2012/4/12 Alain Frisch <al...@frisch.fr>
> On 04/12/2012 04:16 PM, Philippe Veber wrote:
>
>> Isn't this a good use case for polymorphic variants too ?
>>
>
> I don't see how to use polymorphic variants here. The message bus itself
> need to provide functions like:
>
> val dispatch: message -> unit
> val register_listener: (message -> unit) -> unit
>
>
> How do you define the message type without having access to all possible
> message constructors?
I reckon you can't, but I mistakenly thought you could at least define the
bus without writing the full message type:
module Bus : sig
type 'a t
val create : unit -> 'a t
val dispatch : 'a t -> 'a -> unit
val register : 'a t -> ('a -> unit) -> unit
end
=
struct
type 'a t = ('a -> unit) list ref
let create () = ref []
let dispatch bus msg =
List.iter (fun f -> f msg) !bus
let register bus f =
bus := f :: !bus
end
let bus = Bus.create ()
let () = Bus.register bus (function `a -> print_char 'a')
let () = Bus.register bus (function `b n -> print_int n)
However this is not a legal program:
let () = Bus.register bus (function `b n -> print_int n);;
^^^^
Error: This pattern matches values of type [< `b of 'a ]
but a pattern was expected which matches values of type [< `a ]
These two variant types have no intersection
Well, I mentionned polymorphic variants because you can at least define
listeners without knowing the full message type and even reuse them for
buses with different message types. So you have a certain flexibility,
compared to monomorphic types.
But ok, maybe the remark wasn't even worth 2 cents :o).
Cheers,
ph.
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