From: Dario Teixeira <darioteixeira@yahoo.com>
To: caml-list@yquem.inria.fr
Subject: Phantom types: transparency vs rogue unification
Date: Wed, 25 Jun 2008 07:26:30 -0700 (PDT) [thread overview]
Message-ID: <767974.3892.qm@web54607.mail.re2.yahoo.com> (raw)
Hi,
I've been experimenting with phantom types since I've read Richard Jones'
excellent intro to the subject [1]. I've now come across a tricky problem,
however. Essentially, I need to reconcile two seemingly incompatible goals:
making a module's phantom type opaque to avoid the "rogue unification" issue,
and making it transparent so that external modules can produce values of
that type.
So, suppose I have a "Stringies" module, which uses a phantom string to
parametrise floats. Because the phantom type is opaque, the rogue unification
is prevented in the last line:
_______________________________________________________________
module Stringies:
sig
type 'a t
val make: float -> string t
val add: string t -> string t -> string t
end =
struct
type 'a t = float
let make x = x
let add a b = a +. b
end
open Stringies
let foo = add (make 1.0) (make 2.0);;
let bar = add (make 1.0) 2.0;; (* Error! *)
_______________________________________________________________
But suppose you wish to create an external module with a function to parse
strings into Stringies.t. The phantom must become visible, but that allows
for rogue unification to take place:
_______________________________________________________________
module Stringies:
sig
type 'a t = float
val make: float -> string t
val add: string t -> string t -> string t
end =
struct
type 'a t = float
let make x = x
let add a b = a +. b
end
module Stringies_IO:
sig
val parse: string -> string Stringies.t
end =
struct
let parse = float_of_string
end
open Stringies
let foo = add (make 1.0) (Stringies_IO.parse "2.0");;
let bar = add (make 1.0) 2.0;; (* Rogue unification! *)
_______________________________________________________________
How would you go about solving this problem?
Thanks in advance!
cheers,
Dario Teixeira
[1] http://camltastic.blogspot.com/2008/05/phantom-types.html
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next reply other threads:[~2008-06-25 14:26 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-06-25 14:26 Dario Teixeira [this message]
2008-06-25 15:02 ` [Caml-list] " Jacques Carette
2008-06-25 16:29 ` Dario Teixeira
2008-06-26 21:41 ` Dario Teixeira
2008-06-27 20:35 ` Dario Teixeira
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