[M3devel] reference to globals in globals?

[Rodney M. Bates]

On 08/15/2012 07:55 PM, Hendrik Boom wrote:
> On Wed, Aug 15, 2012 at 09:51:41AM -0500, Rodney M. Bates wrote:
>>
>>
>> On 08/14/2012 10:04 PM, Jay K wrote:
>>>
>>> Heck, even if these weren't global, is it that unreasonble,
>> >from the programmer's point of view, for the language/compiler
>>> to do some pointer escape analysis and let me take the address
>>> of a local, as long as I don't store it somewhere that outlives
>>> the local?
>>>
>>
>> This is ultimately an undecidable problem and even conservative
>> approximations of reasonable sophistication are far too involved
>> for a language to require of every compiler.
>
> Not to mention the obscurityof the language definition.
>
> But isn't there something like that with the addresses of top-level
> proocedures?  the ones whose environments are completely static?
> Or am I thinking of another type-safe language?  Or am I thinking of
> assigning the procedures themselves?
>
> -- hendrik
>
>> Of course, safe languages occasionally make you unnecessarily write a bit more
>> code to do it the safe way.  E.g., the famous fake pointer to the root of a
>> linked list example.
>
> You mean something like (in C with probable errors)
>
>    list **p, *q;
>    make q point to some linked list
>
>    for ( p = &q; *p != NIL; p = &((*p)->next))
>    {
>     And here you can easily delete elements by *p = (*p) -> next
>    }

Hmm, this is not quite what I had in mind, but it's close.
I like to code things like this with a pointer to the interesting
node _and_ one that lags one node behind, in case list surgery is
needed.  Also, a more complete example would refer to one or more
data fields of a node too.  But I think I can illustrate my intentions
with your example.

>
> This can be done prefectly type-safe in languages that allow
>
>     REF REF list

Plus, an ADR/& operation.  In the case of C's &, it's type at
least remembers the type of the object whose address is taken.
Modula-3's ADR does not, so would require a LOOPHOLE as well.

But no matter, because an operator that can take the address of anything
other than a heap object (or maybe a component of one?) is unsafe.

>
> Or is there some other fake pointer trick you're talking about?
>
> -- hendrik
>