Re: [Caml-list] Not letting channels escape.

["Frédéric Bour" <frederic.bour@lakaban.net>]

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But you can't do any meaningful computation with this value.
It's just a blackbox.

In your example, the (ST.input_line c) inside the closure just build a 
value of type (string option, '_s) t for a specific s.
In particular you will never be able to execute any action on the input 
channel.

This is not the case with the proposed implementation, because effects 
are executed immediately.
To ensure isolation, all effects should be "frozen":

module ST : sig
  type ('a, 's) t
  val return : 'a -> ('a, 's) t
  val bind : ('a, 's) t -> ('a -> ('b, 's) t) -> ('b, 's) t
  type 's chan
  type 'a f = { f : 's . 's chan -> ('a, 's) t }
  val with_file : string -> f:'a f -> 'a

  val input_line : 's chan -> (string, 's) t
end = struct
  type ('a, 's) t = unit -> 'a
  let return x = fun () -> x
  let bind x f = fun () -> f (x ()) ()

  type 's chan = in_channel
  type 'a f = { f : 's . 's chan -> ('a, 's) t }
  let with_file fp ~f:{ f } =
    let ic = open_in fp in
    try
      let result = f ic () in
      close_in_noerr ic;
      result
    with exn ->
      close_in_noerr ic;
      raise exn
  let input_line c = fun () -> input_line c
end

Le ven. 8 août 2014 à 20:30, Markus Mottl <markus.mottl@gmail.com> a 
écrit :
> The escaping value can still be manipulated through a closure, outside
> of "with_file".  The goal was to prevent this.
> 
> On Fri, Aug 8, 2014 at 2:28 PM, Frédéric Bour 
> <frederic.bour@lakaban.net> wrote:
>>  ST.input_line is just a reified effect, it can't be executed 
>> outside of the
>>  ST monad.
>>  You can make the value escape, but you can't do anything with it.
>> 
>>  And because of the existential variable being propagated, it can't 
>> be
>>  executed outside of this run of the ST value.
>> 
>>  Le ven. 8 août 2014 à 19:23, Markus Mottl 
>> <markus.mottl@gmail.com> a écrit :
>> 
>>  How would you implement this safely with ST here? I wasn't using the
>>  standard input_line but "ST.input_line", which already returns a 
>> monadic
>>  type. The trick here was to use the monadic "return" to return a 
>> closure
>>  that captures the existential variable, allowing me to execute the
>>  computation outside of the safe region. Regards, Markus On Fri, Aug 
>> 8, 2014
>>  at 1:37 PM, Gabriel Scherer <gabriel.scherer@gmail.com> wrote:
>> 
>>  The ST trick only works when all primitives affecting resource are 
>> in the
>>  monadic abstraction (they mention the ST region variable in their
>>  computation type). This is not the case in Markus example as 
>> "input_line" is
>>  a non-typed effect. Using ST safely would be possible in OCaml, but 
>> you
>>  would have to completely eschew the standard library and use a 
>> different
>>  base where all effectful functions have a monadic type. It is the 
>> library,
>>  not the language itself, that allows this. On the contrary, linear 
>> types are
>>  distinctly a language feature. Using monads to encapsulate a form of
>>  linearity is an old trick. If you want to have a taste of built-in 
>> linear
>>  typing, you may want to give Mezzo a try ( 
>> http://protz.github.io/mezzo/ ).
>>  On Fri, Aug 8, 2014 at 7:21 PM, Markus Mottl 
>> <markus.mottl@gmail.com> wrote:
>> 
>>  I see, I was replying to the "reference problem" and hadn't read 
>> your
>>  implementation, which, besides existentials, already requires 
>> monads as
>>  return values. Actually, it just occurred to me that one can even 
>> break the
>>  monadic approach in a purely functional way. You are just one 
>> "return" away
>>  from disaster: let f = ST.with_file "foo.txt" ~f:{ ST.f = fun c -> 
>> ST.return
>>  (fun () -> ignore (ST.input_line c)) } in f () You'd have to 
>> eliminate
>>  "return", in which case it wouldn't be a monad anymore and not 
>> general
>>  enough for realistic uses of "with_file". Regards, Markus On Fri, 
>> Aug 8,
>>  2014 at 12:01 PM, Ben Millwood <bmillwood@janestreet.com> wrote: > I
>>  protected against that in my module by carrying the existential 
>> type >
>>  variable in the result of input_line as well, because I stumbled 
>> into >
>>  exactly that example while originally drafting my e-mail :) > > In 
>> a sense
>>  I'm reinventing monadic IO but in a bit of a half-hearted > way. It 
>> >
>>  wouldn't take much work to make it a bit more fully-hearted, but it 
>> > would
>>>  still be inconvenient to actually use. > > > On 8 August 2014 
>>> 16:44,
>>  Markus Mottl <markus.mottl@gmail.com> wrote: >> >> It doesn't even 
>> require
>>  references to screw things up here. Just >> return the closure 
>> containing
>>  the channel from within "f": >> >> In_channel.with_file "foo.txt" 
>> ~f:(fun ic
>>  () -> input_line ic) >> |> fun f -> f () >> >> The initial 
>> Stream-example is
>>  basically just an instance of this >> "returning a closure" 
>> problem. >> >>
>>  But the availability of references and exceptions arguably makes >> 
>> things
>>  worse, because you cannot even use monadic I/O + existential >> 
>> types to
>>  achieve guaranteed safety. >> >> Regards, >> Markus >> >> On Fri, 
>> Aug 8,
>>  2014 at 10:49 AM, Ben Millwood >> <bmillwood@janestreet.com> >> 
>> wrote: >> >
>>  It's been pointed out to me that the above certainly isn't 
>> perfectly >> >
>>  secure. >> > E.g. >> > >> > let f = ref (fun () -> ()) in >> > 
>> with_file
>>  "safe.ml" ~f:{ f = fun c -> >> > return (f := fun () -> >> > 
>> Fn.ignore (map
>>  (input_line c) ~f:print_string_option)) }; >> > !f () >> > >> > gets
>>  Exception: (Sys_error "Bad file descriptor"). Even though the >> > 
>> channel
>>>>  > and any operations on it can't escape the closure, the type of 
>>>> a >> >
>>  function >> > which uses them needn't mention them at all. >> > >> 
>> > It's
>>  pretty hard to do anything about this in the presence of >> > 
>> unrestricted
>>>>  > side effects, so perhaps there's a reason why the Haskellers 
>>>> are >> >
>>  excited >> > about this sort of thing and you don't see it in OCaml 
>> so much
>>  :) >> > >> > That said, you do seem to be forced to make a bit more 
>> of an
>>  effort >> > to >> > break >> > things here, so I don't think the 
>> technique
>>  is completely without >> > merit, >> > perhaps in cases where you'd 
>> be
>>  defining all your own operations >> > anyway, >> > so >> > the 
>> duplication
>>  isn't an issue. >> > >> > >> > On 8 August 2014 12:30, Ben Millwood
>>  <bmillwood@janestreet.com> >> > wrote: >> >> >> >> There's a trick 
>> with
>>  existential types, as used in e.g. Haskell's ST >> >> monad. It 
>> uses the
>>  fact that an existentially-quantified type >> >> variable >> >> 
>> can't >> >>
>>  escape its scope, so if your channel type and results that depend 
>> on >> >>
>>  it >> >> are >> >> parametrised by an existential type variable, the
>>  corresponding >> >> values >> >> can't >> >> escape the scope of the
>>  callback either. >> >> >> >> Something like: >> >> >> >> module ST 
>> : sig >>
>>>>  type ('a, 's) t >> >> include Monad.S2 with type ('a, 's) t := 
>>>> ('a, 's) t
>>>>  >> type 's chan >> >> type 'a f = { f : 's . 's chan -> ('a, 's) 
>>>> t } >>
>>>>  val with_file : string -> f:'a f -> 'a >> >> >> >> val input_line 
>>>> : 's
>>  chan -> (string option, 's) t >> >> end = struct >> >> module T = 
>> struct >>
>>>>  type ('a, 's) t = 'a >> >> let return x = x >> >> let bind x f = 
>>>> f x >>
>>>>  let map x ~f = f x >> >> end >> >> include T >> >> include 
>>>> Monad.Make2(T)
>>>>  >> type 's chan = In_channel.t >> >> type 'a f = { f : 's . 's 
>>>> chan ->
>>  ('a, 's) t } >> >> let with_file fp ~f:{ f } = In_channel.with_file 
>> fp ~f >>
>>>>  let input_line c = In_channel.input_line c >> >> end >> >> ;; >> 
>>>> >> >> >>
>>  match ST.with_file "safe.ml" ~f:{ ST.f = fun c -> ST.input_line c } 
>> >> >>
>>  with >> >> | None -> print_endline "None" >> >> | Some line -> 
>> print_endline
>>  line >> >> >> >> >> >> On 8 August 2014 11:23, Philippe Veber
>>  <philippe.veber@gmail.com> >> >> wrote: >> >>> >> >>> Dear all, >> 
>> >>> >>
>>>>>  many libraries like lwt, batteries or core provide a very nice 
>>>>> >> >>>
>>  idiom >> >>> to >> >>> be used when a function uses a resource 
>> (file,
>>  connection, mutex, >> >>> et >> >>> cetera), for instance in
>>  Core.In_channel, the function: >> >>> >> >>> val with_file : 
>> ?binary:bool ->
>>  string -> f:(t -> 'a) -> 'a >> >>> >> >>> opens a channel for [f] 
>> and
>>  ensures it is closed after the call to >> >>> [f], >> >>> even if 
>> it raises
>>  an exception. So these functions basically >> >>> prevent >> >>> 
>> from >> >>>
>>  leaking resources. They fail, however, to prevent a user from using 
>> >> >>>
>>  the >> >>> resource after it has been released. For instance, 
>> writing: >>
>>>>>  >> >>> input_char (In_channel.with_file fn (fun x -> x)) >> >>> 
>>>>> >> >>>
>>  is perfectly legal type-wise, but will fail at run-time. There are 
>> >> >>> of
>>>>  >>> course less obvious situations, for instance if you define a 
>>>> >> >>>
>>  function: >> >>> >> >>> val lines : in_channel -> string Stream.t 
>> >> >>> >>
>>>>>  then the following will also fail: >> >>> >> >>> Stream.iter f
>>  (In_channel.with_file fn lines) >> >>> >> >>> My question is the 
>> following:
>>  is there a way to have the compiler >> >>> check >> >>> resources 
>> are not
>>  used after they are closed? I presume this can >> >>> only >> >>> 
>> be >> >>>
>>  achieved by strongly restricting the kind of function passed to >> 
>> >>>
>>  [with_file]. >> >>> One simple restriction I see is to define a 
>> type of
>>  immediate >> >>> value, >> >>> that >> >>> roughly correspond to 
>> "simple"
>>  datatypes (no closures, no lazy >> >>> expressions): >> >>> >> >>> 
>> module
>>  Immediate : sig >> >>> type 'a t = private 'a >> >>> val int : int 
>> -> int t
>>>>  >>> val list : ('a -> 'a t) -> 'a list -> 'a list t >> >>> val 
>>>> tuple :
>>  ('a -> 'a t) -> ('b -> 'b t) -> ('a * 'b) -> ('a * >> >>> 'b) t >> 
>> >>> (*
>>  for records, use the same trick than in >> >>>
>>  http://www.lexifi.com/blog/dynamic-types *) >> >>> ... >> >>> end 
>> >> >>> >>
>>>>>  and have the type of [with_file] changed to >> >>> >> >>> val 
>>>>> with_file
>>  : string -> f:(in_channel -> 'a Immediate.t) -> 'a >> >>> >> >>> 
>> I'm sure
>>  there are lots of smarter solutions out there. Would >> >>> anyone 
>> >> >>>
>>  happen to know some? >> >>> >> >>> Cheers, >> >>> Philippe. >> >>> 
>> >> >> >>
>>>  >> >> >> >> -- >> Markus Mottl http://www.ocaml.info
>>  markus.mottl@gmail.com > > -- Markus Mottl http://www.ocaml.info
>>  markus.mottl@gmail.com -- Caml-list mailing list. Subscription 
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>> 
>>  --
>>  Markus Mottl http://www.ocaml.info markus.mottl@gmail.com
>>  --
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> 
> 
> 
> -- 
> Markus Mottl        http://www.ocaml.info        
> markus.mottl@gmail.com
> 
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