Re: Ada design bug or GNAT bug?

[Shark8 <onewingedshark@gmail.com>]

On Tuesday, June 23, 2015 at 5:51:13 AM UTC-6, vincent....@gmail.com wrote:
> Le lundi 22 juin 2015 20:27:17 UTC+2, Shark8 a écrit :
> 
> > * Ada95's modular types combine the orthogonal idea of a cyclic numeric type and an unsigned type -- conceptually it could have been better to separate the two.
> 
> Could you tell us a bit more on that ? Why would it be better to separate Unsigned Integer and modular types ? I had the impression that a compiler could use a unsigned integer representation for such integer declaration :
> 
> type Unsigned is range 0 .. 2**32 -1;

Well, the simplest example would be clock time:
    Type CIVIL_HOUR is range 1..12 --...with some way to indicate cyclic.

Obviously here we have a numeric type that needs to cycle, but which has a 'First of non-zero and lies completely in POSITIVE.

> Since converting to a Standard.Integer requires anyway to use an explicit
> conversion in the Ada language because of strong typing, not mentionning run-time tests on range, I thought it would be easy for the compiler to insert conversion between signed and unsigned.

"Easy" might not be the right word, but 'doable' I think fits.

> But it seems that Ada implementations prefer to use 
> 
> type Unsigned is mod 2**32;
> 
> But what is the problem with this ? Detection of overflow ?

No, overflow is not a problem here, the MOD is indicating that the values of the type are cyclic... the problem is that this implicitly makes 'First 0 and there's no way to indicate that the cycle doesn't start at 0.

> > * The lack of a "abstract type" that could be used to unify (e.g.) [[Wide_[Wide_]]String into a single "generic"/abstract type; ditto on Character.
> 
> Same question : what would be your proposal ?

I'm not great [or even good] at language-design, but I would think something along the lines of how we contract generics via the generic's parameters only instead of operating on subprograms/packages it would be types themselves.

Perhaps something like:

   -- Analog of:
   -- Generic
   --   Type Actual is (<>);
   -- ...
   Abstract Type Abstract_Character is (<>);
   
   -- Analog of:
   -- Generic
   --   Type Element is (<>);
   --   Type Index is (<>);
   --   Type Actual is Array(Index range <>) of Element;
   -- ...
   Abstract Type Abstract_String( Positive Range <> ) of Abstract_Character;