> I guess you won't be able to find such a solution. In the Ref-based
> implementation, the result arrow type is stateful. You won't be able to
> _implicitly_ copy a state _inside_ the language, otherwise first-class
> continuation would already appear.

That's why I asked this to the Caml-list : to find a black magic recipe :-)

> Since the corresponding mechanics in Ref and CPS are mutation and
> closure (de)construction, no wonder the former is faster. One ad-hoc
> solution, if you only have to provide high-level functional API to
> outside, is to make states explicit, like
>
> type ('a,'b) arrow = {mutable state: 'c; eval: 'c -> 'a -> 'b}
>
> This is not valid OCaml, since the 'c type is out of scope. You can
> either encode it with existential type (which also involve closure
> (de)construction, but I'm not sure about the cost). Or simply using
> Obj.t instead of 'c as far as you won't expose it, at least it won't be
> unsafer than Marshal and faster.

This solution looks promising, I will look at it and see if it suits my needs.

Thanks,

-- 
Pierre-Evariste DAGAND