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From: Fraser Wilson <newsfraser@blancolioni.org>
Newsgroups: comp.lang.ada
Subject: Re: Announce: Grace project site operational
Date: 17 May 2002 17:03:39 +0200
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Date: 2002-05-17T15:02:20+00:00
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Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov> writes:

> Darren New <dnew@san.rr.com> writes:
> 
> > Well, it does just seem to me pretty trivial to write a program that
> > takes something like
> > 
> > =distance
> > =time
> > velocity=distance/time
> > area=distance*distance
> > volume=distance*distance*distance
> > 
> > and turn such a file into the appropriate set of Ada type and function
> > declarations, whether it's 50 lines or 5000 lines, that when "with"ed
> > and "used" could make for safe unit conversions. 
> 
> I agree, it could be done. Please do it! 

Well, I happened to have some tools lying around that make writing
this sort of thing really easy, so I knocked together a prototype this
afternoon.  It takes definitions like "unit Length in Metres",
"Velocity is Length / Time" and "Force in Newtons is Mass *
Acceleration".  The result is an Ada package with a bunch of type X is
new Float declarations and all the given operations implemented (using
pragma Inline; maybe Intrinsic works too).

Unfortunately, there's a wee problem with magnitude changes; say I add
the following:

   function "*" (Left : Float; Right : Metres) return Metres;

Then for M : Metres, M := 2.0 * M is ambiguous, because of Standard."*"

What's more, Standard."*" has no meaning in this domain, but there's
no way (as far as I know) of getting rid of it.

If I don't define a magnitude changing function, then M := 2.0 * M
works, but M := F * M doesn't (for F : Float).

The way to fix all this would be to make the types private, and supply
conversion functions.  I've always found that ugly though.

Fraser.