generics - 返回映射的通用函数

Question

在以下示例中，函数创建并返回有序映射:

with Ada.Containers.Ordered_Maps;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

procedure Main is
   package My_Map is new Ada.Containers.Ordered_Maps
     (Key_Type     => Natural,
      Element_Type => Unbounded_String);

   function Create_Map return My_Map.Map is
      Map_Instance : My_Map.Map;
   begin
      Map_Instance.Insert (1, To_Unbounded_String ("Foo"));
      Map_Instance.Insert (2, To_Unbounded_String ("Bar"));

      return Map_Instance;
   end Create_Map;
begin
   null;
end Main;

我想知道是否可以将函数 Create_Map 转换为更通用的函数？下面的代码显示了我的想法，但不幸的是我真的不知道正确的形式类型 ???:

   generic
      type Key_Type is ???;
      type Element_Type is ???;
      type Map_Type is ???;
   function Create_Map_Generic return Map_Type is
      Map_Instance : Map_Type;
   begin
      -- Inserts calls would be based on parsing data ...

      return Map_Instance;
   end;

   package My_Map_One is new Ada.Containers.Ordered_Maps
     (Key_Type     => Natural,
      Element_Type => Unbounded_String);

   function Create_Map_One is new Create_Map_Generic
     (Key_Type     => Natural,
      Element_Type => Unbounded_String,
      Map_Type     => My_Map_One.Map);

   package My_Map_Two is new Ada.Containers.Ordered_Maps
     (Key_Type     => Unbounded_String,
      Element_Type => Positive);

   function Create_Map_Two is new Create_Map_Generic
     (Key_Type     => Unbounded_String,
      Element_Type => Positive,
      Map_Type     => My_Map_Two.Map);

形式类型Key_Type和Element_Type被指定两次(包声明和泛型函数的实例化)。这些可以从包本身的声明中提取吗？

Answer 1

西蒙给出了更好、更完整的答案。我只是提供一种替代方案，其中 map 包作为形式参数传入。为此，您可以将包作为形式参数传递，只要它的参数也作为形式参数发送即可:

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Containers.Ordered_Maps;

procedure Hello is

    generic
        type Key_Type is private;
        type Element_Type is private;
        with function "<"(L,R : Key_Type) return Boolean is <>;
        with function "="(L,R : Key_Type) return Boolean is <>;

        with package Map_Package is new Ada.Containers.Ordered_Maps
            (Key_Type     => Key_Type,
             Element_Type => Element_Type,
             "<"          => "<",
             "="          => "=");
    function Create_Map_Generic return Map_Package.Map;        

    function Create_Map_Generic return Map_Package.Map is
    begin
        return Result : Map_Package.Map do
            null;  -- replace with your input based creation
        end return;
    end Create_Map_Generic;

    -- Just a nonsense type to test out compilation
    type My_Key is null record;
    function "<"(L,R : My_Key) return Boolean is (True);
    function "="(L,R : My_Key) return Boolean is (True);

    package Maps is new Ada.Containers.Ordered_Maps
        (Key_Type     => My_Key,
         Element_Type => Integer);

    function Create is new Create_Map_Generic
        (Key_Type     => My_Key,
         Element_Type => Integer,
         Map_Package  => Maps);

begin
  Put_Line("Hello, world!");
end Hello;

作为额外的花絮，如果您不关心函数内的键和元素类型是什么(我认为您关心，但以防万一)，那么您不必指定它们，只需执行以下操作:

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Containers.Ordered_Maps;

procedure Hello is

    generic
        with package Map_Package is new Ada.Containers.Ordered_Maps
            (others => <>);
    function Create_Map_Generic return Map_Package.Map;        

    function Create_Map_Generic return Map_Package.Map is
    begin
        return Result : Map_Package.Map do
            null;  -- replace with your input based creation
        end return;
    end Create_Map_Generic;

    -- Just a nonsense type to test out compilation
    type My_Key is null record;
    function "<"(L,R : My_Key) return Boolean is (True);
    function "="(L,R : My_Key) return Boolean is (True);

    package Maps is new Ada.Containers.Ordered_Maps
        (Key_Type     => My_Key,
         Element_Type => Integer);

    function Create is new Create_Map_Generic
        (Map_Package  => Maps);

begin
  Put_Line("Hello, world!");
end Hello;

您也可以根据您想要在创建函数中使用的内容进行折中:

generic
    type Key_Type is private;
    type Element_Type is private;
    with package Map_Package is new Ada.Containers.Ordered_Maps
        (Key_Type     => Key_Type,
         Element_Type => Element_Type,
         others       => <>);
function Create_Map_Generic return Map_Package.Map;