🚀 go-pugleaf

In article <1992Aug11.133926.28840@actrix.gen.nz> Bruce.Hoult@bbs.actrix.gen.nz writes:
>In article <1992Aug10.200814.12809@ntmtv> hildum@ntmtv.UUCP (Eric Hildum) writes:
>> A language feature that I would like to see is support for inheritance upon
>> the builtin type enumerator. That is, I would like to define a number of
>> enumerators for an enumeration type, then use that enumeration type as a base
>> for a second enumerator with an expanded enumerator set, incorporating all the
>> base types enumerators and adding a number of new enumerators.
>> 
>> Has something like this extension been suggested anywhere?
>
>When Wirth was adding object-oriented features to Modula-2 in creating
>Oberon he used the term "type extension" where we C++ers would use
>"derivation".
>
>Being a fan of orthogonality (where sensible) he wanted to be able to apply
>type extension to enumerated types.  It seems that he could find no good way
>to do it and instead dropped enumerated types from the language entirely!
>
>Perhaps there is something nice that Wirth missed (and I seem to
>remember something about the problems in the Oberon documentation) but I
>wouldn't bet the farm on it...  :-)
>
>-- Bruce

We considered supporting enumeration type extension in Ada 9X, but
ultimately dropped it.  One issue (among others) was that values from distinct
enumeration extensions of a parent enumeration type
might have the same associated integer "code,"
making direct comparison between them less meaningful.  That is, equality
could be defined, but less-than/greater-than became relatively meaningfless.

One reason we lost interest was the observation that record 
extension/derivation tends to make enumeration types less necessary.  
Rather than having a variant record/union type with a value of an enumeration
type distinguishing the variants, one would simply define distinct
derivations.  A virtual function could return a string name
for each distinct derivation, corresponding to the enumeration literal.
A separate virtual function could return an integer code, if needed.

>Bruce.Hoult@bbs.actrix.gen.nz   Twisted pair: +64 4 477 2116
>BIX: brucehoult                 Last Resort:  PO Box 4145 Wellington, NZ
>"Cray's producing a 500 MIPS personal computer with 256MB RAM and 8 GB
>hard disk that fits in your pocket!"   "Great!  Is it PC compatible?"

S. Tucker Taft    stt@inmet.com
Intermetrics, Inc.
Cambridge, MA  02138