Re: Safe Units Handling in Ada

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

Fraser Wilson <newsfraser@blancolioni.org> writes:

> After a recent discussion, I put together Macks, a small program that
> generates Ada packages for dealing with physical units in a type safe
> way.

I've downloaded it and read the README and example.macks, and compiled
everything; looks good!

One quibble; example.macks says "Energy is Force / Second;". In the
real world, there is no simple relationship among Energy, Force, and
Second. Energy is Joules = kg m^2 / s^2; Force is Newtons = kg m /
s^2.

You can have "Energy is Kilogram * Square_Velocity or Force * Meter;".

(And yes, I did actually get out my physics text to check this :).

Also, is there a description of the grammar allowed in .macks files? I
assume I could work it out by reading the Ada code, but a
user-readable description would be useful. Hmm, looks like the only
thing missing from example.macks is "or", and if you use my definition
of Energy above, you get that (assuming I used "or" correctly). So
maybe that's enough; just add some comments to example.macks.

Looking at the output of "macks example.macks", I'd like a way to put
in a copyright notice; maybe have macks copy the first contiguous
comment in the source file?

Did you try using "pragma Import (Intrinsic, "*")", instead of
providing a body? That sounds like a good idea to me. Looking at the
assembly code generated by

gnatmake -O3 -gnatn example_success -cargs -save-temps

there are no function calls for "*", so pragma Inline is enough. But
'pragma Import (Intrinsic)' seems more elegant, and may save
compilation time, which may or may not be an issue.


In example_fail.adb, the statements 

N := Kg * MpS / S;
N := Kg * M / S / S;

can be made to compile by adding parens:

N := Kg * (MpS / S);
N := Kg * ((M / S) / S);

I agree it would be nice if the parens were not necessary, but I don't
mind them.


Finally, what is the derivation of the word "Macks"?

-- 
-- Stephe