From: Ken Friis Larsen <kfl@it.edu>
To: caml-list@inria.fr
Subject: [Caml-list] Re: two unrelated questions
Date: 01 May 2001 13:45:31 +0100 [thread overview]
Message-ID: <yruu235b5lg.fsf_-_@swordfish.cl.cam.ac.uk> (raw)
In-Reply-To: Jacques Garrigue's message of "Tue, 01 May 2001 10:31:00 +0900"
"Jacques" == Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp> writes:
Jacques> Particularly, local exception definitions are one of the
Jacques> rare features present in SML and absent in Caml, and that
Jacques> some SML developpers think that Caml is right about :-)
But some of us really like that feature because it allows an elegant
*user* implementation of dynamic typing.
The task is to implement a structure with the signature DYN:
module type DYN =
sig
type dyn
val wrap: unit -> ('a -> dyn) * (dyn -> 'a)
end
The cannonical SML solution is to use exceptions for this. Here is a
Caml version of that solution:
module Dyn1 : DYN =
struct
type dyn = exn
let wrap () =
let module D = struct exception DYN of 'a end in
(function x -> D.DYN x),
(function D.DYN x -> x)
end
(But I'm unfortunately not strong enough in O'Caml because this gives
the error "Unbound type parameter 'a". How do I fix that? Note that
the implementation works as expected if we make 'a a fixed type.)
You can argue that you don't need that feature of exceptions just to
implement dynamic typing, because DYN can be implemented using
closures and references:
module Dyn2 : DYN =
struct
type dyn = unit -> unit
let wrap () =
let r = ref None in
(fun x -> fun () -> r := Some x),
(fun f -> ( r := None
; f()
; match !r with
Some x -> x
| None -> raise (Failure"Using the wrong unwrapper")
))
end
Cheers,
--Ken
-------------------
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next prev parent reply other threads:[~2001-05-02 11:21 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-04-25 21:08 [Caml-list] " Chris Hecker
2001-04-26 0:38 ` Patrick M Doane
2001-04-26 6:04 ` Judicaël Courant
2001-04-26 12:06 ` John Max Skaller
2001-04-27 9:12 ` Anton Moscal
2001-04-29 22:24 ` Brian Rogoff
2001-04-30 18:57 ` Fergus Henderson
2001-05-01 1:31 ` Jacques Garrigue
2001-05-01 12:45 ` Ken Friis Larsen [this message]
2001-04-27 15:09 ` Brian Rogoff
2001-04-27 17:49 ` John Max Skaller
2001-04-26 8:22 ` Andreas Rossberg
2001-04-26 1:13 ` Jacques Garrigue
2001-04-26 13:47 ` Xavier Leroy
2001-04-26 22:34 ` Chris Hecker
2001-04-26 16:57 ` Mark Seaborn
2001-04-26 22:20 ` John Max Skaller
2001-05-01 21:08 ` Brian Rogoff
2001-05-01 23:30 ` John Max Skaller
2001-05-02 0:03 ` John Max Skaller
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