From: Remi VANICAT <vanicat@labri.u-bordeaux.fr>
To: caml-list@inria.fr
Subject: Re: [Caml-list] weak type variables in module type declarations
Date: 23 May 2001 15:57:55 +0200 [thread overview]
Message-ID: <ya3heycyxp8.dlv@serveur3-1.labri.u-bordeaux.fr> (raw)
In-Reply-To: <200105231324.f4NDOgc28482@mail.iis.sinica.edu.tw>
Tyng-Ruey Chuang <trc@iis.sinica.edu.tw> writes:
> Dear all,
>
> O'caml compiler does not seem to recognize user-specified
> weak type variables. The compiler, however, will automatically
> produce weak type variables when they are needed.
> This behavior has been observed and reported as "Id 246"
> in the "Known bugs" list (and classified as "not a bug"). See
> http://www.ocaml.org/bin/caml-bugs/not%20a%20bug?expression=weak;user=guest
>
> What do you do to get around this? In my case, the week type variables
> are indeed useful, and I want to keep them. But since I have no way
> in putting weak type variables in module type declarations, I am prevented
> to program in a modular way. Consider the following code segment:
>
>
> let ($) f g x = f (g x) (* functional composition *)
> let id x = x (* the identity function *)
>
> module Sum =
> struct
> type ('a, 'b) t = Pink of 'a | Blue of 'b
> let pair = (id $ (fun a -> Pink a), id $ (fun b -> Pink b))
> end
i really don't believe you want weak type here : weak type are that
will be determine latter :
if you make a
Sum.pair 1 2
then after this, Sum.pair will have the mostly monomorphic type
val pair : (int -> (int, '_b) t) * (int -> (int, '_d) t)
you may have something very similar, but without weaktype, thanks to
an eta-transformation :
module Sum =
struct
type ('a, 'b) t = Pink of 'a | Blue of 'b
let pair = ((fun x -> (id $ (fun a -> Pink a)) x),(fun x -> (id $ (fun b -> Pink b)) x));;
end
which have as type:
module Sum :
sig
type ('a, 'b) t = Pink of 'a | Blue of 'b
val pair : ('a -> ('a, 'b) t) * ('c -> ('c, 'd) t)
end
Which is probably what you want.
The problem with weak type is mostly a FAQ, you should read the FAQ
at this subject
--
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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next prev parent reply other threads:[~2001-05-23 13:57 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-05-23 13:24 Tyng-Ruey Chuang
2001-05-23 13:57 ` Remi VANICAT [this message]
2001-05-28 8:03 ` Francois Pottier
2001-05-28 12:50 ` Xavier Leroy
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