Re: Discriminants of tagged types

[Adam Beneschan <adam@irvine.com>]

On Oct 27, 8:58 am, Yannick Duchêne (Hibou57)
<yannick_duch...@yahoo.fr> wrote:
> Indeed, this show very well how can default discriminants be a mess (and  
> there are other reasons too to add to the ones given here).
>
> A question then come : do someone know something to which default  
> discriminants are unavoidable ? I feel default discriminants can simply be  
> dropped (kind of sugar?), but I would like to be sure.

I think that *modifiable* discriminants (which means default
discriminants in Ada syntax) would be hard to do without, in variant
record cases.  Suppose, for instance, you were using Ada to write an
interpreter for Perl or some other language that uses variables whose
type could change between an integer, a floating-point value, or a
string.  It seems natural to define a variable's value as a variant
record:

   type Value (Kind : Value_Kinds := None) is record
      case Kind is
         when Integer_Value =>    Int : Long_Integer;  -- or your
choice of integer type
         when Float_Value   =>    Flt : Float;         -- ditto
         when String_Value  =>    Str :
Ada.Strings.Unbounded.Unbounded_String;
      end case;
   end record;

Then, if the program assigns a new value to the variable and the value
has a different Kind, you'd want to be able to rewrite the value
that's stored in the symbol table entry for the variable.  In Ada you
can do this by reassigning the entire record.  Doing this without
modifiable discriminants would be, I believe, impossible without
adding some access types and a level of indirection, adding the
headaches involved with allocation, deallocation, memory leakage,
dangling references, etc.

Variant records are the main reason I use default discriminants in Ada
code.  Another typical use is to allow arrays whose size can change
dynamically:

    MAX_LENGTH : constant := 1000;
    subtype String_Length is 0 .. MAX_LENGTH;
    type Variable_String (Len : String_Length) is record
        Value : String (1 .. Len);
    end record;

                               -- Adam