OO design
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Date:   (:) 
From:  Jacques Garrigue <garrigue@m...> 
Subject:  Re: [Camllist] Re: OO design 
From: Dan Grossman > A monadic approach (where each > operation would return a "new" file) or linearity appears necessary for > the latter. And you can perfectly encode the monadic approach in ocaml. In our case, we need the type of the monad to keep information about open and closed files. I include such a solution, which ensures the safety of file accesses, at the end of this post. Note that file handles are indexed statically, but you can use as many as you wish. It should be safe with references (there is no file handle that you can keep around, everything stays in the monad.) But beware of fancy extensions, like continuations, that would allow you to capture your environement, included files that were open when you created the continuation... From: Geoffrey Alan Washburn > The only "solution" that I've seen that seems to actually solve the > "problem" is to write a small interpreted language for performing I/O, > but that is a bit heavy weight for general use. I see indeed no other easy option if you have continuations in the language. Jacques (* safeio.mli *) type ('a,'b) monad (* A monad, with action 'a, returning 'b *) type ('a,'b) handle (* A file handle to be used/modified *) type input = [`In] (* Permissions *) type output = [`Out] type active = [`In`Out] type closed = [`Closed] type all_closed = <c: closed; n: all_closed> (* All files closed *) val ch1 : (<c:'a;n:'b> > <c:'c;n:'b>, 'a > 'c) handle val succ : ('a > 'b, 'c) handle > (<c:'d;n:'a> > <c:'d;n:'b>, 'c) handle val ch2 : (* shorthand for [succ ch1] *) (<c:'a; n: <c:'b; n:'c> > > <c:'a; n: <c:'d; n:'c> >, 'b > 'd) handle val run : (all_closed > all_closed, 'a) monad > 'a val open_in : ('a, closed > input) handle > string > ('a, unit) monad val open_out : ('a, closed > output) handle > string > ('a, unit) monad val close : ('a, [< active] > closed) handle > ('a, unit) monad val input : ('a, input > input) handle > ('a, char option) monad val output : ('a, output > output) handle > char > ('a, unit) monad val return : 'a > ('b > 'b, 'a) monad val bind : ('a > 'b, 'd) monad > ('d > ('b > 'c, 'e) monad) > ('a > 'c, 'e) monad val ( $ ) : ('a > 'b, unit) monad > ('b > 'c, 'd) monad > ('a > 'c, 'd) monad (* safeio.ml *) type channel = Closed  In of in_channel  Out of out_channel type channels = {mutable c: channel; mutable n: channels option} type ('a,'b) monad = channels > 'b type ('a,'b) handle = int (* Implementation left as exercise... *) (* Example of use *) open Safeio ;; let rec copy2 () = bind (input ch1) (function None > return ()  Some c > bind (output ch2 c) copy2);; val copy2 : unit > (< c : input; n : < c : output; n : 'a > > > < c : input; n : < c : output; n : 'a > >, unit) monad let copy_file f1 f2 = open_in ch1 f1 $ open_out ch2 f2 $ copy2 () $ close ch1 $ close ch2 ;; run (copy_file "a" "b") ;;