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From: bluestorm <bluestorm.dylc@gmail.com>
Date: Tue, 3 Aug 2010 08:57:19 +0200
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Subject: Re: [Caml-list] Conditionals based on phantom types
To: Joseph Young <ocaml@optimojoe.com>
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X-Spam: no; 0.00; conditionals:01 ocaml:01 sig:01 val:01 val:01 wrote:01 abstract:01 abstract:01 caml-list:01 module:03 module:03 float:03 float:03 unit:03 let:03 

On Tue, Aug 3, 2010 at 8:15 AM, Joseph Young <ocaml@optimojoe.com> wrote:
> module Units : sig
> =A0 =A0type t
> =A0 =A0val to_feet : float -> [`Feet]*t
> =A0 =A0val to_meters : float -> [`Meters]*t
>    [..]
> end

With this type, the value representation of units is not abstract
anymore (and it is not very useful to have still t abstract). The user
can analyse the values you give him and access the unit member. This
is the difference with the ('a t) representation were the relation
between 'a and values is hidden outside the module.

For example, your user can write something like that, wich could be
indesirable :

let convert [ `Feet ] * Units.t -> [ `Meters ] * Units.t =3D
  function (`Feet, t) -> (`Meters, t)