Re: Announce: Grace project site operational

[tmoran@acm.org]

> That's the biggie. If you try to make separate types for velocities
> and accelerations, you will suddenly find that you also need separate
> types for times and distances, and perhaps even Hz and mass and forces
> and rotational orientations and velocitites and accelerations. Then
> you'll find how annoying it is to have to do type conversions on every
> factor in every formula. A more attractive option may be to write your
> own math operators that return the correct type given their two input
> types. But that causes an exponential growth in operators that you
> have to create every time you add a new unit.
  How about "usually use Standard.Float but rarely use Standard.Integer".
Most of the times expressions mix types they are physical things, and
typically floating point, while integers are usually used for things
like counts and subscripts, where mixed expressions are rare.

> ... automaticly generates the proper new type given the operator and
> the types of its operands.
  How about a(nother) new Ada 0x construct:
     dimension <real type> <multiplying operator> <real type>
optionally appended to a type declaration, eg
type Seconds is new Float;
type Centimeters is new Float;
type Grams is new Float;
type Velocity is new Float dimension Centimeters/Seconds;
type Acceleration is new Float dimension Velocity/Seconds;
type Dynes is new Float dimension Grams*Acceleration;
 t : Seconds;
 d : Centimeters range 0.0 .. 100.0;
 v : Velocity;
 f : Dynes;
  Then have the compiler allow an expression like "v*t" and have it
do the dimensional analysis at compile seconds to know the result is
of some declared type, in this case Centimeters.  v+t is still
caught as an error and, since neither a square centimeter nor an erg
type have been declared, expressions like d*d and f*d are caught.

> Generally, what I see Ada folk do is use *subtypes* for all this
  That's really orthogonal.  In a program about electric cars you
should be able to say
type Miles is digits 4 range 0.0 .. 100.0;
type Hours is new Float range 0.0 .. 2.0;
type Speed is new Float range 0.0 .. 60.0 dimension Miles/Seconds;