Ada 访客设计模式和泛型

问：

我正在实现一个访问者模式，并且我有一些元素可以使用泛型实现，但 GNAT 编译器抱怨泛型类型。 我有一个使用通用混合的解决方案，但它不太直观。

我该如何解决？

我提供了以下最小的可重复示例：

1. 访客.ads

with ConcreteElements;
with Instantiation;

package Visitors is

  type Visitor_t is interface;

  procedure pVisit (this : in Visitor_t;
                    element : ConcreteElements.ConcreteElement_t) is abstract;

  procedure pVisit (this : in Visitor_t;
                    element : Instantiation.GenericElement_t) is abstract;

end Visitors;

2. 元素.ads

limited with Visitors;

package Elements is

  type Element_t is abstract tagged null record;

  procedure pAccept
    (this : in Element_t;
     visitor : in Visitors.Visitor_t'Class) is abstract;

end Elements;

3. concreteelements.ads/adb

limited with Visitors;
with Elements;

package ConcreteElements is

  type ConcreteElement_t is new Elements.Element_t with null record;

  overriding
  procedure pAccept
    (this : in ConcreteElement_t;
     visitor : in Visitors.Visitor_t'Class);

end ConcreteElements;

with Visitors;

package body ConcreteElements is

  procedure pAccept
    (this : in ConcreteElement_t;
     visitor : in Visitors.Visitor_t'Class) is
  begin
    visitor.pVisit(this);
  end pAccept;

end ConcreteElements;

4. genericelements.ads/adb

with Elements;
limited with Visitors;

generic
  type Parent_t (<>) is abstract new Elements.Element_t with private;
package GenericElements is

  type GenericElement_t is new Parent_t with null record;

  overriding
  procedure pAccept (this : in GenericElement_t;
                     visitor : in Visitors.Visitor_t'Class);

end GenericElements;

with Visitors;

package body GenericElements is

  procedure pAccept (this : in GenericElement_t;
                     visitor : in Visitors.Visitor_t'Class) is
  begin
    visitor.pVisit(this);
  end pAccept;

end GenericElements;

5. 实例化.ads

with GenericElements;
with Elements;

package instantiation is new GenericElements
  (Parent_t => Elements.Element_t);

编译器在 4） 的正文中抱怨，在第 9 行：

预期类型“实例化。GenericELement_t“在 genericelements.ads：8 中定义

找到在 genericelements.ads：8 中定义的类型“GenericElements.GenericElement_t”

我的解决方案是执行混合，使GenericElement_t抽象，因此这将是 1）、4） 和 5）：

1)

with ConcreteElements;
with Instantiation;

package Visitors is

  type Visitor_t is interface;

  procedure pVisit (this : in Visitor_t;
                    element : ConcreteElements.ConcreteElement_t) is abstract;

  procedure pVisit (this : in Visitor_t;
                    element : Instantiation.Instantiation_t) is abstract;

end Visitors;

with Elements;

generic
  type Parent_t (<>) is abstract new Elements.Element_t with private;
package GenericElements is

  type GenericElement_t is abstract new Parent_t with null record;

end GenericElements;

private with GenericElements;
limited with Visitors;
with Elements;

package instantiation is 

  type Instantiation_t is new Elements.Element_t with private;

  overriding
  procedure pAccept (this : in Instantiation_t;
                     visitor : Visitors.Visitor_t'Class);

private

  package instantiation_pck is new GenericElements 
    (Parent_t => Elements.Element_t);

  type Instantiation_t is new instantiation_pck.GenericElement_t with null record;

end instantiation;

with Visitors;

package body instantiation is

  procedure pAccept (this : in Instantiation_t;
                     visitor : Visitors.Visitor_t'Class) is
  begin
    visitor.pVisit(this);
  end pAccept;


end instantiation;

我可以正确实现第一个选项，还是应该使用混合来实现它？

提前感谢您，并对代码的大量使用表示歉意。