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:

    
    
        
pragma Ada_2022; 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:

    
    
        
pragma Ada_2022; 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