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From: Mark Biggar <mark.a.biggar@attbi.com>
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Subject: Re: Announce: Grace project site operational
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Fraser Wilson wrote:
> 
> Mark Biggar <mark.a.biggar@attbi.com> writes:
> 
> > No, your problem is not with Standard."*", it is with the implicit
> > definition of 'function "*"(Left, Right: Metres) return Metres' that
> > you get when you defined the type Metres.
> 
> My bad; sloppy terminology.
> 
> > Ada 95 provides a way to
> > turn off such implicit definitions like so:
> >
> > type Metres is new Float;
> > -- kill unwanted ops
> > function "*"(Left, Right: Metres) return Metres is abstract;
> > function "/"(Left, Right: Meters) return Metres is abstract;
> > -- define wanted ops
> > function "*"(Left, Right: Metres) return Area;
> > function "*"(Left: Float, Right: Metres) return Metres;
> 
> This doesn't resolve the ambiguity of
> 
>    X : Metres;
>    X := 2.0 * X;
> 
> ... it just prevents you from compiling a program that makes an
> unambiguous call to the unwanted operations.

Yes it does.  The above is ambiguous because it can't decide between

(1) function "*"(L,R: Metres) return Metres;

and

(2) function "*"(L: Float, R: Metres) return Metres;

But a abstract function in not callable and the overloading
resolution in the compiler will therefor not consider it a valid
option and thus pick (2) of the above functions.  The whole
reason why Ada 95 allows direct declarations of abstract functions
of non-tagged types is to provide a way to kill off unwanted operations
like (1) above.  See the discussion in the Rational.


--
Mark Biggar
mark.a.biggar@attbi.com