The problem with this solution, if I understand it correctly, is that it
amounts to losing type information. Your values begin with some type
information attached to them, that allows more fine-grained typing of some
information (subscription), but then you coerce them into a big magma of
"everything" to put them in the array, and the type information is lost.
When you get back an element of this array, you don't know anymore if it was
a `Pair or a `Sub for example. You probably won't be able to use the more
restrictive operations of your interface.

A solution to this problem would be to tag each item with its provenance
type, so that you can regain fine-grained types dynamically :

type any_poll =
  | Pair of pair poll
  | Sub of sub poll
  | ...

Then, given an array of `any_poll` elements, you can pattern match and, if
it's a `Sub`, you have a `sub poll` available. I think this is more precise
than your current solution.
More generally, I don't think the polymorphic variants bring anything more
that what you could have with simple abstract types here. For example to
coerce everything to a common type with the `pair poll`, `sub poll` solution
you could define a `any` abstract datatype and provide a `val lose_type_info
: 'a poll -> any poll` function. Unless I'm mistaken, this is equivalent to
your current solution, and shields you of any superfluous polymorphic
variant oddities.

On Mon, May 16, 2011 at 3:02 PM, Joel Reymont <joelr1@gmail.com> wrote:

> Issue solved thanks to Anil Madhavapeddy.
>
> val pair   : [>`Pair] kind
> val pub    : [>`Pub] kind
>
> etc.
>
> and then
>
> type poll_socket = [`Pair|`Pub|`Sub|`Req|`Rep|`Dealer|`Router|`Pull|`Push]
> Socket.t
> type poll_item = (poll_socket * event_mask)
>
> val of_poll_items : poll_item array -> t
>
> This does it!
>
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>
>
>