ST.st_ref 1 >>= fun r ->
hole := Some r;
ST.return ()
};
!hole;;
Error: This field value has type unit -> ('a, unit) ST.st
which is less general than 'b. unit -> ('b, 'c) ST.st
On Tue, Aug 11, 2015 at 11:36 AM, Goswin von Brederlow
<goswin-v-b@web.de> wrote:
> On Thu, Aug 06, 2015 at 07:50:38PM +0000, Malcolm Matalka wrote:
>> Nicolas Ojeda Bar <n.oje.bar@gmail.com> writes:
>>
>> > Hi,
>> >
>> > No magic needed in OCaml:
>> >
>> > let with_open path f =
>> > let oc = open_out path in
>> > match f oc with
>> > | x -> close_out_noerr oc; x
>> > | exception e -> close_out_noerr oc; raise e
>> >
>> > and then use it as
>> >
>> > with_open "foo" (fun oc -> output_string oc str)
>> >
>> > Cheers
>> >
>> > Nicolas
>> >
>>
>> The weakness with this (which I'm not sure if Python offers protection)
>> is returning something that references the file created. It can lead to
>> some confusing error messages.
>
> Yeah. I was thinking that that might be avoided. Something on the line
> of putting the file descriptor into a local module so ocaml would
> complain about it escaping its scope or something.
>
> At least unlike the other suggestion this one closes the FD on exception.
>
> MfG
> Goswin
>
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