Re: [Caml-list] Phantom types and variants

[Jeremy Yallop <yallop@gmail.com>]

On 22 August 2014 04:17, Daniel Bünzli <daniel.buenzli@erratique.ch> wrote:
> Sorry the coerce function type was wrong in my preceding message. Here is the example:
>
> -----
> module M : sig
>   type kind = [ `A | `B ]
>   type +'a t constraint 'a = [< kind ]
>
>   val a : unit -> [> `A] t
>   val b : unit -> [> `B] t
>   val coerce : ([< kind] as 'a) -> 'b t -> 'a t
> end = struct
>   type kind = [ `A | `B ]
>   type +'a t =
>     { kind : kind }
>     constraint 'a = [< kind]
>
>   let a () = { kind = `A }
>   let b () = { kind = `B }
>   let coerce k v = if v.kind <> k then invalid_arg "" else v
> end

The following definition does the trick (and avoids the allocation):

  let coerce (#kind as k) ({kind} as v) = if v.kind <> k then
invalid_arg "" else v