Re: [Caml-list] generic data type -> int function

[Kim Nguyen <Kim.Nguyen@lri.fr>]

Le jeudi 24 mars 2005 à 08:38 -0800, Hal Daume III a écrit :
> Hi all --
> 
> Is there a straightforward way (or a built in function, or...) to 
> automatically map an enumerated data type to integers (and back, if 
> possible, but that's not strictly necessary).  In particular, I need 
> something like:

Hi,
	you can use the polymorphic function : Hashtbl.hash_param
	which happens to map constructors to their internal tag.
	You should be aware that this is only a (cool) side-effect of 	
	the current implementation and could change in the future.
	This is a bit of a hack but prevents you from using Obj.magic
	or automatically generating pattern matching  (which you should
	regenerate every time you change the type).


# type t = A | B | C | D of int | E of string | G;;
type t = A | B | C | D of int | E of string | G

# let to_int x = Hashtbl.hash_param 1 1 x;;
val to_int : 'a -> int = <fun>

# to_int A;; (* first constant constructor *)
- : int = 0

# to_int B;;
- : int = 1

# to_int C;;
- : int = 2

# to_int (D(42));; (* first non-constant constructor *)
- : int = 0

# to_int (E("foo"));;
- : int = 1

# to_int G;;
- : int = 3	

Regards,
-- 
Kim