Re: Expressing physical units (Was: Reprise: 'in out' parameters for functions)

[Jacob Sparre Andersen <sparre@nbi.dk>]

Wojtek Narczynski wrote:
> Jacob Sparre Andersen wrote:

> > The type system is not _unable_ to express physical units.  It
> > just has some limitations in how you can do it.
> 
> It depends on how exactly you phrase out the task. If you define it
> as having a static check on physical units, I believe Ada type
> system is not able to do this. You are left with paper and pencil.

One of my programs disagrees somewhat with your statement.  I switched
from run-time to compile-time checking of unit mismatches, when a
recent version of Macks made it practical to start using compile-time
checks.

It is though correct that physical units are not a specific Ada type,
but rather have to be implemented using the basic type system.  With a
tool like Macks, it is reasonably fast to implement the necessary
types for compile-time checking of unit mismatch.  Before Macks
matured sufficiently, I either used run-time checks or hand-coded the
types I needed.

> > And although I am annoyed by the limitations, I still haven't seen
> > a description of how the language can make it easier, without
> > introducing problems outweighing the benefits. I have tried to
> > formulate such a modification to Ada myself, but ended up with
> > something that appeared to be (provably) impossible to compile.
> 
> (I converged your note here)
> 
> I find this subject fascinating. I remember matchcad doing units
> check for me 10 years ago when I was in secondary school, yet I have
> not seen any compiler capable of doing the same. I was thinking that
> a term rewriting system for partial evaluation of the program would
> do the job. Or maybe are there problems with modularization? Could
> you point me at your results, if they are public?

They are unfortunately not published (and they are written in Danish).
Also, I stopped once _I_ was certain it couldn't be done, so I never
wrote down a complete proof, but just enough to convince a colleague.
It should be noted that my analysis was tied rather strongly to Ada,
and a more relaxed programming language might be able to do something
that would be sufficiently close to full compile-time unit checking.

Are you sure that "matchcad" actually did full compile-time unit
checking?

Jacob
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