>> class virtual figure () =
>>   object
>>     method virtual to_string: string
>>   end
>[...]
>> class screen () =
>>   object
>>     val mutable figures = []
>>     method add_figure f =
>>       figures <- (f#to_string ) :: figures
>>   end
>[...]
>> The method add_figure has type < to_string : 'a; .. > -> unit where ..
>> is unbound
>> 
>> I understand the problem, but i don't know solution, help, please.
>
>Are you sure you understood ?-)
>There's nothing in your program to tell the compiler that the argument

>    method add_figure : 'a. (#figure as 'a) -> unit = fun f ->
>      figures <- f#to_string :: figures
>
>Jacques Garrigue
thanks, i'd try (f:figure) but a need inherit.

and now is a second problem:

    method add_figure : 'a. (#figure as 'a) -> string -> unit = fun f k ->
      Hashtbl.add figures k f#to_string

work correctly, but

    method add_figure : string -> 'a. (#figure as 'a) -> unit = fun k f ->
      Hashtbl.add figures k f#to_string

return syntax error,
can i define method
add key figure = ... 
???

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