Re: [Caml-list] Destructive use of file descriptors

[Ben Millwood <bmillwood@janestreet.com>]

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See also the following thread last year, "Not letting channels escape",
which (after some incomplete attempts from me) illustrates a fully safe
with_file function.

https://sympa.inria.fr/sympa/arc/caml-list/2014-08/msg00024.html

On 11 August 2015 at 12:14, Gabriel Scherer <gabriel.scherer@gmail.com>
wrote:

> > 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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>
> --
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