User-Defined Literals

Note

User-defined literals are supported by

  • GNAT Community Edition 2020

  • GCC 11

In Ada 2022, you can define string, integer, or real literals for your types. The compiler will convert such literals to your type at run time using a function you provide. To do so, specify one or more new aspects:

  • Integer_Literal

  • Real_Literal

  • String_Literal

For our example, let's define all three for a simple type and see how they work. For simplicity, we use a Wide_Wide_String component for the internal representation:

    
    
    
        
with Ada.Wide_Wide_Text_IO;
with Ada.Characters.Conversions;

procedure Main is

   type My_Type (Length : Natural) is record
      Value : Wide_Wide_String (1 .. Length);
   end record
     with String_Literal => From_String,
       Real_Literal      => From_Real,
       Integer_Literal   => From_Integer;

   function From_String (Value : Wide_Wide_String) return My_Type is
      ((Length => Value'Length, Value => Value));

   function From_Real (Value : String) return My_Type is
      ((Length => Value'Length,
        Value  => Ada.Characters.Conversions.To_Wide_Wide_String (Value)));

   function From_Integer (Value : String) return My_Type renames From_Real;

   procedure Print (Self : My_Type) is
   begin
      Ada.Wide_Wide_Text_IO.Put_Line (Self.Value);
   end Print;

begin
   Print ("Test ""string""");
   Print (123);
   Print (16#DEAD_BEEF#);
   Print (2.99_792_458e+8);
end Main;

As you see, real and integer literals are converted to strings while preserving the formatting in the source code, while string literals are decoded: From_String is passed the specified string value. In all cases, the compiler translates these literals into function calls.

Turn Ada into JavaScript

Do you know that '5'+3 in JavaScript is 53?

> '5'+3
'53'

Now we can get the same result in Ada! But before we do, we need to define a custom + operator:

    
    
    
        
with Ada.Wide_Wide_Text_IO;
with Ada.Characters.Conversions;

procedure Main is

   type My_Type (Length : Natural) is record
      Value : Wide_Wide_String (1 .. Length);
   end record
     with String_Literal => From_String,
       Real_Literal      => From_Real,
       Integer_Literal   => From_Integer;

   function "+" (Left, Right : My_Type) return My_Type is
     (Left.Length + Right.Length, Left.Value & Right.Value);

   function From_String (Value : Wide_Wide_String) return My_Type is
      ((Length => Value'Length, Value => Value));

   function From_Real (Value : String) return My_Type is
      ((Length => Value'Length,
        Value  => Ada.Characters.Conversions.To_Wide_Wide_String (Value)));

   function From_Integer (Value : String) return My_Type renames From_Real;

   procedure Print (Self : My_Type) is
   begin
      Ada.Wide_Wide_Text_IO.Put_Line (Self.Value);
   end Print;

begin
   Print ("5" + 3);
end Main;

Jokes aside, this feature is very useful. For example it allows a "native-looking API" for big integers.

References