[M3devel] Oops, forgot to ask

[Rodney M. Bates]

Tony Hosking wrote:
> On 16 Dec 2009, at 15:48, Peter Eiserloh wrote:
>
>   
>> Hi Gang,
>>
>> 0 - CARDINALs are an integral type defined by the language
>> spec to be from 0 (zero) to MAXINT (not MAXCARD).  Recently,
>> a change was made to CM3 to extend the language it recognizes
>> beyond the original language spec (SPWM3), now CM3 understands
>> a LONGINT.  
>>
>> Was there a corresponding LONGCARD defined?
>>     
>
> No.  But you can write:
>
> [0L..LAST(LONGINT)]
>
> just as
>
> CARDINAL=[0..LAST(INTEGER)]
>   
Actually, the line above was once true, long long ago, when dinosaurs
roamed unfettered.  CARDINAL was changed to be "just like"
(but not equal to) [0..LAST(INTEGER)].  This would have been
maybe early 1990s. 

It took me years to figure out what this actually meant, and more
years to figure out why, but I believe I now know.  CARDINAL is not an
equal type to [0..LAST(INTEGER)], but otherwise has all the same
properties.  This makes the two mutually assignable, which is close
enough to equal to not matter in most contexts.  The exact consequences
follow from other language rules. 

The motivation is this:  [0..LAST(INTEGER)] on a 32-bit machine is not
equal to [0..LAST(INTEGER)] on a 64-bit machine (or any combination of
machines with different values of LAST(INTEGER)), because the numeric
value of LAST(INTEGER) is part of the expanded type.  The one place this
matters is if you use pickles to transfer data between machines of different
word sizes.  If the type is [0..LAST(INTEGER)], the type signature (a hash
of the complete, expanded type definition) is different on the two machines,
and values of or containing this type cannot be transferred.  Exact type
signature matches are used in pickles to find corresponding types when
reading. 

But pickles handle different sized INTEGER values fine, expanding/shortening
as needed.  Being a leaf in a type definition, INTEGER is always the same
type on any machine.  So CARDINAL was changed to be its own unique
type too, so it would be possible also to transfer CARDINAL values in 
pickles
between different word sizes. 

So, we really should have a LONGCARD, parallel to CARDINAL.

Note that this also affects network objects, which use pickles to transfer
values of the objects.


 
>
>   
>> Can is use all 64-bits, or is it restricted to 63, like
>> the CARDINAL is only 31-bits.
>>     
>
> 63 bits, since its underlying base type is LONGINT.
>
>
>   
>>
>>
>>
>> +--------------------------------------------------------+
>> | Peter P. Eiserloh                                      |
>> +--------------------------------------------------------+
>>
>>
>>
>>     
>
>
>