Re: [Caml-list] do i need a private row type?

[Goswin von Brederlow <goswin-v-b@web.de>]

Joel Reymont <joelr1@gmail.com> writes:

> https://github.com/wagerlabs/ocaml-zmq/blob/master/src/ZMQ.ml
>
> Looking at the Poll module at the bottom and the definition of poll_item as
>
>   type 'a poll_item = ('a Socket.t * event_mask)
>
> I would like to be able to put different 'a in the same poll_item array. 
>
> This is not possible as written, e.g. 
>
> Error: This expression has type
>          ZMQ.Socket.pub ZMQ.Socket.t * ZMQ.Poll.event_mask
>        but an expression was expected of type
>          ZMQ.Socket.pull ZMQ.Socket.t * ZMQ.Poll.event_mask
>
> How do I type poll_item and poll_item array so that the array is polymorphic with respect to 'a?
>
> Does the definition of Socket a the top of ZMQ.ml need to be changed to make this possible?
>
> 	Thanks, Joel

How clean do you want this to be? Lets go with easy and dirty.

First thing is that if you have different 'a poll_items then you need to
include a callback that accepts a matching 'a Socket.t. E.g.:

type 'a poll_item = ('a Socket.t * event_mask * ('a Socket.t -> event_mask -> unit))

Then you can make an unit poll_item array and use

let set i (item : 'a poll_item) =
    array.[i] <- Obj.magic item

let call i =
    let (sock, mask, fn) = array.[i]
    in
    fn sock mask

The important part here is that you ensure the 'a Socket.t is only
accessed through the callback. Only that will have the right type.


If you want to make it clean then you need a 2 layer approach with 2
records and 'a. 'a -> 'b style types. I can't remember how to do this
from memory but search the list archive for something like universal
container. I've asked the same a few years back.

MfG
        Goswin