Re: Disallowing Pre-Defined Operations

[Charles Hixson <charleshixsn@earthlink.net>]

Re: Units:
mks? cgs? or English System? (And is a gallon an Imperial Gallon or a US
gallon?)  Are angles measures in degrees, radians, or mils? (etc.)

This feels like a very important issue to me, but if there is a generally
applicable solution, I still don't know it.  The closest I've come it to
defining, say, Meters to be a particular data type descended from (Numeric...
choose the appropriate flavor) that implements a toString feature.
Of course then one doesn't have the operators defined until one does something
like
function "/"(x : Meters; y : Numeric) return Meters;
et-multitudinous-cetera.

Paul Graham wrote:

> Tucker Taft wrote:
> >
> > "Charles H. Sampson" wrote:
> > > ...
> > > >You'll have to post the exact code if we're to understand what's going
> > > >on there (all of the ops declared, and the supposedly ambiguous
> > > >statement).  And the error message.
> > >
> > >      O. K., here's an example.  It's not meant to meaningful, just to
> > > show the problem, although I hope that the name for the integer variable
> > > suggests how it could be used meaningfully.  This example has been
> > The techniques using discriminants are unsatisfactory in my view,
> > and too "heavy."  VHDL has the notion of "units" built into the language,
> > and its approach should probably be evaluated.
>
> In VHDL units are mainly used for type TIME.  Things get complicated
> when you
> try to map the physical world onto unit data types.  For instance:
>
>     type time is ...
>     type distance is ...
>     type velocity is ...
>     type acceleration is ...
>     type mass is ...
>
>     function "/"(x : distance; y : time) return velocity;
>     function "/"(x : velocity; y : time) return acceleration;
>     function "*"(x : mass; y : acceleration) return force;
>     function "*"(x : force; y : distance) return work;
>     ...
>
> Anyway, you can see that the set of units and possible operations
> between units
> grows very quickly.  I wonder how big a complete physical types package
> would
> be (complete enough to represent all the equations in a physics or
> electonics
> textbook for instance)?
>
> Paul