On Tue, 3 Aug 2010, bluestorm wrote:
On Tue, Aug 3, 2010 at 8:15 AM, Joseph Young <oc...@optimojoe.com> wrote:
module Units : sig
type t
val to_feet : float -> [`Feet]*t
val 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 =
function (`Feet, t) -> (`Meters, t)
You are correct and that could be problematic. Combining the two
ideas from above gives:
type units=[`Feet | `Meters];;
let unit_to_string=function
| `Feet -> "ft"
| `Meters -> "m"
;;
module Units : sig
type 'a t
val to_feet : float -> ([`Feet] as 'a)*'a t
val to_meters : float -> ([`Meters] as 'a)*'a t
val add : (([<units] as 'a)*'a t as 'b) -> 'b -> 'b
val print : ([<units] as 'a)*'a t -> unit
end = struct
type 'a t=float
let to_feet x=`Feet,x;;
let to_meters x=`Meters,x;;
let add (u,x) (_,y) = u,x +. y;;
let print (u,x)=Printf.printf "%f (%s)" x (unit_to_string u);;
end
which hopefully insures that the exposed and hidden unit are required to
be the same in order to use these functions. To be sure, functions such
as:
let convert (u,x:[ `Feet ] * 'a Units.t) : [ `Meters ] * 'a Units.t =
`Meters,x
are possible. However, I believe that the type checker should throw an
error if the above functions are called on converted values. Mostly, I'm
trying to force the type checker to insure there are valid values without
using runtime assertions as you suggested above.
Thanks again for the help.
Joe_______________________________________________
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