Re: Effective use of derived types

[Shark8 <OneWingedShark@gmail.com>]

On 25-Jul-14 10:40, Victor Porton wrote:
> I write an Ada library (in fact thick bindings to a C library), which (among
> other) deals with some strings, including URIs.
>
> Should I introduce new type for URI strings?
>
> type URI_String is new String;
>
> Or it is an overkill?


It depends on how you are going to use it.
One thing to keep in mind us that you cannot overload on subtypes, but 
can on new [derived] types.

Example:

-- The Compiler cannot distinguish which function should be called.
Function "+"(Left, Right : Natural) return Natural;
Function "+"(Left, Right : Integer) return Integer;

-- If you're dealing with new types the compiler can.
Type ShortEx is new Integer range 0..128;
Function "+"(Left, Right : Natural) return Natural;
Function "+"(Left, Right : ShortEx) return ShortEx;

-----------------------
--  Strings Example  --
-----------------------

     Package Tax is
         Type Tax_ID(<>) is private;

         -- SSN format: ###-##-####
         Subtype Social_Security_Number is String(1..11)
         with Dynamic_Predicate =>
           (for all Index in Social_Security_Number'Range =>
              (case Index is
                 when 4|7 => Social_Security_Number(Index) = '-',
               when others => Social_Security_Number(Index) in '0'..'9'
              )
           );

         -- EIN format: ##-#######
         Subtype EIN is String(1..10)
         with Dynamic_Predicate =>
           (for all Index in Social_Security_Number'Range =>
              (case Index is
                 when 3 => EIN(Index) = '-',
               when others => EIN(Index) in '0'..'9'
              )
           );

     Private
         Type Tax_ID is new String
           with Type_Invariant =>
             String(Tax_ID) in EIN or
             String(Tax_ID) in Social_Security_Number or
             raise Constraint_Error;
     End Tax;