Re: Safe Units Handling in Ada

[Fraser <newsfraser@blancolioni.org>]

Stephen Leake wrote:

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

Glad to hear it.

> 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.

Whoops.  What's Force / Second mean?  That should be Force * Meter I 
guess, and that evaluates to Joules directly.  I wonder if there's a 
clean way of handling constants, such as the 0.5 in the kinetic energy 
formula.

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

I googled the basics, and did the derived from memory.  The physics 
doesn't change, but the memory gets a bit 2Fe + 3O2 => 2Fe2O3.   :)

> Also, is there a description of the grammar allowed in .macks files?

Oh, good idea.  It's not at all complicated, but I'll add a description. 
  Here's a quick version:

macks-file  ::= { unit-specification ';' }

unit-specification ::=
	fundamental-unit-specification | derived-unit-specification

fundamental-unit-specification ::= unit-name [ 'is' 'fundamental' ]

derived-unit-specification ::=
	unit-name 'is' dimension-expression
		{ 'or' dimension-expression }

dimension-expression ::=
	unit-name operator unit-name

opereator ::= '*' | '/'

> 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?

Yes, that would be useful.  Comments are the same as Ada, by the way.  I 
can't remember off-hand whether it's easy to extract comment text from 
the lexer; if not, I'll come up with some other mechanism.

I agree about Intrinsic being more elegant, though I haven't tried it 
yet.  If it works properly (and I'm sure it will), I'll switch over, or 
at least add an option.

> In example_fail.adb, the statements 
> 
> N := Kg * MpS / S;
> N := Kg * M / S / S;
> 
> can be made to compile by adding parens:

Indeed.  It's unfortunate that one has to remember which units have been 
explicitly defined in order to place the parentheses properly, but I 
can't think of a clean way around that right now, and at least it's 
still type safe.

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

(Meters/Moles), Amperes, Candela, (Kilograms/Kelvin), Seconds.  I 
probably spent too much time thinking about that. :-)

Thanks for the comments!

cheers,
Fraser.