Re: Types with physical dimension

[mab@dst17.wdl.loral.com (Mark A Biggar)]

In article <EACHUS.94Oct4115107@spectre.mitre.org> eachus@spectre.mitre.org (Robert I. Eachus) writes:
>In article <SAL714.94Oct3133110@rs710.gsfc.nasa.gov> sal714@rs710.gsfc.nasa.gov (Stephen A. Leake) writes:
> > Very early on, I started to write a package that enforced
> > dimensionality; I defined METERS, METERS_PER_SECOND, etc. I quickly
> > discovered that even simple equations need up to cubic dimensions,
> > in all combinations of length, mass and time.  Defining all these
> > types and the associated operators was far too time-consuming to be
> > worth it. So now I rely on code reviews and careful testing.
> > The moral; strong typing is nice, but is no substitute for careful design
> > and thorough testing.
>  Paul Hilfinger did a nice paper on using discriminants to check
>dimensionality.   I don't have the paper in front of me but it works
>something like this:
>   type Measure(Length,Mass,Time: Integer) is record
>     Value: Float;
>   end record;
>   subtype Meters is Measure(1,0,0);
>   subtype Kilograms is Measure(0,1,0);
>   subtype Time is Measure(0,0,1);
>   ...etc.
>     The only problem I had was with square roots.  I thought about
>changing the package to double all the dimensional values, so length
>would be (2,0,0), but I never did.


Unless you are doing some really wierd physics, the square root of a base
dimension doesn't really make any sense.  So just raise CONSTRAINT_ERROR
whenever any of the dimensions is odd.

--
Mark Biggar
mab@wdl.loral.com