interface IConverter<T>
{
    T Convert(T x);
}

class IntConverter : IConverter<int>
{
    public int Convert(int x)
    {
        return x * 2;
    }
}

class StringConverter : IConverter<string>
{
    public string Convert(string x)
    {
        return x.ToUpper();
    }
}

static void Foo<T>(IConverter<T> conv, T x)
{
    T y = conv.Convert(x); // здесь может быть вызван и IntConverter.Convert(int), и StringConverter.Convert(string)
}

open Printf

let o1 = object 
  method put = printf "O1\n"
  end
  
  
let o2 = object 
  method put = printf "O2\n"
  end


let put o = o#put


let _  = put o1
let _  = put o2

let put o = o#put o

N>let put o = o#put o
N>

Error: This expression has type < put : unit >
       but an expression was expected of type < put : 'a -> 'b; .. > as 'a
       Types for method put are incompatible

method put x = printf "O2\n"