Re: Disallowing Pre-Defined Operations

[Bryce Bardin <bbardin@home.com>]

Tucker Taft wrote:
> ... Personally, I would like a full-up
> "units" capability, where generally A * A => A is not defined, since
> that doesn't balance from a units point of view.  I have some ideas
> how to get there, and would be interested in opinions from others.
> 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.

I have always felt that a full-up, type-safe, compile-time-checked, 
units capability is the one potential "killer" capability that has 
the potential to make Ada the language of choice for engineers and 
scientists world-wide.  But it must be *easy* to use (not feel 
cumbersome) and have no run-time cost.
  
I believe that it should be based on the International System of Units 
(SI), but also incorporate units conversion capabilities, so that 
commonly used systems of engineering units can be gracefully 
accommodated.  I don't think that the number of underlying relations 
would be prohibitive; see for instance:
  http://physics.nist.gov/cuu/Units/units.html,
et seq.  

If all units checking is performed in SI and the relationship
to the various systems of engineering units is implicitly known to
the compiler (or processor), dimensional checking is straightforward.  
Mixed units calculations should be forbidden, absent appropriate 
conversions. 

What remains then is to determine how to specify the units on types 
and values, and how to specify unit conversions.  It is not clear to 
me whether the basis of such a capability needs to be supplied within 
the confines of the current language, or whether a "preprocessor" 
acting on annotated source would be adequate.

Bryce Bardin