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From: Patrick Hohmeyer <pi3_1415926536@yahoo.ca>
Subject: Re: Another Idea for Ada 20XX
Newsgroups: comp.lang.ada
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Date: Sun, 2 Dec 2001 16:19:08 -0500
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Date: 2001-12-02T16:19:08-05:00
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James Rogers wrote :

> In scientific terms a measurement is done in terms of some unit.
> For instance, distance is in meters, microns, feet or furlongs.
> Speed is then calculated as a distance per unit time. Acceleration
> is the second derivative of speed.

Acceleration is the second derivative of *distance*. ;-)

> The advantage for the software developer is in the usage:
> 
> Vehicle_Speed : Speed;
> Track_Distance : Meters;
> Lap_Time : Hours;
> 
> Speed := Track_Distance / Lap_Time;
> 
> This approach would allow the appropriate mixing of data types, which
> is currently forbidden by Ada, in a meaningful and correct manner.
> 

Hmm, this can very easly be achived with overloading of "/" :

function "/" (Dist : Meters; Time : Hours) return Speed is
--...

And I have a huge problem with the name of the type Speed.
Speed is not a unit, but an concept.

Would be more interesting :

type Distance is basic concept with basic unit Meters;
type Time is basic concept with basic unit Seconds;
type Speed is concept (Distance / Time);

Vehicle_Speed : Speed; -- ok
Track_Distance1 : Meters; -- ok
Track_Distance2 : Meters := 12.5; -- ok
Lap_Time1 : Time; -- ok
Lap_Time2 : Time := Seconds(34.3); -- ok

Lap_Time3 : Time := 34.3; 
-- 2 possibles reactions :
-- 1. won't compile as the unit isn't specified !!! (my favor)
-- 2. uses the basic unit for the concept (Seconds)
--     (but generates a compiler warning)

Vehicle_Speed := Track_Distance1 / Lap_Time1; --ok

Internally a concept type would be something as
record
  Value : Double;
  Unit : Unit_type;
end record;

Unit somehow describes how we can transform the value
into a value in the basic units. (an access to a function?)
and vice-versa.

The great part are unit extentions :

type Hours is Time unit 3600 Seconds;

S : Seconds := 3954.3;
H : Hours := S;

And the compiler would automagicly put 3954.3 / 3600 into H.
As would the cast Hours(S) return 3954.3 / 3600;

Especially for Space programs between US and Europe this would
be a *huge* benefit.
Just declare :

type Inchs is Distance unit x Meters;

and the compiler will do all unit conversions for you.

And of course :
type Temperature is basic concept with basic unit Kelvin;
type Celsius is Temperature unit 1 Kelvin + 273.15;

And a non-basic concept : 
type Mass is basic concept with basic unit Kilogram;
type Acceleration is concept Speed / Time;
type Force is concept Mass * Acceleration;

type Newton is Force unit 1 (Kilogram * Meters * Seconds**(-2)); 
(Note that :
  type Newton is Force unit 1
  (Kilogram * Meters * Seconds**(-1) * Hours**(-1));
would also be syntaxly correct, you can mix 2 time units in a
composed unit.)

I think that this could be fun, but may be to complicated to
implement and if you really need it, can already be done with
a record and an access to fonction for the unit type,
and a lot of overloaded functions.

But what do you think ?

-- 
Patrick Hohmeyer