Re: Dimensions and fixed point types

["Robert I. Eachus" <rieachus@comcast.net>]

Martin Dowie wrote:

> "Duncan Sands" <baldrick@free.fr> wrote in message
> news:mailman.77.1086765132.391.comp.lang.ada@ada-france.org...
> 
>>Fixed point types have a remarkable property:
>>you can multiply any two of them to get a third.
>>You can multiply apples and oranges and get
>>bananas.  This goes against type safety.

I agree, and I objected to the original Ada95 rule that allowed the 
explict type conversion from universal fixed to be omitted.  In addition 
to the problems created when you do want to explicitly define a multiply 
operation, it creates the serious problem Duncan refers to:

>> Suppose I have several fixed point types, for
>> example
>> 
>> type Price is delta 0.01 digits ...;
>> type Volume is delta 1.0 digits ...;
>> type Value is delta 0.01 digits ...;

P: Price; V: Volume; Val: Value;
...
P := V * Val; -- the compiler won't object!

> I think this is being addressed by the ARG in AI-364
> 
> http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00364.TXT?rev=1.6
> 
> Still a work item though...

What is being worked on is actually different problem, where you do want 
to declare your own multiplication or division operators, and find that 
they can't be called in infix notation.

That having been said, the current version exactly fixes Duncan Sands 
problem.  According to the new draft, if you declare a multiplication 
operation for Apples, all of the "special" Apple * fixed and fixed * 
Apple operations returning universal_fixed will be hidden where the user 
defined operators are visible.

So to go back to Duncan's original complaint add:

function "*"(Left: Price; Right: Volume) return Value;

This will hide ALL the predefined operations that Duncan wants to get 
rid of, and any new ones that involve Price, Volume, or Value, so the line:

P := V * Val; -- The compiler will complain that no visible "*" matches.

Glad to be of service. ;-)


-- 

                                           Robert I. Eachus

"The terrorists rejoice in the killing of the innocent, and have 
promised similar violence against Americans, against all free peoples, 
and against any Muslims who reject their ideology of murder. Their 
barbarism cannot be appeased, and their hatred cannot be satisfied. 
There's only one way to deal with terror: We must confront the enemy and 
stay on the offensive until these killers are defeated." -- George W. Bush