使用variant记录的子类型访问实例化包时出现问题

Question

我尝试用Ada2012编译器编译Ada95程序。但是，通用包Garbage_Collector的实例化存在问题。包实例化中不接受子类型A_Term：

prolog.adb:27:15: designated type of actual does not match that of formal "Link"
prolog.adb:27:15: instantiation abandoned

我曾尝试将A_Term更改为type A_Term is access A_Node;。然后，程序包将实例化，但其余代码会中断。从Ada 95开始有什么改变了吗?我如何让它在Ada 2012中工作？

procedure Prolog is

   generic
      type Item is limited private;
      type Link is access Item;
   package Garbage_Collector is
      procedure Get (L : in out Link) is null;
   end Garbage_Collector;

   type Node_Tag is (A, B);

   type Node (Tag : Node_Tag);
   type Term is access Node;

   type Node (Tag : Node_Tag) is
      record
         case Tag is
            when A => null;
            when B => null;
         end case;
      end record;

   subtype A_Node is Node (A);
   subtype A_Term is Term (A);
   package Gc_A is new Garbage_Collector
     (Item => A_Node,
      Link => A_Term);

   T : Term;
begin
   Gc_A.Get (T);
end Prolog;

代码来自斯坦福大学的Prolog模块。The project on GitHub

Answer 1

我不能说到底是什么导致了你的错误。这很可能是一个编译器错误。为了以最小的侵入性方式解决这个问题，我建议使access关系对通用单元不可见：

procedure Prolog is

   generic
      type Item is limited private;
      type Link is private;
      with function Deref (L : Link) return Item is <>;
   package Garbage_Collector is
      procedure Get (L : in out Link) is null;
   end Garbage_Collector;

   type Node_Tag is (A, B);

   type Node (Tag : Node_Tag);
   type Term is access Node;

   type Node (Tag : Node_Tag) is
      record
         case Tag is
            when A => null;
            when B => null;
         end case;
      end record;

   subtype A_Node is Node (A);
   subtype A_Term is Term (A);
   
   function Deref (L : A_Term) return A_Node is (L.all);
   
   package Gc_A is new Garbage_Collector
     (Item  => A_Node,
      Link  => A_Term);

   T : Term;
begin
   Gc_A.Get (T);
end Prolog;

现在编译器没有抱怨，因为形式类型Link不再是一种访问类型，并且与Item没有关系。通过赋予函数Deref，您仍然能够取消对Link类型的对象的引用。如果你需要给它赋值，你需要一个额外的函数。

Answer 2

在上面的代码示例中，声明了Node和Term类型

type Node (Tag : Node_Tag);
type Term is access Node;

然后，您尝试声明两个子类型：

subtype A_Node is Node (A);
subtype A_Term is Term (A);

因为Node是一个判别式类型，所以声明subtype A_Node是有意义的。subtype A_Term的声明没有任何意义。Term类型为access Node，与Node类型不同。尝试将subtype A_Term的声明更改为：

subtype A_Term is access A_Node;