[M3devel] what to do about file sizes being 32bits?

[hendrik at topoi.pooq.com]

On Fri, Jan 08, 2010 at 01:45:03AM +0000, Jay K wrote:
> 
> I understand "full range" is a problem, because, something like,
> the set of operations isn't closed.

What does this mean?  What exactly do you mean by "full range"?  And 
what do you mean by "closed"?

> 
> I believe you need to define multiple types and/or
> multiple operations.
> 
> You need an ability to trap/fail on overflow.
> 
> You need an ability for silent wraparound on overflow.
> You need perhaps a way to add precision as needed. Slow.
> You need perhaps a way to specify arbitrarily high precision,
>   and then, again, either trap/fail or silent wraparound.
> 
> Basically, in my opinion, having just "INTEGER" and just "+"
> isn't nearly sufficient.
> 
> Not having operator overloading makes pretty much any solution painful to use.

+ is defined on integers and reals.  If that's not an overload, why not 
have + defined on longint as well?

> We and C both have a compromise that covers most cases and
> when people really need higher fixed or arbitrary precision, they
> either give up the convenient "operator" syntax or use C++.
>  
> 
> As I understand, in C, unsigned integers are defined to "silently wraparound"

The wraparound unsigned integers are extremely useful.  The most common 
operations, +, *, and - have a clean, well-defined semantics as 
arithmetic modulo 2^n.  It satisfies a lot of well-known algebraic 
identifies.

Suppose you have an expression involving +, -, * and integers in 
0..2^n - 1 and its correct result is in this 0..2^n - 1 range as well.
If you evaluate it with nonwraparound arithmetic, you may get overflow.
Nonetheless, using wraparound arithmetic under these conditions will 
still give the right answer.

This makes wraparound integers useful for indexing strange arrays with, 
say, multiple subscripts in the rante 1000000000..1000000003.  
Intermediate results partway through subscript evaluations can overflow 
to their hearts' content, and you still get the right element in the 
end.

> 
> and signed integers are implementation defined, could "trap" but in reality
> all implementations "silently wraparound".
> It is a point of unsafety though, beyond the more well known
> buffer overflows, leaks, etc.
> 
>  - Jay
> 
>  
> > Date: Thu, 7 Jan 2010 19:22:00 -0600
> > From: rodney_bates at lcwb.coop
> > To: m3devel at elegosoft.com
> > Subject: Re: [M3devel] what to do about file sizes being 32bits?
> > 
> > Full-range unsigned integers are a language designer's headache, because
> > there are conflicts no matter what you do. The Modula-3 approach is to
> > use the operations in interface Word.i3 (and now Long.i3) on type
> > INTEGER (not CARDINAL, which has only half the range). These apply
> > unsigned interpretation to values of type INTEGER.

The problem with Word.i3 is that expressions involving these integers 
are so unreadable as to be seriously susceptible to error.

-- hendrik