From: "Chris King" <colanderman@gmail.com>
> Okay, that (and Christophe's explanation) explains why hiding
> everything in a module works.  Thanks everyone for your help!  It
> seems I will have to use Obj.magic to get the type I want but at least
> its use in this situation seems both safe and warranted.

There are no "safe" uses of Obj.magic. It just happens to work in some
cases, but it is strongly discouraged to use it to get around the type
system, particularly when there are other ways to reach the same
result.

type 'a bar_t = Bar of <bar: ('a -> unit) -> unit> ;;
let create_bar () = Bar (object method bar (_: 'a) = () end);;
let b = create_bar ();;
let Bar b = b;;

Note that you need two definitions, as mixing the application with the
extraction would loose the polymorphism.
If you still want to use Obj.magic, remember to put complete type
annotions on both its input and output, as the lack of type
information may break the compiler.

Jacques Garrigue