Re: [Caml-list] define incompatible type

[David Rajchenbach-Teller <David.Teller@univ-orleans.fr>]

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     Hi Grégoire,
 It's not directly possible in OCaml, but there are at least three methods for doing what you want.

The first one is to wrap your integers behind a constructor, e.g.

   type user_id = User of int
   type movie_id = Movie of int

   let a = User 57 and b = Movie 80 in
   if a = b then ...

This is the technique often used by Haskellites. Variant on the topic: use singleton records instead of singleton sums.


The second one is to make use of modules and abstract types, e.g.

  module User =
  struct
     type id = int
     let id_of_int x = x
   ...
  end :
  sig
     type id (*Abstract type*)
     val id_of_int : int -> id
  end
  (*same for b*)
  let a = User.id_of_int 57 and b = User.id_of_int 80 in
  if a = b then ...

 This is probably the most common technique in OCaml, as it fits well with functorization.


Finally, you can use a phantom type, e.g.

  type 'a id = {id: int} (*Type argument used only to differentiate between various kinds of ids*)
  type user                 (*This type has no inhabitant, don't worry, it's only for coercions*)
  type movie              (*same here*)

  let a = {id:57} : user id and b = {id:80}: movie id in
  if a = b then ...

It's an elegant technique, which I personally like, but which can sometimes cause puzzling error messages if you forget coercions.


I hope this helps,
 Regards,
  David


On Feb 12, 2010, at 7:32 AM, Grégoire Seux wrote:

> hello !
> 
> i would like to create two types and use the type checker to verify the "meaning" of my programs:
> 
> type user_id = int
> type movie_id = int
> 
> i'd like if the type checker would warn me if i write something that is non-sense:
> let a:user_id = 57 and b:movie_id = 80 in
> if a=b then ...
>  
> because this is obvioulsy a mistake
> 
> do you know if is this possible ?
> thanks by advance !
> 
> 
> -- 
> Grégoire
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