> 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
to add_figure is supposed to be a figure.

    method add_figure (f : figure) =
      figures <- f#to_string :: figures

Or, if you want to avoid coercions when calling #add_figure,

    method add_figure : 'a. (#figure as 'a) -> unit = fun f ->
      figures <- f#to_string :: figures

Jacques Garrigue
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