From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Original-To: caml-list@sympa.inria.fr Delivered-To: caml-list@sympa.inria.fr Received: from mail3-relais-sop.national.inria.fr (mail3-relais-sop.national.inria.fr [192.134.164.104]) by sympa.inria.fr (Postfix) with ESMTPS id DE5CF7F747 for ; Fri, 8 Aug 2014 19:21:48 +0200 (CEST) Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender authenticity information available from domain of markus.mottl@gmail.com) identity=pra; client-ip=209.85.213.173; receiver=mail3-smtp-sop.national.inria.fr; envelope-from="markus.mottl@gmail.com"; x-sender="markus.mottl@gmail.com"; x-conformance=sidf_compatible Received-SPF: Pass (mail3-smtp-sop.national.inria.fr: domain of markus.mottl@gmail.com designates 209.85.213.173 as permitted sender) identity=mailfrom; client-ip=209.85.213.173; receiver=mail3-smtp-sop.national.inria.fr; envelope-from="markus.mottl@gmail.com"; x-sender="markus.mottl@gmail.com"; x-conformance=sidf_compatible; x-record-type="v=spf1" Received-SPF: None (mail3-smtp-sop.national.inria.fr: no sender authenticity information available from domain of postmaster@mail-ig0-f173.google.com) identity=helo; client-ip=209.85.213.173; receiver=mail3-smtp-sop.national.inria.fr; envelope-from="markus.mottl@gmail.com"; x-sender="postmaster@mail-ig0-f173.google.com"; x-conformance=sidf_compatible X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AjcBAI8G5VPRVdWtlGdsb2JhbABag19XBIJzsjiXQIdIAYENCBYQAQEBAQcLCwkSK4QEAQEEEhEEGQEUBx0BAwwGBQsDCgICJgICIQEBEQEFARwGExsHiAsBAxENo1xqiymBcoMQikQKGScNZoR6EQEFDoEei3OBRWgHgnmBUgWSEYMqhGWCCYFWjHCEQBgphSQhLw X-IPAS-Result: AjcBAI8G5VPRVdWtlGdsb2JhbABag19XBIJzsjiXQIdIAYENCBYQAQEBAQcLCwkSK4QEAQEEEhEEGQEUBx0BAwwGBQsDCgICJgICIQEBEQEFARwGExsHiAsBAxENo1xqiymBcoMQikQKGScNZoR6EQEFDoEei3OBRWgHgnmBUgWSEYMqhGWCCYFWjHCEQBgphSQhLw X-IronPort-AV: E=Sophos;i="5.01,826,1400018400"; d="scan'208";a="74338127" Received: from mail-ig0-f173.google.com ([209.85.213.173]) by mail3-smtp-sop.national.inria.fr with ESMTP/TLS/RC4-SHA; 08 Aug 2014 19:21:47 +0200 Received: by mail-ig0-f173.google.com with SMTP id h18so1368204igc.12 for ; Fri, 08 Aug 2014 10:21:46 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=9aPncLz5EitK4+yw8GtrYHCl+N/bWT9yNqHxTkbsVvw=; b=X9O0cGDF0b1oa8KCMIQOtOtjJPIMkX1kNyh1P73W66rvmsdUMzkDCtYPIjdJPxnwTi fnpltF/RXwyYc1peTkf9Ye4wB68P/3iaoQ/k2gRmyPtnukDjS75Q89Lx7X5XSOG1Qycn PXpU/MYs6ioXHxWL7dSEXV8XrtFuoCuCSXpaEaOXeVqXH0Nhk5hIIOkgNaeSNN9Ek0Oh PGvrNryU+u1wRqL4YQQRAiR7GpNcZzo09N3XMdk3RmYId2FZ//0tyv8YERlo5f7hrVc8 w+/t7QZyUFLKMwAPCLHr3Z1wVdU9vieL4npA5a0W51ZUZ7sKMkaGLn010A4ZZp8myJty Qgrw== MIME-Version: 1.0 X-Received: by 10.50.137.73 with SMTP id qg9mr7106965igb.19.1407518506108; Fri, 08 Aug 2014 10:21:46 -0700 (PDT) Received: by 10.107.135.133 with HTTP; Fri, 8 Aug 2014 10:21:46 -0700 (PDT) In-Reply-To: References: Date: Fri, 8 Aug 2014 13:21:46 -0400 Message-ID: From: Markus Mottl To: Ben Millwood Cc: Philippe Veber , caml users Content-Type: text/plain; charset=UTF-8 Subject: Re: [Caml-list] Not letting channels escape. 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 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 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 >> 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 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 >> >> 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