Re: Question about base types

[Mats Weber <Mats.Weber@elca-matrix.ch>]

> << << type Real is digits 7 range 0.0 .. 1.0;
> << << package Actual is new Example (Real =<< Real);
> << <<
> << << The implementation of Op needs to be able to do calculations independent
> << << of any range constraint on the actual parameter associated with the
> << << formal parameter Real. So it does its calculations using Real'Base:
> << <<
> << << function Op (Arg : Real) return Real is
> << <<    Result : Real'Base := Arg + 27.0;
> <<
> << This could fail (i.e. raise Constraint_Error), since 27.0 is not
> << required to be in the range of Real'Base. But it will work on most
> << machines because on reasonable implementations, floating point types are
> << mapped to representations that the hardware can handle efficiently (e.g.
> << IEEE). >>
> 
> This is a little misleading. While it is true that, unlike the case in
> Ada 83, where of course the base type of type Real must have a big range,
> the definitions of Ada 95 DO allow a narrow range IF that is what the
> hardware provides (I am assuming annex G is implemented). Since there are
> no machines with floating point types with 7 digits of precision and a
> range that does not include 27.0, nor could there ever be such machines,
> to worry about this eventuality is as silly as worrying about the fact
> that a legal Ada implementation could raise storage_Error for every
> possible program.

True. But if you know that you are going to need larger intermediate
results, then I think it is wiser to declare

   type Real_Base is digits 7;
   subtype Real is Real_Base range 0.0 .. 1.0;

than

   type Real is digits 7 range 0.0 .. 1.0;