[Caml-list] Objects and polymorphic variants

[Reed Wilson <cedilla@gmail.com>]

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Greetings list,

I've been trying to make object methods that return polymorphic variants,
but it looks a bit stricter than I'm used to.
For example, take the following methods:

method private method_12 = function
| 1 -> `One
| _ -> `Two

method method_123 = function
| 3 -> `Three
| x -> self#method_12 x

method method_124 = function
| 4 -> `Four
| x -> self#method_12 x

This returns the type:
method private method_12 : int -> ([ > `Four | `One | `Three | `Two ] as 'a)
method method_123 : int -> 'a
method method_124 : int -> 'a

and complains that method_123 uses 'a, which is undefined. That's fair
enough, since it's an open type in a non-private method.
However, if I give method_123 or method_124 a closed type signature it will
give the same exact type to the other two methods.

What I really want is a signature like this:
method private method_12 : int -> [ > `One | `Two ]
method method_123 : int -> [ `One | `Two | `Three ]
method method_124 : int -> [ `One | `Two | `Four ]

If I replace method_12 with a function outside the class it works fine, but
for whatever reason method_12 really wants to be the exact same type as
method_123 and method_124.

Is there any way around this typing requirement for methods?


Thanks for any help,
Reed Wilson


PS. I'd rather avoid doing any verbose "post-processing" of the method_12
result, like this:
method_123 = function
| 3 -> `Three
| x -> (match self#method_12 with `One -> `One | `Two -> `Two)


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