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Date: Mon, 2 Aug 2010 09:07:04 +0200 (CEST)
From: Joseph Young <ocaml@optimojoe.com>
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Subject: Conditionals based on phantom types
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X-Spam: no; 0.00; ocaml:01 conditionals:01 sig:01 val:01 val:01 struct:01 printf:01 printf:01 module:03 module:03 float:03 float:03 unit:03 let:03 let:03 

Hi,
 	Is there any way to write a conditional based on the type 
information of a value?  Specifically, if we use phantom types to write a 
module such as

module Units : sig
     type 'a t
     val to_feet : float -> [`Feet ] t
     val to_meters : float -> [`Meters] t
     val add : 'a t -> 'a t -> 'a t
     val print : 'a t -> unit
end = struct
     type 'a t = float
     let to_feet x=x
     let to_meters x=x
     let add x y = x +. y
     let print x = Printf.printf "%f (units)" x
end;;

is there anyway to modify the print statement to correctly denote which 
units are used?

 	Thanks.

Joe