Re: Fixed point multiplication ambiguity

[robert_dewar@my-dejanews.com]

In article <F696u2.Cp.0.-s@inmet.camb.inmet.com>,
  stt@houdini.camb.inmet.com (Tucker Taft) wrote:

> The whole fixed-point multiplication model is pretty
> obscure, unfortunately.

I agree with Tuck's analysis in its entirety. Note that
the model may be a bit obscure, but the normal practice
of qualifying intermediate results returned by
multiplication using an explicit conversion removes
this obscurity.

I actually find the implicit use of Duration MOST
unfortunate. GNAT finds it worrying enough to emit a
warning:

  1. procedure Tf is
  2.     D : Duration := 5.0;
  3.     Dec_Delt : Integer := Integer (D * 10.0 + 0.5)
                                                 |
   >>> warning: Universal-fixed expression interpreted as
       type "Standard.Duration"

  4. begin
  5.    null;
  6. end;

Since it really seems strange to have Duration creeping
in here. Note that this is NOT related to the declaration
of D as being of type Duration, it would happen if D were
of some other fixed point type, as long as it was not a
visible type at the point of the expression.

If two fixed-point types *are* visible, then GNAT says

  1. procedure Tf2 is
  2.     type r is delta 1.0 range -10.0 .. +10.0;
  3.     D : r := 5.0;
  4.     Dec_Delt : Integer := Integer (D * 10.0 + 0.5);
                                                 |
     >>> ambiguous universal_fixed_expression
     >>> possible interpretation as type Standard.Duration"
     >>> possible interpretation as type "r" defined at
         line 2

     5. begin
     6.    null;
     7. end;

It could still confuse a beginner :-) but it is hard to
see how one can do better error-message wise!


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