From: Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>
To: c_bauer@informatik.uni-kl.de
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Type inference + optional parameters
Date: Wed, 03 Sep 2003 11:08:13 +0900 [thread overview]
Message-ID: <20030903110813O.garrigue@kurims.kyoto-u.ac.jp> (raw)
In-Reply-To: <m3ptij88n1.fsf@diebuntekuh.de>
From: Christoph Bauer <c_bauer@informatik.uni-kl.de>
> ocamls type inference uses information of optionl arguments. This
> results in a strange behaviour.
Brian Rogoff already answered this one. Do you seriously expect a
type-safe compiler to simply ignore the type of the default value :-)
> Here is an example:
>
> Objective Caml version 3.07+beta 2
>
> # let do_with_opt_conv ?(conv = fun s -> s) a = conv a;;
> do_with_opt_conv ~conv:string_of_int "1";;- : int = 1
> # val do_with_opt_conv : ?conv:('a -> 'a) -> 'a -> 'a = <fun>
> # do_with_opt_conv ~conv:string_of_int "1";;
>
> Is there a solution? It is not possible to
> add the "right" type information:
>
> # let do_with_opt_conv ?(conv : 'a -> 'b = fun s -> s) (a:'a) : 'b = conv a;;
> val do_with_opt_conv : ?conv:('a -> 'a) -> 'a -> 'a = <fun>
The answer is that the function you want cannot be defined with
optional arguments.
Actually, we need a slightly more expressive type to allow this.
module Default : sig
type ('a,'b) t
val default : ('a,'a) t
val custom : 'a -> ('a,'b) t
val get : (unit -> 'b) -> ('a,'b) t -> 'a
end = struct
type ('a,'b) t = Custom of 'a | Default
let default = Default
let custom x = Custom x
let get def = function
Default -> Obj.magic (def ())
| Custom x -> x
end
Note that you need to use Obj.magic to define the above module, but
the type of [default] ensures that this is sound.
Then you can define your function:
# let do_with_opt_conv ~conv a =
let conv = Default.get (fun () x -> x) conv in
conv a ;;
val do_with_opt_conv : conv:('a -> 'b, 'c -> 'c) Default.t -> 'a -> 'b
# do_with_opt_conv ~conv:Default.default 1;;
- : int = 1
# do_with_opt_conv ~conv:(Default.custom string_of_int) 1;;
- : string = "1"
Jacques Garrigue
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next prev parent reply other threads:[~2003-09-03 2:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-09-02 16:20 Christoph Bauer
2003-09-02 17:06 ` brogoff
2003-09-03 2:08 ` Jacques Garrigue [this message]
2003-09-03 8:08 ` Alain.Frisch
2003-09-06 0:53 ` Jacques Garrigue
2003-09-03 9:56 ` Michal Moskal
[not found] <m3fzjf825m.fsf@diebuntekuh.de>
2003-09-02 19:01 ` brogoff
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