Re: [Aldor-l] non-constant constants was: Re: [Axiom-developer] Re: exports and constants

[Christian Aistleitner]

Hello,

> > [ Generators ]
>
> Hmmm... I see. A constant that is not constant...

For exactly this reason, you should not define a Generator to be constant.  
It's bad style ;)
You can modify every connstant that has destructive functions...

Is there any language forbidding this?

> Generators are "mutable"? That seems like a very peculiar and  
> questionable semantics
> to me!

No. It's the whole point of generators...


> So if I write:
>
>   for x in const repeat
>     ...
>   for x in const repeat
>     ...
>
> The 2nd loop is guarranteed never to execute?!

I assume, you have some
  const == someGenerator;
before the for loops. Then segfaults happen (occasionally) with such  
constructs.
But after all, you should not write such code.

> But if I have some other constant constant like a list
>
>   const2: List Integer == [1,2,3]
>
>   for x in const2 repeat
>     ...
>   for x in const2 repeat
>     ...
>
> or even
>
>   for x in 1..10 repeat
>     ...
>   for x in 1..10 repeat
>     ...
> this works as expected even though in Aldor iteration over a list
> is implemented as a generator.

yes. Because the you call the generator FUNCTION once your every for loop.
Your loops *magic* are translated into
for x in generator const2 repeat
{
   ...
}
for x in generator const2 repeat
{
   ...
}
(or something equivalent). So the generatos are NOT constants. But the  
List you take the generator of is constant.

> > If you like you could also say something like
> >    a: Record(x: Integer) == [1];
> >    a.x := 0;
> > and the "constant" a changes. But you should note that the
> > last line is
> > actually
> >    set!(a, x, 0)
> > so that is a destructive function and thus you should expect strange
> > things. The BANG functions (like set!) are only for people
> > who know what they do.
>
> Of course but I do not see the relevance of this to a generator.
> Yes I understand that a generator is always in a certain "state"
> and what the generator yields depends on that state, but surely
> there are reasonable contexts in which the state of the generator
> is reset?
>
> Further what does this have to do with the type of the constant,
> i.e. whether it is>
>   const: Generator Integer
>
> or
>
>   const: () -> Generator Integer

I already explained things above. If it's still a mystery to you ... tell  
me, and I'll be more explicit.

> Do you know of any other language where Generators have the
> same semantics as in Aldor?

For example Java:
http://java.sun.com/j2se/1.3/docs/api/java/util/Iterator.html

For example C++:
http://www.sgi.com/tech/stl/Iterators.html

--
Kind regards,
Christian