From: Martin Jambon <martin_jambon@emailuser.net>
To: yminsky@cs.cornell.edu
Cc: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>, <caml-list@inria.fr>
Subject: Re: [Caml-list] When is a function polymorphic?
Date: Wed, 30 Mar 2005 17:26:38 -0800 (PST) [thread overview]
Message-ID: <Pine.LNX.4.44.0503301719080.3339-100000@localhost> (raw)
In-Reply-To: <891bd339050330165142478f37@mail.gmail.com>
On Wed, 30 Mar 2005, Yaron Minsky wrote:
> I think I screwed up the original examples a bit. I think the effect
> I'm looking at doesn't depend on the "as" notation in particular.
> Here's another example that doesn't use "as":
>
> # function Some x -> Some () | None -> None;;
> - : 'a option -> unit option = <fun>
> # function Some x -> Some () | x -> x;;
> - : unit option -> unit option = <fun>
>
> The reason I want this is for the following example. Consider some
> complicated union type with a single parameter:
>
> type 'a foo = A of 'a | B of int | C of string * string | ... | ZZ of float
>
> I want a function that converts an 'a foo to a unit foo. I tried to
> write it this way:
>
> function A _ -> A () | x -> x
>
> But this ends up having type: unit foo -> unit foo, which isn't what I
> want at all. Any idea of how to achieve this cleanly?
With polymorphic variants, you can do something like this:
# type const = [ `B of int
| `C ];;
type const = [ `B of int | `C ]
# type 'a poly =
[ `A of 'a
| const ];;
type 'a poly = [ `A of 'a | `B of int | `C ]
# let f : 'a poly -> unit poly= function
`A _ -> `A ()
| #const as x -> x;;
val f : 'a poly -> unit poly = <fun>
Martin
--
Martin Jambon, PhD
http://martin.jambon.free.fr
next prev parent reply other threads:[~2005-03-31 1:26 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-03-30 22:31 Yaron Minsky
2005-03-31 0:37 ` [Caml-list] " Jacques Garrigue
2005-03-31 0:51 ` Yaron Minsky
2005-03-31 1:15 ` Eric Cooper
2005-03-31 1:26 ` Martin Jambon [this message]
2005-03-31 2:42 ` Jacques Garrigue
2005-03-31 4:04 ` Yaron Minsky
2005-03-31 8:32 ` Jacques Garrigue
2005-03-31 12:04 ` Yaron Minsky
2005-03-31 12:48 ` Fermin Reig
2005-03-31 12:16 ` Jon Harrop
2005-04-01 16:26 ` Luc Maranget
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