Container Aggregates

Note

Container aggregates are supported by

  • GNAT Community Edition 2021

  • GCC 11

Ada 2022 introduces container aggregates, which can be used to easily create values for vectors, lists, maps, and other aggregates. For containers such as maps, the aggregate must use named assоciations to provide keys and values. For other containers it uses positional assоciations. Only square brackets are allowed. Here's an example:

    
    
    
        
with Ada.Text_IO;
with Ada.Containers.Vectors;
with Ada.Containers.Ordered_Maps;

procedure Main is

   package Int_Vectors is new Ada.Containers.Vectors
     (Positive, Integer);

   X : constant Int_Vectors.Vector := [1, 2, 3];

   package Float_Maps is new Ada.Containers.Ordered_Maps
     (Integer, Float);

   Y : constant Float_Maps.Map := [-10 => 1.0, 0 => 2.5, 10 => 5.51];
begin
   Ada.Text_IO.Put_Line (X'Image);
   Ada.Text_IO.Put_Line (Y'Image);
end Main;

At run time, the compiler creates an empty container and populates it with elements one by one. If you define a new container type, you can specify a new Aggregate aspect to enable container aggregates for your container and let the compiler know what subprograms to use to construct the aggregate:

    
    
    
        
procedure Main is

   package JSON is
      type JSON_Value is private
        with Integer_Literal => To_JSON_Value;

      function To_JSON_Value (Text : String) return JSON_Value;

      type JSON_Array is private
        with Aggregate => (Empty       => New_JSON_Array,
                           Add_Unnamed => Append);

      function New_JSON_Array return JSON_Array;

      procedure Append
        (Self  : in out JSON_Array;
         Value : JSON_Value) is null;

   private
      type JSON_Value is null record;
      type JSON_Array is null record;

      function To_JSON_Value (Text : String) return JSON_Value
        is (null record);

      function New_JSON_Array return JSON_Array is (null record);
   end JSON;

   List : JSON.JSON_Array := [1, 2, 3];
   ------------------------------------
begin
   --  Equivalent old initialization code
   List := JSON.New_JSON_Array;
   JSON.Append (List, 1);
   JSON.Append (List, 2);
   JSON.Append (List, 3);
end Main;

The equivalent for maps is:

    
    
    
        
procedure Main is

   package JSON is
      type JSON_Value is private
        with Integer_Literal => To_JSON_Value;

      function To_JSON_Value (Text : String) return JSON_Value;

      type JSON_Object is private
        with Aggregate => (Empty     => New_JSON_Object,
                           Add_Named => Insert);

      function New_JSON_Object return JSON_Object;

      procedure Insert
        (Self  : in out JSON_Object;
         Key   : Wide_Wide_String;
         Value : JSON_Value) is null;

   private
      type JSON_Value is null record;
      type JSON_Object is null record;

      function To_JSON_Value (Text : String) return JSON_Value
        is (null record);

      function New_JSON_Object return JSON_Object is (null record);
   end JSON;

   Object : JSON.JSON_Object := ["a" => 1, "b" => 2, "c" => 3];
   ------------------------------------------------------------
begin
   --  Equivalent old initialization code
   Object := JSON.New_JSON_Object;
   JSON.Insert (Object, "a", 1);
   JSON.Insert (Object, "b", 2);
   JSON.Insert (Object, "c", 3);
end Main;

You can't specify both Add_Named and Add_Unnamed subprograms for the same type. This prevents you from defining JSON_Value with both array and object aggregates present. But we can define conversion functions for array and object and get code almost as dense as the same code in native JSON. For example:

    
    
    
        
procedure Main is

   package JSON is
      type JSON_Value is private
        with Integer_Literal => To_Value, String_Literal => To_Value;

      function To_Value (Text : String) return JSON_Value;
      function To_Value (Text : Wide_Wide_String) return JSON_Value;

      type JSON_Object is private
        with Aggregate => (Empty     => New_JSON_Object,
                           Add_Named => Insert);

      function New_JSON_Object return JSON_Object;

      procedure Insert
        (Self  : in out JSON_Object;
         Key   : Wide_Wide_String;
         Value : JSON_Value) is null;

      function From_Object (Self : JSON_Object) return JSON_Value;

      type JSON_Array is private
        with Aggregate => (Empty       => New_JSON_Array,
                           Add_Unnamed => Append);

      function New_JSON_Array return JSON_Array;

      procedure Append
        (Self  : in out JSON_Array;
         Value : JSON_Value) is null;

      function From_Array (Self : JSON_Array) return JSON_Value;

   private
      type JSON_Value is null record;
      type JSON_Object is null record;
      type JSON_Array is null record;

      function To_Value (Text : String) return JSON_Value is
        (null record);
      function To_Value (Text : Wide_Wide_String) return JSON_Value is
        (null record);
      function New_JSON_Object return JSON_Object is
        (null record);
      function New_JSON_Array return JSON_Array is
        (null record);
      function From_Object (Self : JSON_Object) return JSON_Value is
        (null record);
      function From_Array (Self : JSON_Array) return JSON_Value is
        (null record);
   end JSON;

   function "+" (X : JSON.JSON_Object) return JSON.JSON_Value
     renames JSON.From_Object;
   function "-" (X : JSON.JSON_Array) return JSON.JSON_Value
     renames JSON.From_Array;

   Offices : JSON.JSON_Array :=
     [+["name"   => "North American Office",
        "phones" => -[1_877_787_4628,
                      1_866_787_4232,
                      1_212_620_7300],
        "email"  => "info@adacore.com"],
      +["name"   => "European Office",
        "phones" => -[33_1_49_70_67_16,
                      33_1_49_70_05_52],
        "email"  => "info@adacore.com"]];
   -----------------------------------------------------------------
begin
   --  Equivalent old initialization code is too long to print it here
   null;
end Main;

The Offices variable is supposed to contain this value:

[{"name"  : "North American Office",
  "phones": [18777874628,
             18667874232,
             12126207300],
  "email" : "info@adacore.com"},
 {"name"  : "European Office",
  "phones": [33149706716,
             33149700552],
  "email" : "info@adacore.com"}]

References