Hi Jeremy, 
   Thanks for the reference! 
   The work around is a bit complex since this part is FFI code, so we may fall back to use an abstract type `'a promise` instead.
   In your comments "In an imaginary extension to OCaml
with support for groups of mutually-recursive types and classes we
could write something like this:

class ['a] container (v:'a) = object
method map : 'b. ('a -> 'b) -> 'b container_aux =
fun f -> { container = new container (f v) }
end
and 'a container_aux = { container: 'a container }", this is a nice feature I wish we could have in OCaml -- Hongbo

From: yallop@gmail.com At: 06/24/16 14:56:35
To: HONGBO ZHANG (BLOOMBERG/ 731 LEX)
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] typing question: universal type variable in class type

On 24 June 2016 at 19:22, Hongbo Zhang (BLOOMBERG/ 731 LEX)
<hzhang295@bloomberg.net> wrote:
> When we wrote bindings for promise, we want to express things (simplified )
> like below:
>
> class type ['b] promise = object
> method then_ : 'a . ('b -> 'a promise) -> 'a promise
> end
>
> It is not expressible in OCaml, The universal type variable 'a can not be
> generalized, I tried this too:
>
> type 'b promise = < then_ : 'a . ('b -> 'a u) -> 'a u >
>
> It does not work either, thanks -- Hongbo

You might find something useful in this thread:

   https://sympa.inria.fr/sympa/arc/caml-list/2015-10/msg00130.html

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